the photo chemistry of aqueous nitrate ion revisited

8/3/2019 The Photo Chemistry of Aqueous Nitrate Ion Revisited

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JotmmlofANDA : ~ S T R YE L S E V I E R Journal of Photochemistry and Photobiology A: Chemistry 101 (1996) 89-103

The photochemistry of aqueous n itrate ion rev is i tedG e r t r a u d M a r k a, H a n s - G e r t K o r t h b, H e i n z - P e t e r S c h u c h m a n n a , C l e m e n s v o n S o n n ta g a , ,

M ax-P lanc k-lns titut iir Strahlenchemie, Stiftstrasse 34-36. P.O. Box 101365, D-45 413 Miilheim. Germanyh Universit~it-GH Essen, lns titu tfiir Organische Chem ie, P.O. Box 103764, D-451 17 Essen, Germany

Received 6 November 1995; accepted 29 April 1996

Abs t r a c t

A q u e o u s n i t r a te s o l u t i o n s w e r e p h o t o l y s ed a t 2 5 4 n m i n t h e a b s e n c e o f o x i d i z a b l e a d d it i v e s, i n t h e p r e s e n c e o f m e t h a n o l o r p r o p a n - 2 - o la n d o x y g e n a n d i n t h e p r e s e n c e o f c y c l o p e n t a n e u n d e r a n a e r o b i c c o n d i t i o n s . T h e m a i n n i t r o g e n - c o n t a i n i n g produc t s a r e n i t r i t e a nd pe roxyni t r i t e .T h e q u a n t u m y i e l d s d e p e n d o n t h e p H , n i t r a te c o n c e n t r a t io n , n a t u r e o f t h e a d d i t i v e a n d t h e l i g h t i n t en s i t y , T h e i n t r i n s ic n i tr i t e y i el d i n a l k a l i n es o l u t i o n s c o u l d n o t b e d e t e r m i n e d d i r e c t l y b e c a u s e , u n d e r t h e c o n d i t i o n s o f t h e n i t r it e a s sa y , t h e a c c o m p a n y i n g p e r o x y n i t r it e d e c o m p o s e s t oform n i t r i t e a nd n i t r a te ; i t is sm a l le r tha n the a ppa re n t n i t r i t e y ie ld . In the a c id ic (pH 4- 7) r a nge , the in t r ins ic n i t r it e qua ntu m yie ld i s e qua lto the a ppa re n t n i t r i t e y ie ld be c a use the re i s no bu i ldup of pe roxyni t r i t e unde r the se c ondi t ions . The a ppa re n t n i t r i t e qua nt um yie ld inc re a se sf r o m 0 .0 1 ( n o o x i d i z a b l e a d d i ti v e ) t o a p p r o x i m a t e l y 0 . 0 3 ( c y c l o p e n t a n e ( m i l l i m o l a r r a n g e ) , o x y g e n f r ee ) t o 0 . 0 6 ( m e t h a n o l ( m i l l i m o l a rr a ng e ) , a i r s a tura te d) . A t pH 13 a nd in the a bse n c e of ox id iz a b le a ddi t ive s , the a ppa re n t n i tr i t e qua ntu m yie ld inc re a se s to a bout 0 . 1 , w he re a sf r o m m a t e r i al b a l a n c e c o n s i d e r a t i o n s t h e i n t r i n s i c n i t ri t e q u a n t u m y i e l d i s e s t im a t e d t o b e 0 . 0 6 , t w i ce t h e o x y g e n q u a n t u m y i e l d o f 0 . 0 3.Spe c t rop hotom e t r i c a l ly , pe roxyn i t r i t e i s de tec te d in the a lka l ine ra nge on ly , be c a use i t s p ro tona te d form i s uns ta b le .

I n t h e a b s e n c e o f o x i d i z a b l e a d d i t i v e s, t h e q u a n t u m y i e l d o f p e r o x y n i tr i te i s a b o u t 0 . 1 , i .e . o n l y a b o u t t w o - t h i r d s o f t h e q u a n t u m y i e l d i nt h e p r e s e n c e o f o x i d i z a b l e a d d i t iv e s . M e c h a n i s t i c c o n s i d e r a ti o n s o n t h e b a s i s o f t h e p H d e p e n d e n c e o f t h e q u a n t u m y i e l d s o f t h e p r o d u c t sn i t r i t e , pe roxyni t r i t e a nd oxyge n , a s w e l l a s the i r de pe nde nc e on the k ind of a ddi t ive , ind ic a te tha t the de c i s ive f a c tor o f photo lys i s in thea b s e n c e o f a d d i t i v e s i s t h e f o r m a t i o n o f t h e n i t r i c o x i d e p e r o x y l r a d i c al , O N O O , f o r m e d b y r e a c t i o n o f p e r o x y n i t r i t e w i t h t h e p r i m a r i l yg e n e r a t e d O H r a d i ca l . T h e d e c a y o f O N O O i s t h e s o u rc e o f 0 2 i n th i s s y st e m . N i tr i c o x i d e , N O , t h e o t h e r f r ag m e n t o f t h is d e c ay r e a c t io n ,r e a c t s w i th n i t rog e n d iox ide , w h ic h i s one of the pr im a r i ly form e d in te rm e d ia te s . The l a t t e r r e a c t ion i s one of the pa thw a y s to the produc tn i t ri t e , p a r t i c u la r l y i n t h e a l k a l in e r a n g e . T h e f o r m a t i o n o f N O d u r i n g p h o t o l y s i s h a s b e e n v e r if i ed b y e l e c t r o n s p i n r e s o n a n c e ( E S R )s p e c t ro s c o p i c d e t e c t i o n o f t h e n i t r o x i d e l , l , 3 , 3 - t e t r a m e t h y l - i s o i n d o l i n - 2 - o x y l , h e N O a d d u c t t o 7 , 7 , 8 , 8 - t e t ra m e t h y l - o - q u i n o d i m e t h a n e .

O f the th re e pr im a ry p roc e s se s d i s c us se d in the l i te r a ture , w e c onc lu de tha t r e a c t ions ( 1 ) a nd (2) oc c ur w i th qua ntum yie ld s of a pproxim a te ly0 .09 a nd 0 .1 r e spe c t ive lyN O s + h u - - - ~ N 0 2 + O ( O - + H 2 0 - - - ~ O H + O H ) ( 1 )N O r + h v ~ O N O O - ( 2 )I t a ppe a r s tha t n one o f the pe roxyn i t r i t e a n ion i s fo rm e d in a c a ge r e a c t ion th rough the r e c om bina t ion of the pr im a ry f r a gm e nts f rom re a c t ion( 1 ) . T h e p r i m a r y p r o c e s s s h o w n i n r e a c t i o n ( 3 ) i s o f r e la t i v e ly m i n o r i m p o r t a n c e , w i th a q u a n t u m y i e l d o f n o m o r e t h a n 0 . 0 01N O r + h v ~ N O 2 + O ( 3)

I n th e p r e s e n c e o f m e t h a n o l ( o r p r o p a n - 2 - o l ) a n d o x y g e n u n d e r a ci d i c c o n d i ti o n s , fo r m a l d e h y d e ( o r a c e t o n e ) i s f o rm e d i n a n a m o u n te quiv a le n t to n i t r i t e v ia pe roxyl r a d ic a l r e a c t ions ( qua ntu m yie ld of a pproxim a te ly 0 .06 for bo th a lc oho l s ) . In the a lka l ine r a nge , the a ppa re n tf o r m a l d e h y d e q u a n t u m y i e l d d e c r e as e s w i t h i n c r e a s in g p H , w h i l e f o rm i c a c i d i s p r o d u c e d i n i n c r e a s i n g a m o u n t s . T h e f o r m a t i o n o f f o r m i ca c id i s a s c r ibe d to the r e a c t ion of pe roxyni t r i t e a n ion w i th photo ly t i c a l ly ge ne ra te d form a lde hyde . The a c e tone qua ntum yie ld doe s no td e c r e as e w i t h i n c r e a s i n g p H o v e r t h e w h o l e a l k a l i n e p H r a n g e .

I n t h e p r e s e n c e o f c y c l o p e n t a n e u n d e r o x y g e n - f r e e c o n d i t i o n s , a p a r t f ro m n i t r it e ( a n d p e r o x y n i t r it e w h e n a l k a l i n e ) , t h e c o m p o u n d sn i t r o c y c l o p e n t a n e , c y c l o p e n t y l n i tr a t e, c y c l o p e n t e n e , c y c l o p e n t a n o l a n d c y c l o p e n t a n o n e a r e p r o d u c e d . T h e f o r m a t i o n o f t h e o r g a n i c n i t r o g e nc om pou nds l e a ds to a n inc re a se in the pH a s photo lys i s p roc e e ds . Thi s pH sh i f t is pa r t i c u la r ly pron oun c e d in the ne ut r a l r a nge .K e y w o rd s : Nitrate ion; Photochemistry; Peroxynitrite; Nitric oxide peroxyl radical

* Corresponding author. Tel. : +4 9 208 306 352 9; fax: + 49 208 3063951.1010-6030/96/$15.00 1996 Elsevier Science S.A. All rights reservedP 1 1 S 1 0 1 0 - 6 0 3 0 ( 9 6 ) 0 4 3 9 1 - 2


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G. Mark et al. / Journal of Phowchemistry and Photobiology A: Chemistry 101 (1996) 89-103 91

N o d i s t in c t io n h a s b e e n m a d e to d a t e b e tw e e n r e a c t io n (2 )a n d th e p o s s ib i l i t y th a t p e ro x y n i t r i t e O N O O i s n o t s t r ic t lya p r i m a r y p h o t o p r o d u c t, b u t r a t h er f o r m e d b y r e c o m b i n a t i o no f N O 2 a n d O - i n t h e s o l v e n t c a g e (r e a c t io n ( 7 ) ,S c h e m e 1 ) . O n th e o th e r h a n d , o n th e b a s i s o f r e s u l t s o b ta in e dd u r in g th e p h o to ly s i s o f c ry s t a l l i n e K N O 3 , i t h a s b e e n s u g -g e s t e d r e c e n t ly th a t r e a c t io n ( 2 ) i s in d e e d th e d o m in a n t p r i -m a ry p ro c e s s [ 2 5 ] .

I n r e a c t i o n ( 3 ) , 2 5 4 n m r a d i a t i o n m a y p r o d u c e o x y g e na to m s in th e s in g le t (h ig h - e n e rg y ) o r t r i p l e t ( l o w -e n e rg y )s t a te . T h e s in g le t o x y g e n a to m i s k n o w n to in s e r t r a p id ly in toth e w a te r m o le c u le , fo rm in g H 2 0 2 [ 3 3 , 3 4 ] . H o w e v e r , H 2 0 2h a s n o t b e e n d e te c t e d a m o n g th e p ro d u c t s [ 1 4 , 1 6 ,1 7 ] , w h ic hle a v e s th e p o s s ib i l i t y o f t r ip l e t o x y g e n a to m fo rm a t io n . In c o n -s i s t en t ly , t h i s h a s b e e n b e l i e v e d to b e th e m a in p r im a ry p ro c -e s s [ 1 7 ] , ru l e d o u t a l to g e th e r [ 1 4 ] o r g iv e n a s h are o f a b o u t1 0 % ( 9 0 % O H r a d ic a l f o r m a t i o n ) [ 2 0 ] o n t h e b a s is o f t h ere a c t io n o f O ( 3 p ) w i th c y c lo p e n te n e ( th i s g iv e s r i s e to e th -y le n e [3 5 ] w h ic h is t h e n m e a s u re d ) .

T h e p h o t o c h e m i s t r y c h a n g e s o n g o i n g f r o m a c i d i c / n e u t r a lto a lk a lin e s o lu t io n s [ 13 , 1 4 ,1 7 ] ( s e e F ig . 2 ) . T h i s c a n n o t b ed u e to th e in f lu e n c e o f p H o n th e p r im a ry p ro c e s s e s ; i n st e a d ,i t m u s t b e l i n k e d to th e c o n s e c u t iv e c h e m is t ry o f s h o r t - l iv e din te rm e d ia t e s fo r m e d in th e p r im a ry p ro c e s s e s . T h i s fo l lo w sf ro m th e o b s e rv a t io n th a t a c h a n g e in th e n i t r i t e q u a n tu my ie ld i s o b s e rv e d a t a p H v a lu e a s l o w a s p H 8 , c o r re s p o n d in gt o a n O H - c o n c e n t ra t i o n o f 1 0 - 6 m o l d m - 3 . W i t h r e g a r d t oth e e x c i t e d s t a t e o f t h e n i t r a te i o n , w e e s t im a te a h a l f - l i f e o fth e o rd e r o f 10 4 s fo r t h e q u e n c h in g p ro c e s s b y O H io n s ,a s s u m i n g a m a x i m u m r a t e c o n s ta n t o f l 0 w d m 3 m o l ~ s 1fo r t h e q u e n c h in g ; t h i s i s c e r ta in ly to o lo n g fo r t h e l i f e tim eo f a n e x c i t e d s t a t e u n d e r s u c h c o n d i t io n s . In a d d i t io n , t h ein t e ra c t io n w i th O H - i s ru l e d o u t o n th e b a s is o f e x p e r im e n t s ,s in c e o n f l a s h p h o to ly s i s i t h a s b e e n o b s e rv e d th a t p e ro x y n i -t r i te fo rm a t io n fo l lo w s f i r s t -o rd e r k in e t i c s (h a l f - t im e , 1 5 m s ) ,in d e p e n d e n t o f t h e p H [ 1 5 ] . T h e o n ly in t e rm e d ia t e r e s u l t i n gf ro m th e p r im a ry p ro c e s s e s w h ic h i s l i k e ly to d i s p la y a p ro -n o u n c e d p H - d e p e n d e n t c h e m i s t r y i s p e r o x y n i t r o u s a c i dw h o s e p K a v a lu e h a s b e e n r e p o r t e d to b e 6 . 7 [3 6 ] , 6 . 8 [3 7 ]o r 6 . 5 [3 8 ] .

O th e r a t t e m p t s h a v e b e e n m a d e in th e p a s t t o e x p la in th ep H b e h a v io u r . T h e p H d e p e n d e n c e h a s b e e n l i n k e d to th eh y p o th e t i c a l fo rm a t io n o f p e ro x y n i t r i c a c id a n d i t s s u b s e -q u e n t d e c a y in to O 2 a n d H N O 2 , th e s ig m o id a l c u rv e r e f le c t in gth e p K a v a lu e o f H O O N O 2 [ 1 3 ] , o r t o th e r e a c t io n o f p e r -o x y n i t ro u s a c id w i th th e s u b s t r a t e N O 3 a n d th e p ro d u c tN O , d e p e n d in g o n i t s s t a t e o f d i ss o c ia t io n [ 1 4 ] . C o m p e t in gr e a c ti o n s o f O - , e .g . th e f o r m a t i o n o f O a n d N O 2 f r o m O ~a n d N O 2 , h a v e a l s o b e e n h e ld r e s p o n s ib l e [ 1 8 ] . H o w e v e r ,th i s s e e m s im p o s s ib l e in v i e w o f t he f a s t p ro to n a t io n o f O( re a c t io n (6 ) , S c h e m e 1 ) , a t l e a s t b e lo w p H 1 1.

T h e f o r e m o s t r e a s o n f o r t h e m e c h a n i s ti c c o m p l e x i t y o f th en i t r a te s y s t e m i s t h a t t h e p ro to n a te d fo rm o f th e in it i al p ro d u c tp e r o x y n i t ri t e O N O O ( r e a c ti o n ( 8 ) , S c h e m e 1 ), p e r o x y n i -t ro u s a c id O N O O H (s e e R e f s . [ 1 6 , 1 8 ,3 6 , 3 9 ,4 0 ] ) , i s o m e r i z e sto n i t r ic a c id . T h i s m a y o c c u r d ir e c t ly [3 7 ] ( r e a c t io n (1 0 ) ) ,

b u t a l s o b y w a y o f c l e a v a g e in to " N O 2 a n d O H r a d i ca l s( r e a c t io n ( 9 ) , S c h e m e 1 ) f o l l o w e d b y b u l k ( r e a c ti o n ( 1 4 ) )a n d c a g e ( r e a c t io n (1 3 ) ) r e c o m b in a t io n [ 3 6 ,4 0 -- -4 2 ], w i th ara t e c o n s t a n t fo r t h e d e c a y o f t h e o rd e r o f a f e w re c ip ro c a ls e c o n d s [ 1 6 , 1 8 ,3 6 , 3 9 ,4 0 , 4 3 ] (m o s t r e c e n t v a lu e s : 1 .4 _ +0 .4s J [36 ] ; 1 .3 s -1 a t 25 C [3 7]; 1 .0 s -~ a t 20 C [ 38 ]) ,w h i l e t h e a n io n i t s e l f is t h e rm a l ly s t a b le fo r m in u te s [ 3 9 ] o re v e n s e v e ra l d a y s [ 1 6 , 36 ] ( fo r a r e c e n t r e v ie w , s e e R e f .[4 4 ] ) . I t h a s b e e n p ro p o s e d th a t th e p ro b a b i l i t y th a t O Hra d ic a l s b e c o m e m a n i fe s t i n th i s d e c o m p o s i t io n ( i . e. d o n o te v e n tu a l ly l e a d to th e fo rm a t io n o f n i t r a t e i n a s o lu t io n o fO N O O H ) m a y be a p p r o x im a t e l y 3 0 % - 4 0 % [ 3 6 ,4 1 ] .R e c e n t ly , t h e p e ro x y n i t ro u s a c id m o le c u le in i t s " t r a n s " c o n -fo rm a t io n h a s b e e n im p l i c a t e d in h y d ro x y l - r a d ic a l - l i k e r e a c -t io n s [ 3 7 , 4 5 -4 7 ] .

I t is s h o w n b e lo w th a t t h e g ro s s f e a tu re s o f t h e p H d e p e n d -e n c e o f t h e n i t ri t e a n d o x y g e n q u a n tu m y ie ld s in th e a b s e n c eo f o x id iz a b le a d d i t iv e s m u s t b e e x p la in e d o n th e b a s i s o fth e s e f a c t s , a n d th a t i n th e p re s e n c e o f o x id iz a b le a d d i t iv e sth e r e a c t io n m e c h a n i s m c h a n g e s d ra s t i c a l ly . Me th a n o l , p ro -p a n - 2 - ol a n d c y c l o p e n t a n e w e r e c h o s e n a s m o d e l c o m p o u n d sfo r t h e n a tu ra l p o ly s a c c h a r id e t r a c e c o n s t i t u e n t s a n d h y d ro -c a rb o n im p u r i t i e s i n s u r fa c e w a te r s.

2 . E x p e r i m e n t a l d e t a i l sA q u e o u s s o lu t io n s o f m a g n e s iu m n i t r a te (M e rc k , p . a .;

1 0 -3 _ 1 0 - t to o l d m 3; fo r s o lu t io n s o f p H < 1 1, t h i s s a l t w a sc h o s e n b e c a u s e i t h a n d le s b e t t e r t h a n a lk a l i n i t r a te s ) o r p o ta s -s iu m n i t r a t e (M e rc k , p . a . ; d r i e d a t 1 0 5 C a n d s to re d o v e r d rys i l i c a g e l ) , i n t h e p re s e n c e o r a b s e n c e o f a n o x id iz a b le a d d i -t i v e , w e re p h o to ly s e d u s in g l i g h t f ro m a lo w -p re s s u re m e r -c u r y a rc ( H e r a e u s S t e r is o l N N 3 0 / 8 9 , f l u e nc e r at e s H ( 2 5 4n m ) = 0 . 5 -1 0 J m - 2 s 1, w h ic h c o u ld b e v a r i e d b y c h a n g in gth e d i s t a n c e b e tw e e n th e l a m p a n d s a m p le ; o r G ra e n tz e l(K a r l s ru h e ) U V i r r a d ia to r 1 8 5 -2 5 4 n m , V y c o r - f i l t e r e d ,H ( 2 5 4 n m ) = 2 0 0 J m 2 s - 1 . T h e h ig h e r n i t ra t e c o n c e n t ra -t io n s (w h e re th e r a t e s o f p ro d u c t fo rm a t io n a re h ig h e r ) w e reu s u a l ly c h o s e n w h e n th e a s s a y o f a p ro d u c t w a s r e l a t iv e lyin s e n s i t iv e (w h e re a s th e n i t r i te a s s a y i s v e ry s e n s i t i v e ) .

T h e o p t i c a l c e l l s w e re S u p ra s i l Q S ( H e l lm a ) ( s i z e , 4c m 1 c m ( f a c e ) 1 c m ( d e p t h ) ) o r w e re m a d e f ro m r ec -t a n g u l a r q u a r t z t u b i n g t y p e H S Q - 3 0 0 ( H e r a e u s Q u a r z s c h -m e l z e , H a n a u ) ( s i ze , 15 c m 3 c m ( f a c e ) 0 . 5 c m( d e p t h ) ) . O x y g e n a t i o n o f t he s o l u t io n s w a s a c h i e v e d b yp a s s in g a s t r e a m o f o x y g e n g a s th ro u g h th e s a m p le fo r 5 m in .W h e n s o lu t io n s w e re r e q u i re d to b e o x y g e n f r e e , t h e s a m p le sw e re p u rg e d w i th a s t r e a m o f a rg o n fo r a b o u t 2 0 r a in ( th ec e l l a s s e m b ly u s e d in s u c h c a s e s h a s b e e n d e s c r ib e d e l s e w h e re[ 4 8 1 ) .

T h e f lu e n c e r a t e o f t h e in c id e n t U V l ig h t w a s d e te rm in e db y f e m o x a l a t e a c t i n o m e t r y [ 49 1 ( t h e v i s i b l e p a r t o f t h e l a m pl ig h t w a s t a k e n in to a c c o u n t b y s h u t t in g o u t t h e U V p a r t w i tha g l a s s p l a t e ) o r b y u s in g a p e ro x id i s u lp h a te / t e r t -b u ta n o l /0 2 a c t in o m e te r [5 0 ] (w h ic h r e g i s t e r s o n ly th e 2 5 4 n m l in e ) .


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92 G. Mark e t al . / Journal o f Photochemis t ry and Photobio logy A: Chemis t ry 101 (1996) 8 9-103Th e f l u en ce r a t e ab so rb ed b y t h e n i t r a t e so lu ti o n s was ca l -cu l a t ed acco rd in g t o Beer ' s l aw.

M i i l i p o re M i l l i -Q p u r i f i ed wa t e r was u sed . Th e h ig h es tp o ss ib l e p u r i t y i s d es i rab l e fo r ex p er imen t s t o b e p e r fo rm edwi th o u t o rg an i c ad d i t i v es . Ev en smal l t r aces o f su ch mate r i a lmo d i fy t h e co u r se o f p h o to ly s is ( s ee b e lo w ) . I r r ad ia t i o nswere a l so p e r fo rmed i n t h e p resen ce o f ad d i t i v es (o x y g en ,meth an o l , p ro p an -2 -o l an d cy c lo p en t an e ) . M eth an o l an d p ro -p an -2 -o l co n cen t r a t i o n s were i n th e r an g e 1 0 -4 - -1 8 an d 0 .1 -1 0 t o o l d m - 3 r esp ec t i v e ly ; t h a t o f cy c lo p en t an e was 2 X 1 0 3mo l d m - 3 ( sa tu ra t i o n co n cen t r a t i o n ) .

Ni t r i te w as d e t e rmin ed sp ec t ro p h o to met r i ca l l y b y i ts r edazo d y e fo rmed f ro m su lp h an i l amid e an d N- (1 -n ap h th y l ) e t h y l en ed i am in e [ 5 1 ] ( see a l so G erman S t an d ardM eth o d s , d e t e rmin a t i o n o f n i t ri t e in wa t e r , DIN 3 8 5 0 5 ) . P e r -o x y n i t r i t e is p ro d u c ed t o g e th er wi th n i t r it e wh e n t h e p h o to l -ysis i s carr ied ou t in basic so lu t ion . I t s p resence in ter fereswi th t h e n i t ri t e a s say , i n wh ich i t is d eco m p o sed v i a ( i n p a r t )f ree rad ical path wa ys, g iv ing r i se to fu r ther n i t r ite as wel l asn i t r a te ( see b e lo w ) . T h e n i t r it e a s say i n v o lv es t h e p resen ceo f an o x id i zab l e o rg an i c co m p o u n d i n a so lu t i o n o f l ow p H.

Th e p resen ce o f N O as an i n te rmed ia t e i n n i t ra t e p ho to l -y s i s was u n eq u iv o ca l l y es t ab l i sh ed b y e l ec t ro n sp in r eso -n a n c e ( E S R ) s p e c t ro s c o p y : a s tr e a m o f A r ( 2 0 m l r a i n - ' )was p assed t h ro u g h 3 m l o f a 1 t o o l d m - 3 p o t as s iu m n i t r a teso lu t io n u n d er co n t i n u o u s 2 5 4 n m p h o to ly s i s (Ray o n e t p h o -to reac to r ) fo r 1 h , an d t h e v o l a t i l e p ro d u c t s were t r an s fe r r edin to a 4 mm q u ar t z ES R tu b e co n t a in in g a 1 0 3 t o o l d m -3h ex an e so lu t i o n o f t h e NO sp in t r ap 7 ,7 ,8 ,8 - te t r ameth y l -o -q u in o d imeth an e ( NO CT - 1 ) a t - 3 0 C [ 5 2 ,5 3 ] . ES R sp ec t r awere r eco rd ed a t 2 0 C o n a Bru k er ER-4 2 0 X-b an dsp ec t ro mete r .

P ero x y n i t r i t e io n i n a lk a l i n e samp les [3 9 ] was d e t e rmin edsp ec t ro p h o to m et r i ca l l y b y m easu r in g t h e o p t i ca l ab so rp t i o na t 3 0 2 n m ( E ( 3 0 2 n m ) = 1 6 7 0 d m 3 m o i - l s - l [ 5 4 ] ) a f t e rphoto lysis .

Org an i c p ro d u c t s f ro m th e cy c lo p en t an e sy s t em (cy c lo -p en t an o l , cy c lo p en t a n o n e , cy c lo p en t en e , n i t ro cy c lo p en t an e ,cy c io p en ty l n i t r a te ) were an a ly sed b y g as ch ro mato g rap h y(G C) an d i d en t i f i ed wi th t h e h e lp o f r e f e r en ce mate r i a l [ 5 5 ]an d /o r m ass sp ec t ro met r i ca l l y . F o rm ald eh y d e was d e t e r -min ed b y t h e ace ty l ace to n e meth o d [ 5 6 ] an d ace to n e b y hig hp e r f o r m a n c e l i qu i d c h r o m a t o g r a p h y ( H P L C ) a s d in i tr o p he n -y lh y d raz o n e ( c o lu m n , N u c l eo s i l 5 C 1 8 ; e l u en t , 7 0 % ace to n -i t r i l e / H 2 0 ) [ 5 7 ] . I n s o m e e x p e r i m e n ts , t h e O H - p r o d u c ti o nwas d e t e rmin ed b y t i tr a t i on o r b y m easu reme n t o f th e p Haf t e r p h o to ly s i s . Th e ev o lu t i o n o f 0 2 was fo l l o w ed d u r in g th eco u r se o f i r r ad i a ti o n w i th an o x y g en - sen s i t i v e e l ec t ro d e(Wissen sch af t l i ch -Tech n i sch e Werk s t a t t en , Wei lh e im, Ger -ma n y ) ; b ecau se o f i ts r o u n d sh ap e , t h e q u ar tz v esse l (H eraeu sHan au , H S Q -3 0 0 ) t h a t acco m mo d ates t h e e l ec t ro d e i s n o tsu i t abl e fo r q u an tu m y i e ld measu rem en t s . N i t r i te was meas -u red i n p a ra l l e l w i th ea ch o x y g en d e t e rm in a t i o n , an d t he r a t i oo f t h e o x y g en an d n i t r i t e y i e ld s p e rmi t s an es t imat e t o b ema d e o f q b (O2 ) s i n ce , f o r r easo n s o f s t o ich io met ry ,2 q ~(NO2 ) = ~ (O 2 ) . I t i s n o t ed th a t th e p l as ti c cas in g o f t h e

e l ec t ro d e s l o wly r e l eases smal l amo u n t s o f o rg an i c mate r i a lin to the so lu t ion . This i s manifes ted by an increase in theo p t i ca l ab so rb an ce o f t h e so lu t i o n , e sp ec i a l l y i n t h e wav e-leng th range below 250 nm, on s tand ing in the dark . Thismay i n f l u ence t h e o x y g en r ead in g s, s i n ce f r ee r ad ica l r eac -t i o n s d u r in g t h e co u r se o f p h o to ly s i s cau se o x y g en co n su m p -t i on ( see b e lo w) . Th e c h emica l i d en t it y o f th i s mat e r ia l i sn o t k n o wn .

Key p ro d u c t s i n d i ca t i ng t h e imp o r t an ce o f r eac t i o n (3 ) a r eso u g h t f ro m th e r eac t i o n o f s in g l e t O a to ms w i th wa t e r mo l -ecu l es (k ey p ro d u c t , H2 0 2 ) [3 3 ] an d f ro m th e r eac t io n o ft r i p le t O a to ms wi th cy c lo p en t en e (k e y p ro d u c t , C2 H4) [ 3 5 ] .I r r ad ia t ed , a rg o n -p u rg ed , 0.1 mo l d m -3 n i t r a te so lu t io n s o fp H 5 were t e s t ed fo r H2 0 2 u s in g t h e i o d id e meth o d (A l l en ' sr eag en t ) [5 8 ] , o b se rv in g t h e 13 ab so rb an ce a t 3 5 0 n m. Th ee th y l en e p ro d u ced o n i r r ad i a t io n o f cy c lo p en t en e-co n t a in in g(5 1 0 3 mo l d m -3 ) d eo x y g e n a t ed n it r a te so lut i o n s (1 0 -2mo i d m 3 ) was d e t e rmin ed b y gas ch ro m ato g rap h y ( see Ref .[ 4 8 ] ) .

To g e th er wi th fo rmald eh y d e , fo rmate was fo u n d i n i r r a -d iated , oxygenated , a lkal ine, e thanol -con tain ing n i t rate so lu-t i o n s . I t was d e t e rmin ed b y i o n ch ro mato g rap h y (Dio n ex2 0 0 0 i ; co lu mn , HP IC AS 4 ; e l u en t , 5 10 -4 m o l d m - 3NaHCO 3 ; r e t en t io n time , 2 .5 m in ) .

In o rd er to ex amin e t h e t h e rmal d eco m p o s i t i o n p a th way o fp ero x y n i t ro u s ac id , so lu ti o n s o f p e ro x y n i t r i t e were p rep aredf ro m n i t ro u s ac id an d H 2 0 2 [ 1 2 ,3 9 ] , o r b y p h o to ly s i s o f analkal ine n i t rate so lu t ion (0 .2 m ol dm -3 , pH 13) at a h ighin t en s i t y , wh ereb y p ero x y n i t r i t e co n cen t r a t i o n s o f u p t o3 1 0 -4 mo l d m -3 co u ld b e ach i ev ed . Th e r eac t i v i ty o fsu p ero x id e 0 2 t o ward s NO 3 was s tu d i ed b y su b j ec t in g an eu t r a l d i l u te n i tr a t e so lu t i o n (1 0 -3 mo l d m -3 ) co n t a in in g0 .1 m o l d m -3 fo rm ate , sa tu ra t ed wi th a 4 : 1 (v /v ) mix tu reof N20 and Oz, to 6Co 3 ' rad io lysis , whe reby O 2- i s gener-ated [59] ( fo r an in t roduct ion in to the f ield o f rad iat ionch emis t ry , s ee Ref s . [ 6 0 -6 2 ] ) .

3 . R e s u l t s

Th e ex p e r imen t s d esc r i b ed h ere were p e r fo rm ed u s ing 2 5 4n m rad i a t i o n f ro m a l o w-p ressu re m ercu ry a r c , i . e . w i th t h e1 8 5 n m l i n e sh u t o u t , wh ereb y o n ly t h e 2 5 4 n m l i n e an d av ery smal l co mp o n en t a t 3 1 3 n m remain . Th e sp ec t ru m o fthe n i t rate an ion over laps wi th bo th l ines , bu t the ex t inct ioncoeff icien ts are smal l fo r bo th (F ig . 1 ) . In the p re sen t case,t h e e f f ec t o f ex c i t a ti o n o f t h e l o n g er wav e l en g th b an d i sn eg l ec t ed as t h e emi ss io n o f t he H g l o w -p ressu re a r c i s smal li n t h is wav e l en g th r eg io n ; i n ad d i t io n , t h e e f f ec t i v en ess o ft h e p ro d u c t i o n o f n i t r i t e i o n s may b e l o wer b y a f ac to r o fro u g h ly o n e o rd er o f ma g n i tu d e a t t h i s wav e l en g th [ 1 7 ,2 0 ] .Thus we are deal ing wi th a relat ively s imple s i tuat ion : p rac-t i ca ll y mo n o c h ro m at i c ex c i t a t i o n o f t h e n i t r a te i o n a t 2 5 4 n m .

Th e q u an tu m y i e ld s o f n i t ra t e p h o to ly s i s p ro d u c ts , e sp e-c i a l l y n i tr i te , w ere d e t e rmin ed u n d er d i f f e r en t co n d i t i o n s : n o


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G. Mark et a l . / Journal o f Photochemis t ry and Photobio logy A: Chemis t ry 101 (1996) 8 9-103 93

o x i d iz a b l e a d d i t i v e / ( A r o r 0 2 ) ; m e t h a n o l /O 2 ; p r o p a n - 2 - o l /0 2 ; cy c lo p e n t an e /Ar . T h e r esu l t s a r e p resen t ed in t h is o rd er .3.1. No oxidizable additive

In general ac corda nce w i th the l i teratu re [ 13 ,14] , in theab sen ce o f an o x id i zab l e ad d i t i v e we f i n d a s i g mo id a l p Hd ep en d e n ce o f t h e n i tr i te q u an tu m y i e ld as sh o wn in F ig . 2 ,wi th ~ ( NO ~- ) i n c reas in g f ro m ap p ro x imate ly 0 .0 1 i n t h eac id i c p H r an g e t o ab o u t 0 .1 a t p H 1 0 an d ab o v e . Here , t h esign i f icance of q~(NO~-) i s no t s t raigh t forward , especial lyb ecau se t h e p ro d u c t p e ro x y n i t r i t e (wh ic h accu m u la t es in t hep h o to ly sed sam p les a t su f f i c i en t ly a lk a l i n e p H) co n t r i b u testo the NO2- read ing , s ince i t par t ly decomposes in to n i t r i te( see b e lo w) d u r in g t h e co u r se o f t h e n i tr i te a s say.

Simi lar to ear l ier repor ts [ 13 ,19] , w e obse rve that , wi thth e ex cep t i o n o f t h e a lk a li n e p H ran g e , t h e NO ~ y i e ld v s.d o se p lo t s a r e n o t i ceab ly cu rv ed d o wn ward s (F ig . 3 ) . Th i simpl ies that the quantum y ield q~(NO~-) in th is system isd o se d ep en d en t . Th e p h o to ly s i s u n d er t ak en wi th a ce r t a i nam o u n t o f n i t r i te i n i t ia l l y ad d ed t o t h e samp le sh o ws t h e samet ime cou rse as the photo ly sis o f an in i t ia lly n i t r i te- f ree sam plewh en t h e same amo u n t o f n i t r i t e h as accu mu la t ed (F ig . 3 ) .Th e majo r cau se o f t h i s i s t h e p a r t ia l r e -o x id a t i o n o f n i tr i te( r eac t i o n (1 7 ) ) a s sh o wn b e lo w.O H + N O~ - ~ N O 2 + O H - ( 17 )

By def in i t ion , the ins tan taneous or d i f feren t ial quan tumy ie ld i s ex p ressed as ~b = dN/dD and the in tegral quan tumyield as q~i, t= N/D, wh ere N an d D a re th e n u mb e r o f mo l -ecu l es o f t h e p ro d u c t an d t h e n u mb er o f q u an ta ex p en d ed t oprodu ce them respe ct ively . ~ i . , i s iden t ical wi th ~b on ly i f they ield vs . dose fun ct ion i s l inear , o therwise bo th quant i t ies aredose de pende nt . Th e quant i ty p lo t ted in F ig . 2 i s nei ther . I t i sthe apparen t quan tum y ield , which ref lects the in tegral quan-tu m y i e ld b u t wh ich , wh i l e co in c id in g wi th t h e i n t eg ra l q u an -tum y ield in the acid ic and neu t ral range, in the alkal ine rangeco n t a in s an ad d i t io n a l t e rm d u e t o t h e u n av o id ab l e ' ' a r t e f ac t "o f p e ro x y n i t r i t e d eco mp o s i t i o n u n d er t h e co n d i t i o n s o f t h en i t r i te assay that fu rn ishe s add i t ional n i t r i te . Th e data shownwere o b t a in ed a t f l u en ce r a tes i n t h e ran g e 6 -1 0 W m - 2 an df lu en ces o f u p t o ab o u t 3 0 k J m-2 . Wh e n w id e ly d i f f e r i n gf lu en ce r a t es a r e ap p l i ed , i t b eco m es ap p aren t t h a t t h e r a t e o fn i t ri t e fo rmat io n d ecreases o n g o in g f ro m lo w to h ig h f l u en cerates . Th is ef f ect i s show n in F ig . 4 . I t has been es tab l i shedthat , wi th regard to the be havio ur show n in F ig . 4 , there is nod i sce rn ib le d i f f e r en ce b e tween a rg o n -p u rg ed an d o x y g en a t ed( [0 2 ] = 1 .3 X l0 -3 mo l d m -3 ) so lu ti o n s .

In acco rd a n ce w i th l i t e ra tu re r ep o r ts [ 1 2 ,1 6 ] , a t p H > 1 0,we o b serv e t h e fo rmat io n o f p e ro x y n i t r i t e an io n , t he b u i l d u pof which tends tow ards a s teady s tate (F ig . 5 ) . I t i s seen thatt h e s t ead y s t a t e co n cen t r a t i o n i s l o wer an d i s r each ed so o n erat lower pH values . The in i t ia l s lopes are s imi lar , and in i t ia lp e ro x y n i t r i t e q u an tu m y i e ld s o f ab o u t 0 .0 7 a r e es t imat ed . Inth e p resen ce o f p ro p an -2 -o i , th e q u an tu m y i e ld o f p e ro x y n i -t r i te reaches about 0 .1 at p ropan-2-o l concent rat ions below

0 .100 .08

If O.06e 0 . 0 4

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7

x I- X - - x ~ X / " ~ / /- 0 ~ 0 ~ 0

I I i I i4 6 8 110 1L2p H

F i g . 2 . I n t e g r a l n i t r i t e q u a n t u m y i e l d s f r o m t h e p h o t o l y s i s ( 2 5 4 n m ) o fa q u e o u s N O 3 s o l u t io n s ( 0 . 0 1 m o l d m - 3 , a t d i f f e r e n t p H v a l u e s ) , w i t h o u to r g a n i c a d d i t i v e ( O ) a n d a t s m a l l O H r a d i c a l s c a v e n g e r c o n c e n t ra t i o n s : , 0 . 0 l m o l d m - 3 m e t h a n o l , a i r s a t u r a te d ; , 2 X l 0 3 m o l d m - 3 c y c l o -pen tane , a rgon sa tu ra t ed . F luence ra t e , 0 .5 W m 2 . Data no t co r rec ted fo ra d d i t io n a l n i t r i te f o r m e d f r o m p e r o x y n i t d t e u n d e r t h e c o n d i t i o n s o f t h e n i t r it ea s s a y ( s e e t e x t ) .

T 0 .8E"5 0 .6E,,?,O~ 0 . 4 /Oz 0.2I

,5 J ~ f A /

1 0 2 0 3 0 4 0- - F lu en ce / kJ m 2

Fig . 3 . Fo rm at ion o f n i t r i t e a t pH 6 (a rgon sa tu ra t ed ) : [NO~-] vs . fl uence .F l u e n c e r a t e , 0. 5 W m - 2 . W i t h o u t a d d i ti v e ( O ) a n d w i t h 6 1 0 - 6 m o ld m 3 N O ~ - a d d e d b e f o r e p h o t o l y s i s ( / ~ ) ; s e q u e n c e o f A s y m b o l s s h i ft e dt o t h e fi g h t t o m a t c h t h e se q u e n c e o f s y m b o l s .

TE

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/,0 / . joo i . / j o" " : / ' - " , I , I , I ,2 4 6 8

- - F lu en ce / k J m -aFig . 4 . N i t r i t e y i e ld vs . f l uence a t d i f feren t pH va lues (c i rc l es , pH 11 ;t r i a n g le s , p H 5 ) a n d d i f f e r e n t f l u e n c e ra t e s ( o p e n s y m b o l s , 1 0 W m - 2 ; f i ll e ds y m b o l s , 0 .5 W m - 2 ) i n a r g o n - s a tu r a t e d n i tr a t e s o l u t i o n s ( 0. 0 1 m o l d m - 3 ) .No ox id izab le add i t ive .


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9 4 G. Mark et a l. / Journal o f Photochemis t ry and Pho tobio logy A: Chemis try , 101 (1996) 89-1 03

I t i . : L .% 1 " 5 l ~ : f ~ " " i . . . I p i l l 3

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j oO" [ / o pill10 .5 r o f o - - -- - -- []o...?., lp . 1 0-4 . . . . . -~ . . . . . . - 7 - - r . . . . . . ; . . . . . . . . . . . ] . . .1 0 2 0 3 0- - F l u en c e / k J m -2 ,,-

F i g . 5 . P e r o x y n i t r i t e b u i l d u p d u r i n g t h e p h o t o l y s i s o f a l k a l in e a q u e o u sN O 3 ( 0 . 1 m o l d m - 3 ) a t d i f f e r e n t a l k a l i n e p H v a l u e s w i t h o u t o x i d i z a b l ea d d i t i v e . F l u e n c e r a t e , 0 . 5 W m - 2 . P e r o x y n i t r i t e c o n c e n t r a ti o n v s . d o s e .I n s e t : p o s t - i r ra d i a t i o n d e c a y o f p e r o x y n i t r i t e a t p H 1 1 : t ~/ 2 = 3 0 m i n .

0 . 1 5

0 . 1 0bozo

6 , 0 . 0 5

2 4 6 8 1o- - [ P r o p a n - 2 - o l ] / t o o l d m - 3 - - - , -

F i g . 6 . P e r o x y n i t r i t e q u a n t u m y i e l d v s . p r o p a n - 2 - o l c o n c e n t r a t i o n d u r i n g t h ep h o t o l y s i s o f o x y g e n a t e d n it r a te s o l u t io n s ( N O 3 = 0 .1 m o l d m - 3 ) c o n t a i n -i n g u p t o 1 0 m o l d m - 3 p r o p a n -2 - o l a t p H 1 3 ( O , w i t h o u t p r o p a n -2 - o l ) .F l u e n c e r a t e , 0 .5 W m - 2 .

Hy d ro g en p ero x id e ( a p o ss ib l e i n d i ca to r p ro d u c t o f t h eo ccu r r en ce o f r eac t i o n (3 ) ) i s n o t o b se rv ed . I t s q u an tu m y i e ldmu st be less than 7 x 10 4 (argo n saturated , 0 .1 mo l dm 3n i t r a t e , p H 5 ) . E th y l en e , a p ro d u c t f ro m th e 3 0 a to m assay[ 3 5 ] , i s an o th er i n d i ca to r p ro d u c t o f t h e o ccu r r en c e o f r eac -t io n ( 3 ) . I ts q u a n tu m y i e ld i s 3 1 0 4 ( 1 0 - 2 t o o l d m 3nitrate, 5 X l 0 - 3 t o o l d m -3 cy c lo p en t en e , a rg o n sa tu ra t ed ,p H 7 ) .

Th e p ro d u c t i o n o f a v o l a t i l e n i t ro g en -co n t a in in g co m-p o u n d [ 1 7] i n t h e ab sen ce o f an o x id i zab l e ad d i t i v e h as b eenco n f i rmed . P u rg in g t h e so lu t i o n d u r in g p h o to ly s i s wi th aniner t carr ier gas leads to i t s par t ia l removal , as demonst ratedby the fo rm at ion of n i t r i te and n i t rate in the t rapp ing so lu t ion ,wh i l e t h e p H o f t h e p h o to ly sed so lu t i o n i s sh i f ted t o ward s t h ealkal ine reg ion . When the purg ing gas s t ream is passed in toa h ex an e so lu t i o n o f t h e ch e l e t ro p i c sp in t r ap NOCT-1[ 5 2 ,5 3 ] , s t ro n g ES R s ig n a ls o f t h e co r r esp o n d in g NO ad d u c t1 ,1 ,3 ,3 - te t r ameth y l - i so in d o li n -2 -o x y l (NO CT -1 N O) a red e t ec t ed , t o g e th er wi th w eak s i g n al s o f t h e co r r esp o n d in gN O 2 a d d u c t ( N O C T - 1 N O 2 ) ( F i g. 7 ) . T h e f o r m a t i o n o f t h eco r r esp o n d in g n i t ro x id e rad i ca l NOC T- 1N O u n a m b i g u o u s lyp ro v es t h a t f r ee n i tr i c o x id e i s p ro d u ced d u r in g t h e co u r se o fphoto lysis .

Us in g t h e p h o to ch em ica l p a th way t o p rep are p e ro x y n i t r i t ein so lu t ion , a lkal ine n i t rate so lu t ions (0 .2 mol dm-3 , pH 13)were i r rad iated at h igh l igh t in tensi ty ; the concent rat ion oft h e p e ro x y n i tr i t e fo rme d was d e t e rm in ed sp ec t ro p h o to met r -i ca l ly . A l iq u o t s o f t h i s so lu t io n w ere s t i r r ed i n to o x y g en a t e dso lu t io n s o f meth an o l (5 t o o l d m -3 ) an d ad ju s t ed t o p H 4 .Th i s r esu l t s i n t h e p ro to n a t i o n o f t h e p e ro x y n i t r i t e an d t h ef o r m a t io n o f O H r a d ic a l s ( r e a c ti o n ( 9 ) ) ( o r a n o t h e rs t ro n g ly o x id i z in g sp ec i es , s ee ab o v e) . F o rmald eh y d e wasfo rmed i n a y i e ld co r r esp o n d in g t o ap p ro x im ate ly 2 0 % -3 0 %o f t h e p e ro x y n i t r i te o r i g in a l l y p resen t .

0 .1 mo l d m-3 , an d s t ay s ro u g h ly a t t h i s l ev e l wh en h ig h erco n cen t r a t i o n s o f t h e a l co h o l , u p t o ap p ro x imate ly 1 0 mo ld m -3 , a r e u sed , a s sh o wn b y t h e d a t a in F ig . 6 . (W i th r eg ardto t h e d eg ree o f a l k a l i n i ty i n t h e case o f so lu t io n s wi th su p ra -mo la r a l co h o l co n cen t r a t i o n s , "p H 1 3 " s i g n i f i e s t h a t t h eso lu t io n i s 0 .1 m o l d m -3 i n so d iu m h y d ro x id e . )

Th e p H t r en d o f t h e o x y g en y i e ld s p a ra l l e ls t h a t o f n i t ri t e(d a t a n o t sh o wn ) . H o we v er , th e 0 2 q u an tu m y i e ld s a r e so me-wh a t l o w er t h an o n e-h a l f o f th e n i t r it e q u an tu m y i e ld s (o nth e b as is o f s t o i ch io met ry , we e x p ec t t h em to b e ex ac t l y o n e-hal f ) . In the alkal ine pH range, th is i s ascr ibed to an "ar te-f ac t " , n ame ly t h e e f f ec t o f t h e p ro d u c t p e ro x y n i t r i te , wh ichu n d er t h e ac id i c co n d i t i o n s o f t h e NO2 - as say g iv es r i s e t oaddi t ional n i t r i te [25 ,36] .

Th e o x y g e n d ef i c i t is r e l a t i v e ly l a rg e r i n t h e ac id i c p H r an g e(0 2 r ead in g p rac t i ca l l y ze ro ) wh ere t h e NO 2 q u an tu m y i e ld(an d t h e re fo re a l so t h e ex p ec t ed o x y g en q u an tu m y i e ld ) i ssmal l compared wi th i t s value at pH 13 . ( I t i s argued belowth a t th e ze ro o x y g en r ead in g i s a co n seq u en ce o f t h e o x y g ene l ec t ro d e cas in g b e in g m ad e o f p l as ti c . )

1

H' 1 mT"

N, O ,,. -O"t~_ j O "NO2 NONOCT-1 NO2 NOCT-1 NOCT-1 NO

F i g . 7 . E S R s p e c t r u m o f t h e c y c l ic n i t r o x i d e r a d i c a l s o b t a i n e d b y r e a c t i o no f N O ( N O C T - 1N O ; i n t e n s e t r ip l e t) a n d N O 2 ( N O C T - 1NO2; wea k t r ip le t ,c e n tr a l l i n e m a s k e d ) w i t h t e t r a m e t h y l - o - q u i n o d i m e t h a n e i n c y c l o h e x a n e a t2 0 (2 . N O a n d N O 2 s t ri p p e d f r o m a p o t a s s i u m n i t ra t e s o l u ti o n ( l m o ld i n - 3 ) u n d e r 2 5 4 n m i r r a d ia t i o n .


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G. Mark et al . / Journal of Photochemistry and Photobiology A: Chem istry 101 (1996) 89-103 95

0 . 1 0

0z 0 . 0 5

0 . 0 0

f

T : : : fO . 0 4 J ~0 . 0 2i t - , I , I , L , I ,

0 . 5 1 . 0 1 . 5 2 . 0[ A l c o h o l ] / 1 0 3 t o o l d m " 3

2 4 6 8- - [P r op an- 2 - o l ] / mo l dm -3 , -

Fig. 8. Dependence of the nitrite quantum yield on the propan-2-ol concen-tration at pH 5 (0.01 m ol dm -3 nitrate, up to 10 mol dm -3 propan-2-ol,oxygen saturated). Fluence rate, 0.5 W m 2. Inset: photolysis of oxyg en-saturated nitrate solutions ( 1 0- 2 mol di n- 3; pH 5 ) in the presence of propan-2-ol (circles) or methanol (triangles) at concentrations up to 0 .01 tooldm 3. Nitri te (O, A ) , acetone ( ) and formaldehyde ( ) quantum yie ldsas a function of the alcoh ol concentration; U]. nitrite yield in the absenc e ofany ox idizable additive.3 .2 . A l c o h o l i c a d d i t iv e s a n d o x y g e n

O x y g e n - s a t u r a t e d n i t ra t e s o l u t i o n s (0 . 0 1 m o l d i n - 3 ) c o n -t a in i n g v a r y i n g a m o u n t s o f m e t h a n o l o r p r o p a n - 2 - o l w e r ei r r a d i a t e d a t pH 5 , a nd t he n i t r i t e qua n t um y i e l d w a s de t e r -m i n e d a s a f u n c t i o n o f t h e a lc o h o l c o n c e n t r a t i o n ( F i g . 8 ) .T he s a l i e n t po i n t he r e i s t ha t , a f t e r a s t e e p r i s e t o a va l ue o fa b o u t 0 . 0 6 w i t h in t h e c e n t i m o l a r a l c o h o l c o n c e n t r a t i o n r a n g e( i n s e t ) , t h e c u r v e t h e n b e n d s i n t o a s e c t i o n w i t h a m u c hs m a l l e r u p w a r d s l o p e t h a t f in a l ly b r i n g s @ ( N O 2 ) u p t o 0 .1 .T h e i n s e t in F i g . 8 s h o w s t h e d e p e n d e n c e o f th e n i t ri te a n df o r m a l d e h y d e q u a n t u m y i e l d s in th e c a s e o f m e t h a n o l , a n dt h e n i tr i te a n d a c e t o n e q u a n t u m y i e l d s in th e c a s e o f p r o p a n - 2 -o l , o n t h e c o n c e n t r a t i o n o f t h e a l c o h o l a d d i t i v e i n t h e m i l li -m o l a r r a n g e a t p H 5 ( [ N O 3 ] = 1 0 - 3 m o l d m - 3 ) . T h e p i c t u r ei s ba s i c a l l y t he s a m e f o r bo t h a l c oh o l s , a nd r e p r e s e n t s a ne x p a n d e d v i e w o f th e s t e e p ri s e s e c ti o n o f th e m a i n g r a p h o fF i g . 8 . I t i s s e e n i n t he i n s e t t ha t t he r e i s a n i nc r e a s e i n t hen i tr i te q u a n t u m y i e l d , w h i c h " p l a t e a u s o u t " a t a b o u t 0 . 0 6b e y o n d a n a l c o h o l c o n c e n t r a t i o n o f 2 1 0 - 3 m o l d m 3 . I t i sa l s o s e en t h a t, a t th i s p H , @ ( N O 2 - ) m a t c h e s q ~ ( c a r b o n y lc o m p o u n d ) . T h i s " p l a t e a u " i s t a k e n to r e f le c t t h e c h e m i s t r yo f t h e s y s t e m ( S c h e m e 1 ) w h e n t h e a d d i t i v e c o n c e n t r a t i o ni s s t i l l l ow e nough s o a s no t t o i n t e r f e r e w i t h t he c a ger e a c t i ons .

I t i s pe r ha ps u s e f u l t o e m p ha s i z e t ha t t he sha r p r i s e ove rt h e re g i o n o f t h e v e r y l o w e s t a l c o h o l c o n c e n t r a t i o n ( i n s e t o fF i g . 8 ) r e fl e c ts t h e c o m p e t i t i o n f o r th e ' O H r a d i c a l b e tw e e nt he a l c oho l a nd t he p r oduc t n i t r i t e a nd , a s e xp l a i ne d e a r l i e r ,i s a c o n s e q u e n c e o f t h e f a c t t h a t t h e d i a g r a m r e p r e s en t s t h ei n t eg r a l q u a n t u m y i e l d a n d n o t th e d i f f er e n t ia l q u a n t u m y i e l da t t he l i m i t o f z e r o pho t o l y s i s t i m e .

T h e q u a n t u m y i e l d s o f n i t ri t e a n d f o r m a l d e h y d e a s a f u n c -t i o n o f p H a r e s h o w n i n F i g . 9 ( m e t h a n o l c o n c e n t r a ti o n , 1 0 - 2

e

0 .08

0 . 0 6

0 .040 .02 5~o

5 6 7 8 9pH ~Fig. 9. Photolysis of oxygen-saturated nitrate solutions ( 10-2 mol d in-3)in the presence of methanol (10 2 mol dm -3). Fluence rate, 0.5 W m -2.Nitrite () and formaldehyde (O ) quantum yields as a function of pH.Buffer: phosp hate, borate.m o l d m 3 ). I n t h e a c i d i c r a n g e , th e q u a n t u m y i e l d s ( a b o u t0 . 0 6 ) a r e e q u a l w i t h i n e x p e r i m e n t a l e r ro r , a s e x p e c t e d f r o mt h e s to i c h i o m e t r y r e p r e s e n t e d b y r e l a t io n ( 1 8 )N O 3 + C H 3 0 H ~ N O 2 + C H 2 0 q - H 2 0 ( 1 8 )I n t he a l ka l i ne r e g i on , t he " p l a t e a u " ( s e e i n s e t, F i g . 8 ) y i e l d so f n it r it e a n d f o r m a l d e h y d e d i v e r g e ( F i g . 9 ) . T h e t r e n d o fN O 2 p r o d u c t i o n w i t h p H ( F i g . 2 ) i s i n a g r e e m e n t w i t h e a r -l i e r f i nd i ngs i n t he p r e s e n c e o f a l c oh o l s [ 18 , 2 0 ] . I t ha s a l s ob e e n r e p o r t e d t h a t a l d e h y d e o r k e t o n e w i l l b e f o r m e d w h e nt h e a d d i t i v e i s a p r i m a r y o r s e c o n d a r y a l c o h o l r e s p e c t i v e l y[ 18 ] . K e e p i n g i n m i nd t ha t t he n i t r i te r e a d i n gs i n a l ka l i nes o l u t i o n a r e b o o s t e d b y p o s t - p h o t o l y t i c a l l y f o r m e d n i t r i t ef r o m t h e d e c o m p o s i t i o n o f p e r o x y n i t r it e i n t h e n i t ri te a s s a y ,w e w o u l d e x p e c t t h e f o r m a l d e h y d e y i e l d t o b e l o w e r th a n t h ea ppa r e n t n i t r i t e y i e l d , bu t no t t o de c l i ne be l ow i t s a c i d i c pHr a nge l e ve l . I n f a c t , t he de c l i ne i s r ough l y ba l a nc e d by t hea p p e a r a n c e o f f o r m a t e w h o s e y i e l d i n c r e a se s w i t h i n c r e a s i n gp H . T h e p r o d u c t i o n o f f o r m a t e i s a p p a r e n t l y d u e t o a t h e r m a lr e a c ti o n , a n d w e c o n f i r m e d i n a s e p a r a te s e r ie s o f e x p e r i m e n t st h a t p e r o x y n i t r i t e p r e p a r e d f r o m n i tr it e a n d H 2 0 2 [ 1 2 ] d o e si n d e e d o x i d i z e f o r m a l d e h y d e t o fo r m a t e a t p H 1 0 .

P e r o x y n i t r i t e h a s a l s o b e e n m e a s u r e d i n a l k a l i n e p r o p a n -2 - o l s o lu t i o n s a b o v e p H 1 0 . I n th e p r o p a n - 2 - o l s y s t e m , t h ep e r o x y n i t r i te i o n i s n o t r e d u c e d i n a p o s t - p h o t o l y s i s t h e r m a lr e a c t i on s i nc e i t is un r e a c t i ve t ow a r ds a c e t one . I t s i n i ti a l qua n -t um y i e l d i s c l o s e t o 0 . 1 , i .e . h i ghe r t ha n t he va l ue i n a dd i t i ve -f r e e n i t r a t e s o l u t i on by a bou t a f a c t o r o f 1 . 5 . T he c u r ve s( s i m i l a r t o t h o s e i n F i g . 5 , d a t a n o t s h o w n ) b e n d t o w a r d s as t e a d y s t a te c o n c e n t r a t i o n w h i c h i n c r e a s e s w i t h p H , a s i t d o e si n th e a b s e n c e o f a n o x i d i z a b l e a d d i t iv e ( F i g . 5 ) .3 . 3. C y c l o p e n t a n e s o l u t i o n s

I n w a t e r a t 2 0 C , c y c l o p e n t a n e i s s o l u b l e t o a c o n c e n t r a t i o no f 2 1 0 3 m o l d m - 3 . A n y f r e e ra d i c a l a tt a c k o n c y c l o p e n -t a n e w i l l y i el d o n l y o n e t y p e o f r ad i c a l, n a m e l y c y c l o p e n t y l .F i g . 2 s h o w s t h e p H d e p e n d e n c e o f t h e q u a n t u m y i e ld o f


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96 G. Mark e t a l . / Journal o f Photochemis t ry and Photobio logy A: Chemist ry 101 (1996) 89-10 3

T a b l e 1P h o t o l y s i s o f n i t r a te ( 1 0 - 2 m o l d m - 3 ) i n t h e p r e s e n c e o f c y c l o p e n t a n e( 2 1 0 - 3 t o o l d m - 3 ) i n w e a k l y a c i d i c a n d w e a k l y b a s i c o x y g e n - f r e e a q ue -o u s s o l u t io n . Q u a n t u m y i e l d s o f o r g a n i c n i t r o g e n c o m p o u n d s i n c o m p a r i s o nw i t h t h e n i t r i te q u a n t u m y i e l dP r o d u c t p H 5 p H 9

N i t r o c y c l o p e n t a n e 0 , 0 2 < 1 0 - 3C y c l o p e n t y l n i t r a t e < 1 0 - 3 0 .0 1Ni t r i t e 0 .03 0 ,07

n i t ri t e i n t h e ab sen ce o f o x y g e n . To g e th er wi th n i t ri t e, cy c lo -p en ty l n i t r a te ( i t s f o rmat io n sh o w s a b r i e f i n d u c t i o n p e r i o d ) ,n i t ro cy c lo p en t an e an d cy c lo p e n t en e a r e o b se rv ed (Tab l e 1 ) .W e d e t ec t a ch an g e i n p H in t h e a lka l i n e d ir ec t i o n wh ich i smo s t co n sp i cu o u s w h en t h e i n i t ia l p H i s ab o u t n eu t ra l . S mal lamo u n t s o f cy c lo p en t an o n e an d cy c lo p en t an o l a r e a l soo b serv ed . In o x y g en a t ed so lu t i o n s , th e fo rm at io n o f cy c lo -p en ty l n i t r a t e an d n i t ro cy c lo p en t an e i s su p p ressed , wh i l e t h eco n t en t s o f cy c lo p en t an o n e an d cy c lo p en t an o l i n c rease asp ero x y l r ad i ca l r eac t i o n s t ake o v e r ( see Ref s . [ 6 3 ,6 4 ] ) .Th e c o mp le t e ab sen ce o f cy c lo p en ty l n i tr i te f ro m th e p ro d -u c t s i s r emark ab l e . Au th en t i c mate r i a l h as b een sy n th es i zed[5 5 ] an d i t s GC re t en t i o n time es t ab l i sh ed . I f i t had b eenfo rme d p h o to ly t icaUy , i t wo u ld ce r t a i n ly h av e b ee n d e t ec t ed ,s i n ce t h e re was n o i n t e r f e r en c e b y o th e r GC p eak s . A l so ,wh en a t r ace amo u n t o f cy c lo p en ty ln i t r i t e was ad d ed t o th eso lu t i o n p r i o r t o p h o to ly si s , i t was seen t o r emain u n af f ec t edby rad iat ion .

4 . Di scu ss io n

4.1. No ox id iza ble addi t ive s

Ni t r a t e p h o to ly s i s i n t h e ab sen c e o f an o x id i zab l e ad di t iv ey ields n i t r i te , oxyge n and pero xyni t r i te ; the f i rs t two are foun din a mo la r r a t io so me wh at i n ex cess o f two (p H 1 3 ) . Th eex cess n i t r i te s t ems f ro m th e d e co mp o s i t i o n o f p e ro x y n i tr i t eunde r the acid ic con di t ions o f the n i t r ite assay , wi th i t s par t ia lco n v er s io n t o n i t r i t e [2 5 ,3 6 ] . 0 2 mu s t b e fo rmed as a co n -seq u en ce o f th e p r imary r eac t i o n s (1 ) an d (2 ) , b u t n o t o fr eac t i o n ( 3 ) , s i n ce an y imp o r t an t co n t r ib u t i o n o f th e l a t te r t oth e p h o to ly s i s can b e ru l ed o u t : a v a lu e o f q ~ (H2 0 2 ) < 1 0 -3d emo n s t r a t es t h e l ack o f imp o r t an ce o f t he p r imary fo rmat io no f t h e s in g l e t o x y g en a to m [3 3 ,3 4 ] . Th e l o w y i e ld o f e th y l -ene, (C2H4) = 3 10 -4 , whe n the i rrad iat ion i s carr ied ou ti n t h e p resen ce o f cy c lo p en t en e ( t h i s i s a r eag en t fo r t h e t r ip l e to x y g e n a to m [ 2 0 , 3 5 , 6 5 ] ) i n di c a te s t h a t ~ ( 3 0 ) d o e s n o tex ce ed 1 0 -3 .

Th e o x y g en d ef i c i t i n t h e ac id i c p H r an g e (0 2 r ead in gp rac t i ca ll y ze ro ) i s a sc r i b ed t o t h e f ac t t h a t smal l amo u n t s o fo x id i zab l e mate r i a l f ro m th e p l as ti c cas in g o f t h e O2 - sen s i ti v ee l ec t ro d e a r e co n s t an t l y b e in g l i b e ra t ed i n to t h e so lu t i o n .Du r in g i r r ad ia t i o n , th i s mat e r i a l ma y u n d erg o f r ee r ad ica lf o r m a t i o n a n d s o m a y b e c o n s u m i n g t h e o x y g e n ( s e e b e lo w )

th a t, a t ac id i c p H, i s p ro d u ced a t a r e l a t i v e ly l o w r a t e ( seeNO 2 y i e ld v s . p H, F ig. 2 ) .

Th ere a r e sev era l a l r ead y ack n o w led g ed i n t e rmed ia t e an dp r o du c t s p ec ie s i n t hi s s y st e m ( O H , N O 2 , N O , N O 2 ,O N O O - , O N O O H ) t h a t a r e m u t u a l ly r e a ct i ve , to g e t h e r w i t hth e p ro b ab l e sp ec i es ONO O' ( see b e lo w ) ; t h e re fo re t h e r eac -t i o n sch eme i s ex p ec t ed t o b e co mp l i ca t ed . Nev er th e l es s , i ti s p o ss ib l e t o g a in an u n d er s t an d in g o f t h e p H d e p en d en c eand , at the same t ime, o f the h i ther to unsat i sfactor i lyex p l a in ed man n er i n wh ich 0 2 i s fo rmed .

Ou r h y p o th es i s i s t h a t , i n t h e ab sen ce o f an o x id i zab l ead d i t iv e , t h e m ain i n f l u en c in g f ac to r reg ard in g t h e p H b eh av -io u r o f t h e n i t r i t e q u an tu m y i e ld i n v o lv es t h e o x id a t i o n o fp ero x y n i t r i t e b y t h e 'OH rad i ca l t o g iv e t h e n i t ri c o x id e p e r -o x y l r a d ic a l O N O O ( r e a c ti o n s (1 9 ) a n d ( 2 0 ) ) , i n c o m p e -t i t ion wi th the i somerizat ion of peroxyni t r i te in to n i t rate v iat h e p r o to n a t e d f o r m ( r e a c t i o n s ( 1 0 ) a n d ( 1 3 ) , S c h e m e I ) .A s imi l a r p ro cess in v o lv in g N O2 in s t ead o f O H as t h e o x i -d i z in g ag en t ( r eac t i o n (2 1 ) ) h as b een es t imat ed t o b e ex o -th e rmic b y ab o u t 1 3 k ca l mo l - 1 [ 3 7 ] an d t h e re fo re can n o tbe ru led ou t .O H + O N O O - ~ O N O O ' + O H -O H + O N O O H - - * O N O O " + H 2 0" N O 2 + O N O O - --* O N O O + N O (

( 1 9 )( 2 0 )( 2 1 )

T h e e f f e c t o f t h e d e c a y o f O N O O H i s n o t o n l y r e fl e c te d int h e p H d e p e n d e n c e o f th e N O r q u a n t u m y i e l d ( F ig . 2 ) , b u tby the data show n in F ig . 5 : the photo ly t ic s teady-s tate con-c e n t ra t io n o f O N O O - d e c r e a s e s as t h e t e n d e n c y t o f o r m th ep ero x y n i t ro u s ac id mo lecu l e f ro m i t s an io n i n c reases wi thd ecreas in g p H.

W e p r o p o s e [ 2 9 ] t h a t o x y g e n 0 2 i s re l e a s e d f r o m O N O O 'main ly t h ro u g h r eac t i o n (2 2 ) , i t s o ccu r r en c e b e in g s t ro n g lysu p p o r t ed b y t h e d isco v ery o f N O wh ich b eh av es as a r eac -t i v e i n t e rmed ia t e i n t h i s sy s t em, b u t can b e p u rg ed f ro m th eso lu t io n u n d er p h o to ly s i s . Th e sec o n d -o rd er r eac t i o n (2 3 )o f f e r s a fu r t h e r p o ss ib i l i ty fo r an o x y g e n - fo rm in g p ro cessi n v o lv i n g O N O O 'O N O O ~ 0 2 + N O ( 22 ,- 2 2)2 O N O O ~ 2 ' N O 2 + 0 2 ( 2 3 )Th ese r eac t i o n s a r e i n acco rd an ce wi th t h e f ac t t h a t b o tho x y g en a to m s i n t h e p ro d u c t Oz s t em f ro m th e n i t ra t e [ 1 7 ] .M o r e o v e r , t h e se q u e n c e o f r e a c ti o n s ( 9 ) , ( 1 1 ) , ( 1 9 ) a n d(2 2 ) , ( 2 3 ) p ro v id es an ex p l an a t i o n fo r t h e o b se rv a t io n[2 5 ,6 6 ] t h a t t h e d eco mp o s i t i o n o f p e ro x y n i t r i t e in n eu t r a lso lu ti o n i n t h e ab sen ce o f o rg an i c co m p o u n d s g iv es r is e t osome n i t r i te , and by impl icat ion , to 02 , in add i t ion to i som-er i za t io n t o NO~- . Th e i n v o lv em en t o f t h e n i tr i c o x id e p e ro x y lrad ical in th is photo lysis has h i ther to no t been suspected .(Th i s sp ec i es h as b een co n s id e red t o b e t h e p ro b ab l e i n t e r -med ia t e i n th e o x id a t i o n o f N O b y d io x y g e n to "NO2 in t h eg as p h ase [6 7 ,6 8 ] , an d t o n i t ro u s ac id i n aq u eo u s so lu t i o n[ 6 9 -7 1 ] , an d h as b een o b se rv ed i n an a rg o n mat r i x i so l a t io n


9/15

G . M a r k e t a l, / J o u r n a l o f P h o t o c h e m i s t r y a n d P h o t o b i o l o g y A : C h e m i s t r y 1 0 1 ( 1 9 9 6 ) 8 9 - 1 0 3 97

i n f r a re d s p e c t ro s c o p ic s tu d y o f th e r e a c tio n o f N O w i th 0 2[7 2 ] . ) U n d e r o u r e x p e r im e n ta l c o n d i t io n s , i . e . i n t h e p re s -e n c e o f u p to 1 .3 X 1 0 - 3 m o l d m - 3 ( s a tu ra t io n a t a tm o s p h e r i cp re s s u re ) o x y g e n , an d r e l a t iv e ly sm a l l N O s t e a d y -s t a t e c o n -c e n t ra t i on s , r ea c t io n ( - 2 2 ) d o e s no t y e t s e e m t o p l a y as ig n i f ic a n t ro l e a s i n d ic a t e d b y th e f a c t t h a t o x y g e n - f re e a n do x y g e n a te d s a m p le s o f a d d i t iv e - f l e e n i t ra t e s o lu tio n s b e h a v ei n th e s a m e w a y ( n o t s h o w n ) w i t h in t h e l im i t s o f e x p e r i m e n -t a l e r ro r . R e g a rd in g th e q u e s t io n o f t h e s t a b i l it y o f O N O O ,i t i s n o te w o r th y th a t , a c c o rd in g to q u a n tu m c h e m ic a l e s t i -m a te s , t h i s s p e c ie s i s e n d o th e rm ic b y u p to 1 1. 5 k c a l m o l - 1re l a t iv e to i t s c o n s t i t u e n t s N O a n d 0 2 [7 3 , 7 4 ] . B e lo wa p p ro x im a te ly p H 1 0, o w in g to th e i s o m e r i z a t io n o f p e ro x -y n i t ro u s a c id ( r e a c t io n s (1 0 ) a n d (1 3 ) ) , w e e x p e c t a s t e a d ys ta t e c o n c e n t ra t io n o f p e ro x y n i t r i t e t o b e r e a c h e d f a i r ly so o na f t e r t h e o n s e t o f p h o to ly s i s ( s e e F ig . 5 ) . T h e r e c o m b in a t io no f N O a n d N O 2 r e p r es e n t s a f u r t h er p a th w a y t o n it r it e ( r e a c -t i on ( 2 4 ) ) , t o g e t h e r w i t h re a c t io n ( 1 6 )N O + "NO 2 + H 2 0 ~ 2 N O ( + 2 H + ( 2 4 )

A p a r t f ro m th e p H d e p e n d e n c e , a c o n c e n t ra t io n d e p e n d -e n c e a n d a f lu e n c e r a t e d e p e n d e n c e a re , i n p r in c ip l e , a l s oe x p e c te d fo r t h i s p h o to ly s i s . T h e e n e rg y d e p o s i t i o n d e n s i ty ,a n d th e re fo re th e r a d ic a l d e n s i ty , i n c re a s e s w i th [ N O 3 ] ; t h u ss e v e ra l o f t h e e l e m e n ta ry r e a c t io n s in v o lv e d in th e r e a c t io nm e c h a n i s m m u s t b e s e c o n d o rd e r i n r a d ic a l c o n c e n t ra t io n .

T h e f a c t t h a t, i n t h e a b s e n c e o f a n o x id iz a b le a d d i t iv e, t h en i t r i t e q u a n tu m y ie ld f a l l s w i th in c re a s in g f lu e n c e r a t e ( s e eF ig . 4 ) i n d ic a t e s th a t s e c o n d -o r d e r r e a c t io n s , u l t im a te lyresu l t ing in the fu l l re s t i tu t ion o f n i t ra te , p lay a pa r t . Reac t ion(2 5 ) a c c o u n t s fo r th i s f ea tu re .N O 2 + O N O O + H 2 0 --* 2 N O 3 + 2 H + ( 2 5 )B o th th e p ro d u c t s n i t r i t e a n d p e ro x y n i t r i t e a re s u b je c t t oa t t a ck b y t h e O H r a d i ca l ( r e a c t i o n s ( 1 7 ) a n d ( 1 9 ) ) ; it m a yb e s a id th a t n i t r i t e p ro te c t s p e ro x y n i t r i t e a n d v i c e v e r s a .

T o s u m u p , t h e p ro d u c t p e ro x y n i t r i t e t o s o m e e x te n t p ro -t e c t s t h e p ro d u c t n i t r it e a g a in s t r e -o x id a t io n b y O H th ro u g hre a c t io n (1 9 ) to 0 2 a n d th e n i t r a t e p re c u r s o r N O 2 ( re a c t io n(1 6 ) ) , t h e p ro te c t iv e e f f e c t b e in g g re a t e s t i n th e a lk a l in era n g e w h e re th e O N O O - s t e a d y - s t a t e c o n c e n t ra t io n i s h ig h -e s t . A t t h e s a m e t im e , p e ro x y n i t r i t e c o n t r ib u te s to n i t ri t e p ro -d u c t io n o n a c c o u n t o f r e a c ti o n s ( 2 2 ) a n d ( 2 4 ) . T h ep ro te c t iv e e f f e c t i s l o c k e d in a s t h e r e s u l t i n g n i t r i c o x id ep e r o x y l r a d i c al i s t r a n sf o r m e d i n to m o l e c u l a r o x y g e n ( r e a c -t io n s (2 2 ) a n d ( 2 3 ) ) (w h ic h i s in e r t u n d e r th e se c o n d i t io n s )a n d in to th e n i t r it e p re c u r s o r N O .

R e g a rd in g th e s i t u a t io n in th e m o re s t ro n g ly a lk a l in e p Hra n g e , a q u e s t io n r e m a in s a s t o w h e th e r p h o to ly t i c d e s t ru c t io no f O N O O - ( E ( 2 5 4 n m ) - - 3 6 0 d m 3 m o l - 1 c m - l , e s t i m a t e do n t h e b a s is o f e ( 3 0 2 n m ) = 1 6 7 0 d m 3 m o l - l c m - 1 ) m a yc o n t r ib u te to s o m e e x te n t t o n i t r i t e fo rm a t io n [4 4 ] , e . g . v i aO N O O - + h v - - * 0 2 - + N O . B o th N O a nd O 2 - ( se eb e lo w ) in e f f e c t r e d u c e N O 2 to n i tr i te . T h e i r r e c o m b in a t io n[ 3 7 , 7 5 -7 7 ] i s p ro b a b ly u n im p o r t a n t u n d e r th e p re s e n t c o n -d i t i o n s. A p p a re n t ly , t h e e f f e c t o f a n y p e ro x y n i t r i t e p h o to ly s i s

is qu i te smal l s ince , in s t rong ly a lka l ine so lu t ions , where thec o m p o u n d i s t h e rm a l ly s t a b le , t h e p e ro x y n i t r i t e y i e ld v s . d o s ep lo t i s o n ly v e ry s l ig h t ly c u rv e d d o w n w a rd s (F ig . 5 ) .

4.2. Oxidizable additives

In th e p re s e n c e o f O H ra d ic a l s c a v e n g e r s , s u c h a s a l c o h o l so r h y d ro c a rb o n s , t h e re is a p ro n o u n c e d c h a n g e in m e c h a n i s m .T h i s i s t h e o b v io u s c o n s e q u e n c e o f t h e r a p id a n d , a t s u f f i -c i e n t ly h ig h a d d i t iv e c o n c e n t ra t io n s , p ra c t i c a l ly e x c lu s iv ere a c t io n o f O H ra d ic a l s w i th th e a d d i t iv e . A p o s s ib l e r e a c t io no f th e p h o to e x c i t e d n i t r a te i o n w i th th e s e c o m p o u n d s c a n b eru led ou t [ 15 ] . Loo king a t the s i tua t ion in the a lka l ine range ,w e s e e th a t t h e a l c o h o l p ro te c t s t h e p ro d u c t s n i t r it e a n d p e r -o x y n i t r i t e . In th e n e u t ra l a n d a c id i c p H ra n g e , i t p ro t e c t s t h en i t r i t e a n d O N O O H , th e l a t t e r b e in g p re s e n t i n a r e l a t iv e lylo w s t e a d y s t a t e c o n c e n t rat io n . T h e f a c t t h a t th e p e ro x y n i t r i t eq u a n tu m y ie ld is g re a t e r i n th e p re s e n c e o f O H ra d ic a l s c a v -e n g e r s th a n in th e i r a b s e n c e l e n d s s t ro n g s u p p o r t t o th e e x i s-t e n c e o f r e a ct io n (1 9 ) . T h e h y d ro x y a lk y l r a d ic a l s w h ic hr e s ul t f r o m r e a c t i o n ( 2 6 ) ( k 2 6 = 9 . 7 1 08 d m 3 m o 1 - 1 s - t[3 2 ] ) a r e r a p id ly c o n v e r t e d in th e p re s e n c e o f o x y g e n to th ec o r re s p o n d in g p e ro x y l r a d ic a l s ( r e a c t io n (2 7 ) ; k27 = 4 10 9d m 3 m o l - ~ s - 1 [ 7 8 ] ) . T h e l a tt e r f u r t he r d e c a y s i n to t h ec a r b o n y l c o m p o u n d a n d s u p e r o x i de r a d i c a ls [ 7 9 ] . I t c a n b es h o w n th a t th e b im o le c u la r s e l f - r e a c t io n o f t h e s e p e ro x y lr a d ic a ls , w h i c h i n t he c a s e o f H O C H 2 O O g i v e s r is e to f o r m i ca c id [8 0 ] , m u s t b e in s ig n i f i c a n t a b o v e p H 5 a t th e l i g h tin t e n s i t i e s a p p l i e d ( fo r a r e v ie w o f th e r e a c t io n s o f p e ro x y lra d ic a l s i n a q u e o u s s o lu t io n , s e e R e f . [6 4 ] ) . T h e s u p e ro x id ee l i m i n a t i o n h a s a s l o w s p o n t a n e o u s c o m p o n e n t ( r e a c t i o n( 2 8 ) ; kz8 < 1 0 s - 1 a n d an O H - - c a t a l y s e d c o m p o n e n t ( r e a c -t i on ( 2 9 ) ; k 2 9 = 1 . 5 1 0 l d m 3 m o l - l s - l ) [ 7 9 ] , i .e . a tp H = 6 ( [ O H - ] = 1 0 - 8 m o l d m - 3 ) , t he O H - - i n d u c e d r e a c -t io n is a l r e a d y q u i t e f a s t (k = 1 0 0 s - l ) , d e s p i t e t h e f a c t t h a t[ O H - ] i s lo w a t t h is p H . T h u s O 2 - / H O 2 i s a m a j o r r a d ic a lin th i s s y s t e m [8 1 ] .OH + CH30 H --~ CH2O H + H20C H z O H + 02 - ' -)H O C H 2 0 ~HOCH202 --) CH20 + H 0 2H O C H 2 0 ~ + O H - - o C H 2 0 + O 2 - + H 2 0H O 2 ~ O ~ - + H + ( p K a = 4 .8 )

(26)(27)(28)(29)(30)

A t l o w e r p H , w h e r e t h e h y d r o x y a l k y l p e r o x y l r a d ic a l s a rein c re a s in g ly lo n g - l iv e d a s th e p a th w a y (2 9 ) b e c o m e s b a r re d ,w e e x p e c t th e re a c t io n o f N O 2 w i t h th e h y d r o x y a l k y l p e r o x y lra d ic a l s t o p l a y a ro l e ( r e a c t io n (3 1 ) [ 8 2 ] ) . T h e h y d ro x y -a lk y lp e ro x y n i t r a t e t h u s fo rm e d i s e x p e c te d to u n d e rg oh y d r o l y si s ( r e a c t i o n ( 3 2 ) ) , r e l e a si n g f o r m a l d e h y d e a n d p er -o x y n i t r i c a cid . T h e l a t t e r i s k n o w n to d e c o m p o s e in to o x y g e na n d n i t r it e [8 3, 8 4 1 ( r e a c t io n (3 3 ) ) .


10/15

98 G . M a r k e t a l . / J o u r n a l o f P h o t o c h e m i s t r y a n d P h o t o b i o l o g y A : C h e m i s t r y 1 0 1 ( 1 9 9 6 ) 8 9 - 1 0 3

H O C H 2 0 2 + N O 2 --~ H O C H 2 O O N O 2 ( 3 1 )HOCHzOONO2 + H20 --) CH20 + 02NOO +H (32)O z N O O - ~ 0 2 + N O ~- ( 3 3 )H e re , t h e q u e s t io n a s to th e f a t e o f t h e s u p e ro x id e r a d ic a lm u s t b e p o s e d ; i n p a r t i c u la r , w h e th e r t h e in c re a s e inq ~ (N O 2 ) w i th in c re a s in g p H m a y p e rh a p s b e p a r t i a l ly d u et o t he r e d u c ti o n o f N O 3 b y O 2 - . S i n c e th e l i f et i m e o f s u p e-ro x id e in c re a s e s w i th in c re a s in g p H , th is r e a c t io n m a y p o te n -t i a l ly h a v e m o re s c o p e . H o w e v e r , s u c h a p ro c e s s c a n b e ru le do u t o n th e b a s i s o f r a d ia t io n c h e m ic a l e x p e r im e n t s w h e res u p e ro x id e r a d ic a l s w e re g e n e ra t e d in th e p re s e n c e o f n i tr a t eb u t w e re in e f fe c t iv e in g iv in g r i s e to n i tr i t e. In s t e a d , O 2 - i se x p e c te d to b e in v o lv e d in a r a d ic a l t e rm in a t io n r e a c t io n w i thN O 2 . I t is se e n ( F i g . 2 ) t h a t ~ ( N O 2 ) , i n t h e p r e s e n c e o fo x id iz a b le a d d i t iv e s a s w e l l a s i n th e i r a b s e n c e , b e g in s toin c re a s e n e a r p H 7 f ro m a lo w e r v a lu e in th e a c id i c r a n g e .H o w e v e r , t h i s i s m u c h h ig h e r in th e p re s e n c e th a n in th ea b s e n c e o f t h e a d d i t iv e m e th a n o l : a b o u t 0 . 0 6 in s t e a d o f l e s sth a n 0 . 01 . T h e p ro b a b le r e a s o n fo r th i s i s t h a t O 2 - r e d u c e sN O 2 to n i t r i t e ( r e a c t io n (3 4 ) ; k34 = 2 X 1 08 d m 3 m o l - ~ s -1[ 8 3 ] ) , a p p a re n t ly v i a th e s h o r t - l i v e d p e ro x y n i t r a t e O 2 N O O( t l / 2 ~ 0 . 7 s [ 8 4 ] ) .0 2 - + N O 2 - - * O 2 + N O 2 ( 3 4 )T h u s a t p H < 7 , t h e s e q u e n c e o f r e a c t io n s ( 1 ) , (1 6 ) , (1 5 ) ,(2 7 ) , (2 8 ) , (3 0 ) a n d (3 4 ) r e s u l t s i n th e s to i c h io m e t r i c r e l a -t i o n (1 8 ) . T h e N O 2 ra d ic a l i s n o t s u f f ic i e n t ly r e a c t iv e [8 5 ]t o a b st r a c t H a t o m s f r o m m e t h a n o l ( e v e n w i t h f o r m a l d e h y d ew h e re th e C - H b o n d s t r e n g th i s s o m e w h a t l e s s t h a n in m e th -a n ol , k ( N O z + C H 2 0 ) i s o n ly 4 1 0 4 d m 3 m o l 1 s i a tro o m te m p e ra tu re [ 8 6 ] ) . In c o n t ra s t, t h e h y d ro x y m e th y l r ad -i c a l c a n r e a c t w i th N O 2 a t a d i f fu s io n -c o n t ro l l e d r a t e[8 7 , 8 8 ] , b u t w i l l e f f e c t iv e ly d o s o o n ly in th e a b s e n c e o fo x y g e n . U n d e r a n o x i c co n d i t io n s , C H 2 O H m a y a c t a s are d u c in g a g e n t t o w a rd s v a r io u s n i t ro g e n -c o n ta in in g in t e r -m e d ia t e s ( a l s o to s o m e e x te n t d ro p p in g o u t o f su c h a n in t e r -a c t io n b y d i m e r i z at i o n [ 8 9 ] ) . S e p a r a t e e x p e r i m e n t s ( d a t an o t s h o w n ) h a v e in d ic a t e d th a t t h e n i t ri t e q u a n tu m y ie ld s a res l ig h t ly lo w e r in o x y g e n - f re e th a n in o x y g e n a te d s o lu t io n s.

In th e a lk a l in e m e th a n o l /O 2 c a s e , th e re a re o b v io u s ly se c -o n d a ry r e a c t io n s o c c u r r in g w h ic h t r a n s fo rm th e p r im a ryp ro d u c t fo r m a ld e h y d e in to fo rm ic a c id . A s th is h a p p e n s o n lyin th e a lk a l in e p H re g io n , w e p ro p o s e th a t t h e p e ro x y n i t r i t ea n io n i s i n v o lv e d ( r e a c t io n (3 5 ) ) . I t i s k n o w n th a t p e ro x y -n i t r it e a n io n c a n a c t a s a p o te n t o x id a n t t o w a rd s c e r t a in c o m -p o u n d s , e . g . t h io l s [3 7 , 9 0 ] a n d m e th io n in e r e s id u e s inp ro te in s [ 9 1 ] . W e fo u n d i t to b e p ra c t i c a l ly u n re a c t iv eto w a rd s N O 2 : i n s o lu t io n s o f p H 1 3, t h e ra t e o f d e c a y o fO N O O - ( t l / 2 = 3 h ) , m o n i to re d a t 3 0 2 n m , i s n o t n o t i c e a b lya c c e le ra t e d o n a d d i t io n o f n i t ri t e a t n i t r it e c o n c e n t ra t io n s u pt o 1 0 - 2 m o l d m - 3 . T h e r e a c t io n o f f o r m a l d e h y d e w i t h n it r o-g e n d io x id e , i f i t l e a d s to fo rm ic a c id , i s m u c h to o s lo w [ 8 6 ] .E q u a l ly , r e a c t io n (3 6 ) [8 0 ] c a n n o t p ro v id e an e x p la n a t io ns in c e, u n d e r a lk a l in e c o n d i tio n s , t he O H --c a ta ly s e d d e c a y

r e a c ti o n ( 2 9 ) o f t h e H O C H 2 O O r a d i ca l s [ 7 9 ] m u s t c o m -p le t e ly e c l ip s e r e a c t io n (3 6 ) [ 8 0 ] . T h e a lk a l i - c a t a ly s e d C a n -n iz z a ro d i s p ro p o r t io n a t io n o f fo rm a ld e h y d e in to fo rm ic a c ida n d m e th a n o l w a s ru l e d o u t o n th e b a s i s o f a c o n t ro le x p e r im e n t .C H 2 0 + O N O O - ~ H C O2 - + H + + N O 2 ( 3 5 )2 H O C H 2 O O ~ 2 H C O 2H + H20 2 ( 3 6 )

I t h a s b e e n s h o w n re c e n t ly th a t t h e p e ro x y n i t r i t e a n io n d o e sin d e e d r e a c t w i th fo rm a ld e h y d e , k3 5 = 1 .4 X 10 3 d m 3 m o l - 1s 1 [3 6 ] . A p p a re n t ly , i t a l s o r e a c t s w i th fo rm a te , a l th o u g hm o r e s lo w l y , k = 2 d m 3 m o l i s ~ [ 7 7 ] . A c o n t r o l e x p e r i-m e n t h a s s h o w n th a t, i n a lk a l in e s o lu t io n , p e ro x y n i t r i t e d o e sn o t r e a c t w i th m e th a n o l o n th e t im e s c a le o f o u r e x p e r im e n t s .

T h e f a l l -o f f (F ig . 6 ) i n th e n i t r it e i n t e g ra l q u a n tu m y ie lda t v e ry s m a l l m e th a n o l c o n c e n t ra t io n s i s p ro b a b ly d u e toin c re a s in g c o m p e t i t i o n o f n i tr i t e a n d p e ro x y n i t r i t e w i th m e th -a n o l fo r t h e O H ra d ic a l a s p h o to ly s i s p ro c e e d s . T h i s i n d ic a t e sa n e x p e r im e n ta l d i f f i c u lty e n c o u n te re d in th e s tu d y o f t h ea d d i t iv e - f r e e (b e lo w n e u t ra l ) n i t r a te p h o to ly s i s . I f t h e w a te ru s e d i s n o t v e ry p u re w i th r e s p e c t t o o x id iz a b le s o lu t e s , i n i ti a ln i t r it e q u a n tu m y ie ld s w i l l b e r e l a t iv e ly h ig h b e fo re th e y s e t t l ed o w n t o th e ir " r e a l " v a l u e s a f t e r t h e i m p u r i t y h as b e e n c o m -p le t e ly o x id iz e d . A t b a s i c p H , th e e f f e c t o f s u c h a n im p u r i tyw i l l b e le s s c o n s p ic u o u s ( s e e F ig . 2 ) .

Wi th th e a d d i t iv e c y c lo p e n ta n e , o n ly a s in g le ty p e o f r a d -i c a l, c y c lo p e n ty i , w il l b e g e n e ra t e d b y O H ra d ic a l a t t a c k .F ig . 2 s h o w s th e p H d e p e n d e n c e o f t h e q u a n tu m y ie ld o fn i t ri t e i n th e p re s e n c e o f c y c lo p e n ta n e a n d in th e a b s e n c e o fO 2 . A p a r t f ro m n i t r it e a n d p e ro x y n i t r i t e , c y c lo p e n ty l n i t ra t e( i t s fo rm a t io n s h o w in g a b r i e f i n d u c t io n p e r io d ) a n d n i t ro -c y c lo p e n ta n e w e re fo u n d (T a b le 1 ) . T h e re i s a l s o a c h a n g ein p H in th e a lk a l in e d i r e c t io n w h ic h i s m o s t c o n s p ic u o u sw h e n th e in i t i a l p H i s a b o u t n e u t ra l . T h e r e a s o n fo r t h i s i sth a t , d u e to th e fo rm a t io n o f t h e o rg a n ic n i t ro g e n c o m p o u n d s(c f . n i tro c y c lo p e n ta n e in r e a c t io n ( 3 7 ) ) , t h e n i t ro g e n -b a s e da n io n i s r e m o v e d a n d th e n e g a t iv e c h a rg e m u s t c o m e to r e s id ee lsewhere , i . e . hydrox ide ion is l ibe ra tedC 5 H9 + N O 2 ~ C s H 9 N O 2 ( 3 7 )T h i s a l s o e x p la in s w h y th e n i t ri t e y i e ld s s h o u ld b e lo w e r th a nin th e m e th a n o l /O 2 c a s e . Mo re o v e r , s in c e th e p h o to ly s i sc h e m is t ry i t s e l f is p H d e p e n d e n t , i t is im p o r t a n t t o b u f fe r t h es o lu t io n o r t o k e e p c o n v e r s io n s v e ry s m a l l i n th e n e u t ra lr e g io n s o th a t t h e p H c h a n g e s a s l it t le a s p o s s ib l e d u r in g th ec o u rs e o f p h o to ly s i s . A n in c re a s e in p H in th e a d d i t iv e - f r e en i t r at e p h o to ly s i s h a s b e e n o b s e rv e d p re v io u s ly [ 1 7 ] , a n dim p u r i t i e s w e re s u s p e c te d a s th e c a u s e o f t h is . N o t e v e ry k in do f o rg a n ic im p u r i ty w i l l r e a d i ly a c t i n th i s w a y , a s c e r t a inty p e s o f r a d ic a ls , s u c h as a -h y d ro x y a lk y l r a d ic a l s (w h ic h a reo x i d i z ed ( r e a c t i o n ( 3 8 ) ) t o th e c a r b o n y l c o m p o u n d , a c c o m -p a n ie d b y th e fo rm a t io n o f n it r i te [9 2 ] ) , m a y n o t g iv e r i s et o n o n - h y d r o l y s i n g o r g a n ic n i t ro g e n c o m p o u n d sC H 2 O H + N O 2 --* C H 2 0 + N O 2 + H + ( 3 8 )


11/15

G. Mark et al. / Journal of Photochemist~ and PhotobiologyA: Chemistry 101 (1996) 89-103 99Table 2Photolysis of aqueous nitrate in the absence and presence of propan-2-ol. Som e product quantum yields under limiting conditions of additive concentrationand pH (see also Figs. 2, 6 and 8)Product No additive Approximately 10 mol dm -~ propan-2-ol (02 saturated)

pH 5a pH 13a pH 5~ pH 13bNO~ 0.006 0. Ic 0.102 - 0.03 (0) (0)ONO O 0 0.06 0 0.1a[NO3] =0.01 moldm 3.b[NO3] =0.1 mold m 3.~Includes ni~ te prod uced from peroxy nitrite und er the cond itions of the nitrite assay.

T h e c o m p l e t e a b s e n c e o f c y c l o p e n t y l n it ri te C s H g O N Oa m o n g t h e p r o d u c t s i s r e m a r k a b l e . I f i t h a d b e e n f o r m e dp h o t o l y t i c a l l y , i t w o u l d h a v e b e e n d e t e c t e d s i n c e t h e r e w a sn o i n t e r f e re n c e b y o t h e r G C p e a k s . A l s o , w h e n a t ra c e a m o u n to f a u t h e n t i c c y c l o p e n t y l n i tr it e w a s a d d e d p r i o r to p h o t o l y s i s ,i t w a s s e e n t o r e m a i n u n a f f e c t e d o n p h o t o l y s i s . S m a l l a m o u n t so f c y c l o p e n t a n o n e a n d c y c l o p e n t a n o l w e r e a l s o o b s e r v e d . A ni n d u c t i o n p e r i o d f o r c y c l o p e n t y l n i t r a t e f o r m a t i o n w a so b s e r v ed . T h i s w o u l d b e e x p e c t e d i f r e ac t io n s ( 3 9 ) - ( 4 2 )w e r e t o o c c u r , s i n c e t h e ra t e o f re a c t i o n ( 3 9 ) i n c r e as e s f r o mz e r o a s t h e p h o t o l y s i s p r o c e e d s . M o r e o v e r , c y c l o p e n t y ln i t r a t e f o r m a t i on s hou l d be f a vou r e d a t a l ka l i ne pH , a s i s i nf a c t the c a s e ( s e e T a b l e 1 ) .R + O N O O ~ R O N ( O O - ) ( 3 9 )R O N ( O O - ) + H + ~ R O N ( O O H ) ( 40 )R O N ( O O H ) + N O 2 ~ + N ( O O ) O R + N O 2 + H ~

( 4 1 )+ N ( O O - ) O R - -* O 2N O R ( 4 2 )

I n o x y g e n a t e d s o l u t i o n s , t h e f o r m a t i o n o f c y c l o p e n t y ln i tr a te a n d n i t r o c y c l o p e n t a n e i s s u p p r e s s e d , w h i l e c y c l o p e n -t a n o n e a n d c y c l o p e n t a n o l i n c r e a s e i n c o n c e n t r a t i o n . T h i s i sa s e x p e c t e d s i n c e , i n t h e p r e s e n c e o f 0 2 , t h e c y c l o p e n t y lr a d i c a l i s c o n v e r t e d i n t o t h e c o r r e s p o n d i n g p e r o x y l r a d i c a lw h o s e t e r m i n a t i o n p r o d u c t s a r e c y c l o p e n t a n o l a n d c y c l o p e n -t a n o n e . ( F o r d e t a il s o f th e c y c l o p e n t y l p e r o x y l ra d i c a l c h e m -i s t ry i n a qu e ou s s o l u t i on , s e e R e f . [ 6 3 ] .)

4.3. The importance of photoreactions (1)-(3)O n t he ba s is o f the r e s u l t s d i s c us s e d a bov e , w ho s e s a l ie n t

po i n t s a r e c o l l e c t e d i n T a b l e 2 , t he que s t i on o f t he r e l a t i vei m p o r t a n ce o f t h e p h o t o r e a c ti o n s ( 1 ) - ( 3 ) ( s e e S c h e m e 1 ,w h e r e t he pos s i b i l i t y o f s o l ve n t c a ge e f f e c t s is t a ke n i n t oc o n s i d e r a t i o n ) c a n n o w b e a d d r e ss e d .

F i r s t, r e a c t i on ( 3 ) , i n w h i c h a n i tr i te i on i s t b r m e d t og e t he rw i t h a n o x y g e n a t o m , s e e m s t o b e o f m i n o r i m p o r t a n c e [ 1 4 ]c o m p a r e d w i t h t h e s u m o f r e a c t io n s ( 1 ) a n d ( 2 ) f o r t h ef o l l o w i n g r e a s o n s .

1 . I ts f r e q u e n c y i s t h o u g h t t o b e a t m o s t 1 0 % o f th a t o fr e a c t i on ( 1 ) [ 20 ] .

2 . T h e p r e s e n c e o f ~O a t o m s a n d a n y r e a c t i o n w i t h t h e su b -s t r a t e c a n b e r u l e d o u t b e c a u s e t h e y w o u l d r e a c t v e r ye f fi c ie n t l y w i t h w a t er t o p r o d u c e H 2 0 2 : 1 0 + H 2 0 ~ H 2 0 2[ 3 3 , 3 4 ] ; h o w e v e r , H 2 0 2 w a s n o t d e t e c t e d b y u s o r o t h e r s[ 1 4 , 1 6 ] ; q ) ( H 2 0 2 ) m u s t b e s m a l l e r t h a n 1 0 - 3 .3 . W h e n d e t e r m i n i n g t h e p r e s e n c e o f O ( 3 p ) a t o m s i n a q u e -o u s s o l u ti o n a c c o r d i n g t o t he m e t h o d o f B r o w n a n d H a r t[ 3 5 ] , w e f o u n d a q u a n t u m y i e l d o f l e ss t h a n 10 -3, i .e .l e ss t h a n a b o u t 1 % o f t h e m a x i m u m n i tr it e c o n t e n t . I t h a sb e e n p r o p o s e d t h a t t h e t ri p le t o x y g e n a t o m m a y r e a c t w i tht h e n i tr a te i o n v i a t h e f o r m a t i o n o f t h e p e r o x y n i t r a t e i o nO 2 N O O [ 1 3 ] . T h i s w o u l d g i v e r i se t o N O 2 a n d O z i n at h e r m a l d e c o m p o s i t i o n r e a c t i o n [ 8 3 , 8 4 ] , i .e . t o th e s a m ep r o d u c t s a s g e n e r a t e d i n t h e r ea c t i o n s s u b s e q u e n t t o t h ep r i m a r y r e a c ti o n s ( 1 ) a n d ( 2 ) ; t h u s i t w o u l d b e im p o s -s i bl e t o j u d g e t h e i m p o r t a n c e o f r e a c t i o n ( 3 ) s o l e ly b yl o o k i n g a t t h e p h o t o l y s i s p r o d u c t s . H o w e v e r , t h e r e a c t iv i t yo f t h e tr ip l e t o x y g e n a t o m w i t h N O 3 s h o u l d b e s m a l l e rt h a n t h at w i t h c y c l o p e n t e n e b y a b o u t t w o o r d e r s o f m a g -n i tu d e ( s e e R e f . [ 9 3 ] i n c o n j u n c t i o n w i t h R e f . [ 6 5 ] ) .T h e r e f o r e , u n d e r t h e c o n d i t io n s o f t h e 3 0 a s s a y [ 3 5 ] ,e s s e n t i a l l y a ll o f t he 30 ge ne r a t e d s hou l d ha ve r e a c t e dw i t h c y c l o p e n t e n e .

T h i s l e a v es re a c t io n s ( 1 ) a n d ( 2 ) a s t h e m a j o r p r i m a r yp r o c e s s e s. P e r o x y n i t r i te m a y b e f o r m e d d i r e c tl y b y r e a c t io n( 2 ) , b u t a l s o f r o m N O 2 a n d O - in t h e s o l v e n t c a g e ( r e a c t i o n( 7 ) ) . I f t h e c a g e r e c o m b i n a t i o n r e a c t io n ( 7 ) i s i m p o r t a n t ,t h e O N O O - y i e l d s h o u l d b e d e p r e s s e d i n t h e p r e s e n c e o fh i g h c o n c e n t r a t i o n s o f a s c a v e n g e r s u c h a s p r o p a n - 2 - o l ,w h i c h r e m o v e s O v i a r e a c t io n ( 5 ) . T h i s s e e m s t o b e t h ec a s e to a s l ig h t e x te n t o n l y ( q ~ ( r e a c t i o n 7 ) - ~ 0 . 0 1 ) a s t h eda t a i n F i g . 6 , ob t a i ne d a t pH 13 , ind i c a t e . I t i s t he r e f o r ec o n c l u d e d t h a t r e a c t io n ( 2 ) i s t h e m a i n p a t h w a y t o p e r o x y -n i tr it e , f o r m e d w i t h a q u a n t u m y i e l d o f 0 .1 . S o l u t i o n s o f p H1 3 h a v e b e e n e m p l o y e d , s i n c e t h e p e r o x y n i t r i te a n i o n i s s u f -f i c i e n tl y s t a b l e a t t h i s pH o n t he t i m e s c a l e o f t he s e e xpe r i -m e n t s [ 1 6 , 3 9 ] .

T h e i m p o r t a n c e o f r e a c t io n ( 2 ) r e l a ti v e t o r e a c t io n ( 1 )c a n n o t b e d i r e ct l y in f e r r ed f r o m t h e N O ~ y i e l d s b e c a u s e o ft h e c o m p l i c a t i o n s i n t r o d u c e d b y t h e s e c o n d a r y r e a c t i o n s o f


12/15

10 0 G . M a r k e t a l . / J o u r n a l o f P h o t o c h e m i s t r y a n d P h o t o b i o l o g y A : C h e m i s t r y 1 0 1 ( 1 9 9 6 ) 8 9 - 1 0 3

t h e p ro d u c t p e ro x y n i t r i t e . N e v e r th e le s s , a n e s t im a te c a n b em a d e o n th e b a s i s o f t h e 0 2 y i e ld m e a s u re d a t p H 1 3( 4 ( 0 2 ) = 0 . 0 3 ) , o r o n t h e b a s is o f t h e d i f f e re n c e b e t w e e nth e p e ro x y n i t r i t e y i e ld s a t t h i s p H in th e p re s e n c e o f p ro p a n -2 -o l ( 4 ( O N O 0 - ) 8 M -P~OH= 4 ( reac t io n 2 ) = 0 .1 ) and in i tsa b s e n c e ( 4 ( O N O 0 - ) no ~ ,~ Uv~= 0 .06 4) .07 , see F ig . 6 ) ; th isd i f f e re n c e s ho u l d b e e q u a l to 4 ( 0 ! ) ( s e e a b o v e ) . I t h a s b e e nn o t e d a b o v e t h a t p r o p a n - 2 - o l , w h ic h s c a v e n g e s ' O H a n d O -r a d ic a l s, p ro t e c t s p e ro x y n i t r i t e a g a in s t O H ra d ic a l a t t a c k . A tp H 1 3, w h e r e O N O 0 - i s s t ab l e, t he on l y s o ur c e f o r t h e O H /O - r a d i c al i s r e a c ti o n ( 1 ) . E a c h O N O 0 i o n o x i d i ze d b y' O H p r o d u c e s o n e m o l e c u l e o f O 2 if r e a c ti o n ( 1 9 ) i s f o l l o w e db y re a c t io n (2 2 ) , b u t o n ly a h a l f - e q u iv a le n t o f O 2 i f fo l lo w e db y r e a c t i o n ( 2 3 ) . T h e v a l u e o f q b ( O z ) = 0 . 0 3 i s i n f a i ra g r e e m e n t w i t h t he v a l u e f o r 4 ( O N O O - ) 8 M i -P rO H --4 ( O N O 0 ) ~o ~ddiU~= A 4 ( O N O O - ) = 0. 04 ( s ee F i g. 6 ),w h ic h m a y b e t a k e n a s an in d ic a t io n th a t r e a c t io n (2 3 ) i s o fm i n o r i m p o r t a n c e c o m p a r e d w i t h r e a c ti o n ( 2 2 ) . 4 ( 0 2 ) , o rA 4 ( O N O O - ) , r ep r e se n t s th e l o w e r l i m it s t o 4 ( r e a c t i o n 1 )s in c e, i n th e a b s e n c e o f p ro p a n -2 -o l , th e O H ra d ic a l s g e n -e r a te d in r e a c t io n ( 1 ) d o n o t o n l y r e a c t w i t h O N O 0 , b u ta r e p a r t it i o n e d b e t w e e n t h e p r o d u c t s O N O 0 - a n d N O 2 .A l lo w in g fo r a c o n c e iv a b le n i t r a t e - fo rm in g c a g e r e a c t io n (4 )a n d a s s ig n in g a v a lu e o f 0 .0 3 to i t ( s e e a l s o b e lo w ) , t h e O Hra d ic a l p a r t i t i o n in g f in d s it s e x p re s s io n in E q . ( 4 3 )

4 / ) ( 0 2 ) ( / ) ( 0 2 )4 ( O H ) - 4 ( r e a c t i o n 1 ) - 4 ( r e a c t i o n 4 ) - 4 ( r e a c t i o n 7 )

k ~9 [ O N O O ] ( 4 3 )k ~ 9[ O N O O ] + k ~ 7 [ N O 2 ]W i t h 4 o a s t h e s u m o f 4 ( r e a c t i o n 1 ) a n d 4 ( r e a c t i o n 2 ) , E q .( 4 3 ) i s equ i va len t

the photo chemistry of aqueous nitrate ion revisited

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