multiphoton dissociation of ions derived from isopropanol and deuterated analogues in a quistor with...

Multiphoton dissociation of ions derived from isopropanol and deuterated analogues in a QUISTOR with low power CW infrared laser radiation'

RICHARD JAMES HUGHES, RAYMOND EVANS MARCH,' AND ALEXANDER BALDWIN YOUNG' Depnrrment of'Chettli.st,lv, Tretlt Ulziversih. Peterbororrgh. Otlt., Crit~rrrlrr K9J 7B8

Received June 30, 1982

This popel- is dedicrrted to Professor Hart? E . G~it1tlitlg otl the oc.c.nsioil of' his 65th birthtlnj

RICHARD JAMES HUGHES, RAYMOND EVANS MARCH, and ALEXANDER BALDWIN YOUNG. Can. J. Chem. 61, 834 (1983). A versatile technique employing pulsed quadrupole ion storage mass spectrometry and a low power CW CO' laser for the

study of slow infrared multiphoton dissociation of gaseous ions under near collision-free conditions is described. Multiphoton dissociation of gaseous ions derived from isopropanol. 2-(1,-isopropanol, isopropan-rl-ol and perdeutcroisopropanol, with irradiation for up to 110 ms with intensities of -20 W cm-', was effected. Irradiation of proton (or D') bound alcohol dimers showed that three dissociative reaction channels are utilized in multiphoton dissociation: the photodissociation yield for the dissociative rcaction channel of lowest activation energy increases and becomes dominant with collisional and radiative relaxation of the ions. Of the three ionic photoproducts of multiphoton dissociation of proton bound alcohol dimers, only the protonated ether is found to be photodissociative under thc prevailing conditions. 'The photochcmical stability of the ion formed by loss of the elements of propene from the proton bound alcohol dimcr contrasts sharply with the multiphoton dissociation of the proton bound alcohol-water mixed dimer, and serves to demonstrate the applicability of multiphoton dissociation to distinguish between gaseous ion isomers.

'The large extent of dissociation observed with low laser flucnce is indicative of both the rapid collisionally-induced migration of ions to the centre of the QUIS'TOR (and hence the laser beam) and also the predominance of multiphoton absorption over radiative and collisional deactivation processes in the time and pressure domains employed here.

RICHARD JAMES HUGHES, RAYMOND EVANS MARCH et ALEXANDER BALDWIN YOUNG. Can. J. Cheni. 61, 834 (1983). On dCcrit une technique versatile qui utilise la spectromitrie de masse i emmaganisage d'ion quadrupolaire pulsC et un laser

CW i COZ B faible puissance pour Ctudier la dissociation multiphoton infrarouge d'ions i I'Ctat gazeux dans des conditions pratiquement sans collisions. On effectue la dissociation multiphoton des ions gazeux provenant de I'isopropanol, de I'isopropanol 0 - d , de I'isopropanol rl-2 et du perdeutCroisopropano1 avec une irradiation allant jusqu'B 110 ms et des intensitCs d'environ 20 W cm-'. L'irradiation d'alcools dimeres liCs par le proton (ou D') a montrC que trois voies de rkaction dissociatives sont utilisCes dans la dissociation multiphoton; le rendement de la photodissociation de la voie de rCaction dissociative de plus faible Cnergie d'activation augmente ct devient pr6pondCrant avec la relaxation collisionnellc et radiative des ions. Des trois photoproduits ioniques de la dissociation rnultiphoton des alcools dimkrcs liCs par le proton, on a trouvC que seul I'Cther proton6 peut &tre photodissociC dans les conditions existantes. La stabilitC photochimique de I'ion form6 par suite de la perte des Cltments du propkne B partir de l'alcool dimere liC par le proton contrastc netternent avec la dissociation multiphoton du dimkre mixte alcool-eau liC par le proton et sert i dCmontrer la validit6 de la dissociation multiphoton dans la distinction des isomkres d'ions gazeux.

Le fort taux de dissociation observC avec un laser de faible puissance est une indication de la migration rapide induite par collision des ions vers le centre du QUlSTOR (et par consequent la faisceau du lascr) et Cgalement dc la p&dominance-de I'absorption multiphoton sur les processus de dCsactivation radiative et collisionnelle dans les domaines de temps et de pression.

[Traduit par le journal]

Introduction The characterization of intramolecular relaxation subsequent

to photochemical excitation requires near collision-free conditions for study; in general, single species chemistry of a molecule or ion cannot be ascertained unambiguously unless the mean time between collisions is greater than that required for excitation and for unimolecular decomposition. Recently, methods have been developed for studying the infrared (ir) multiphoton photochemistry of gas-phase ions under near collision-free conditions, using pulse ion cyclotron resonance (ICR) spectroxopy (1, 2).

Brauman and co-workers (1) have used a pulsed C 0 2 laser to study the photophysics of megawatt ir multiphoton dissociation and to probe vibrational relaxation in gas-phase ions.

'presented initially at the 65th Conference of the Chemical Institute of Canada, May 30 - June 2, 1982 in Toronto.

' ~d junc t Professor, Department of Chemistry, Queen's University, Kingston.

' ~ e ~ i s t e r e d in the Ph.D. programme in Chemistry of Queen's University.

Beauchamp and co-workers (2) have demonstrated that, as dissociation probabilities of isolated molecules depend only on energy fluence and not on peak laser power (3), a relatively low power CW C 0 2 laser (several watts) can effect dissociation processes in gaseous ions provided that an ion is irradiated for sufficiently long times under nearly collision-free conditions. Among the early studies of ion photodissociation originating from Dehmelt's laboratory in the 1960's (4) were a few studies on the photodissociation of H; and C H ~ carried out using radio-frequency quadrupole traps ( 5 ) . The application of such a quadrupole ion store (QUISTOR) has been extended now to slow ir multiphoton induced dissociation of gaseous ions. Pho- todissociation of the protonated dimer of 2-propanol, which was reported earlier (6), was achieved by the absorption of 11 photons in the ir region. A comparison of the characteristics of the QUISTOR with those of ICR is presented in the accompanying paper (12); while the two techniques are similar, ions stored in the QUISTOR are concentrated at the centre of the quadrupole field which has both axial and radial symmetry, and ion photodissociation studies are carried out at 8- 10 Hz compared to 1 Hz with ICR.

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HUGHES ET AL.: 2 835

In view of the extensive work of Bonlse and Beauchamp ( 2 c ) on the multiphoton dissociation of proton bound alcohol dimers, isopropanol was selected for initial study in the QUISTOR and the study was extended to an examination of unimolecular reaction energetics in deuterated analogues of isopropanol. Infrared multiphoton dissociation has been e m - ployed (2,f, g ) in order to distinguish between structural isomers of gaseous ions, and this technique has been used here to distinguish between a protonated alcohol-water cluster ion and the ion formed by loss of alkene from a proton bound alcohol dimer.

T h e ion/molecule reactions of protonated aliphatic alcohols have been studied in some detail, with particular emphasis given to isopropanol using ion cyclotron double resonance (7, 8) and by the application of resonance ion ejection to quadrupole ion-storage mass spectrometry (9). Well-characterized bi- molecular processes occur in the isopropanol system whereby reaction intermediates are readily prepared a s stable species. Primary ions (particularly nz/z 43) formed by electron impact react with neutral isopropanol molecules to form protonated isopropanol, ( C H ~ ) ~ C H O H ~ , m/z 6 1 , which reacts further with isopropanol to form the activated protonated dimer , i n l z 12 1 :

T h e activated proton bound isopropanol dimer may dis- sociate back to reactants, or be collisionally o r radiatively stabilized

where M is a neutral isopropanol molecule. Alternatively, H 2 0 may be eliminated to form a protonated ether

o r propene may be lost

In the absence of other specific data, the enthalpy change for [ I ] is assumed to be comparable to the measured enthalpy changes for the analogous reaction of H , O t , C H , O H ~ , and (CH&OH+ (10). T h e activation energies for [3] and [4] are not known, though [3] is reported a s the decomposition pathway of minimum activation energy (2c ) . Multiphoton dissociation of the proton bound dimer of isopropanol was induced in order to demonstrate the application of the QUISTOR for single ion photochemistry studies.

Experimental Although the apparatus has been described in detail previously (6)

and in the accompanying paper, some modifications have been made. The laser diffraction grating is replaced by a plane gold surfaced mirror and is thus operating at 944 cm-' for all experiments. The laser power is monitored by a homemade thermocouple device mounted outside the second sodium chloride window. The output of the thermocouple is displayed on a microvoltmeter (Digitec 268) and recorded on a chart recorder (Leeds and Northrup, Speedomax W). The thermocouple output is calibrated to the laser power using a Laser Power Probe (Optical Engineering Inc.). Beam shape is monitored by burning business cards. The QUISTOR/tank/pump assembly is por- table and mounted on four adjustable feet. Maximum laser intensity within the QUISTOR is achieved by adjustment of the assembly while

thc cxtcnt of dissociation of thc protonatcd dimcr of 2-propanol is monitored.

The time scalc of thc expcrimcnt, typically 100 ms, is provided by a square wavc generator (Heathkit model SGl8A) also driving a mechanical choppcr (PAR 222), which is phase locked to the input signal. The beam is chopped such that a dark period precedes the irradiation period with the duration of cach being cqual. The square wavc gencrator also triggcrs the creation pulse (Hewlett Packard Pulse Gcnerator, Model 214A) which in turn triggers the extraction pulse (Hewlett Packard, Modcl 214B) which is advanced by - 1 ms. The creation pulse (80 V, 90 ps) is applied to an electron gate situatcd betwecn the filament and the top end cap. The gate is normally held at -90 V to bar electrons from the trap. The extraction pulse (-50 V, 10 ps) triggers a scan dclay generator (Ortec Brookdeal Model 9425) which provides a trigger, or dctection pulse of 100 p s width, 90 ps after the extraction pulsc. The detection pulse drives a Linear Gate (Ortec Brookdcal Model 9415). The QUISTOR is operated in mode I1 as describcd in the accompanying paper, the RF potential (typically 500-550 V) being supplied by an Extranuclear Laboratories Quadru- pole Power Supply, Model 01 1-15, modified for QUISTOR storage work. Following irradiation, product ions are mass analyzed by a quadrupole mass filter (Vacuum Generators. QXK 400) either by running a complcte mass spectrum or, in this work, by single monitoring. The pulse sequence is shown in Fig. 1.

In those experiments in which Quadrupole Resonance ejection was employed, the output signal of the chopper triggers a toneburst (Wavetek, Model 134) applied to the top end cap concomitant with the laser irradiation period. The toneburst of a chosen frequency resonantly ejects ions of a particular mass-to-charge ratio. The ions ejected in this work are either the protonated (or D') parent alcohol or the proton (or D') bound dimer of the alcohol.

In studies of dissociation pathways, a third pulse generator (Berkely Nucleonics Corp., Model 801 0) is used. In this work, the square wave output is uscd to trigger the BNC pulse generator which in turn triggers the creation pulse. Thc extraction pulse is triggered also by the square wave output (negative slope). This mode of triggering allows the creation pulse to be advanced to any position within the 100 ms time frame of the experiment.

The ion structure experiments are performed in two ways, the first being a resonant ejection of m/z 121, the protonated dimer of isopropanol, with concurrent laser irradiation while monitoring the behaviour of m / z 79, the postulated proton bound dimer of water and isopropanol. In the second set of experiments, m/z 83 is synthesized in situ by clustering D,O molecules and deuteronated isopropan-d-ol ions. At low pressures (0.7 to I mPa) clustering is favoured, whereas at higher prissures (5 mPa) m/z 82 is produced via unimolecular dissociation of m/z 124, the deuterated analogue of the proton bound dimer of isopropanol.

Some of the isopropanol experiments are performed using pulse sequences previously described (1). All regions labelled Laser On refer to the mechanically chopped laser previously described, whereas Continuous Laser refers to laser irradiation for the total duration of the experiment.

Isopropan-d-ol was purchased from Aldrich; perdeuteroisopropanol and 2-dl-isopropanol were obtained from Merck, Sharp and Dohme. The samples were used without further purification, other than degassing by several freezelthaw pumping cycles prior to an experiment.

Results and discussion Bomse and Beauchamp have reported ( 2 c ) that at pressure

< 4 X mPa ir multiphoton dissociation of the proton bound dimer of isopropanol, (ROH)2H+ where R is i-C3H,, occurs principally by

[5] (ROH),H' + nhv - R20H' + H,O m/z 121 m/z 103

and that the alternate dissociation pathways

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836 CAN. J . CHEhl. VOL. 61. 1983

Heathkit

Wavetek

- Laser O n >

I I ,a- Resonance Time-~

I

11- Reaction Time > I n I

I I

> I-Irradiation Time -

Linear I Ga te 1 Storage 1 Time >K

Moveab le Creat ion Pulse

c-2- I

nd- Storage I Time 3- I I

<- I I n+ Reaction Time->!

I FIG. I . Pulse sequence used in single ion monitoring with typically 100 ms of ion storage, 50 ms of laser irradiation concurrent with resonance

ejection. The appended moveable creation pulse was employed in studies of multiple dissociation pathways induced by multiphoton absorption with respect to variation in internal energy of the irradiated ion.

[6] (ROH)2H+ + n ' h v - R O H ~ + ROH m/z 121 m/z 61

where n' = 11, and

together account for not more than 2% of the photodissociated (ROH)2H'. In apparent contrast with these findings, multiphoton dissociation of (ROH)?Ht in the QUISTOR at a pressure of 5.3 mPa showed that all three reaction pathways [5], [6], and [7] were open. As the pressure and cycling frequency of the ICR technique are not readily obtained with the QUISTOR, a more circuitous study was carried out in order to verify the observations and to attempt a reconciliation of the conclusions.

In Fig. 2 are shown two single ion monitoring traces of the abundance of proton bound alcohol dimer, m/z 121, throughout the reaction time range 60- 100 ms, at a pressure of 5.3 mPa. Resonance ejection of the precursor ion, m/z 6 1 , was

initiated at 55 ms reaction time. The upper trace, marked toneburst (m/z 61), shows that the rate of formation of proton bound dimers, m/z 12 1, is diminishing during the time 60- 80 ms due to relatively gentle removal of precursor ion; a maximum abundance is reached at -80 ms followed by a slow decline in abundance due mainly to ion losses by ion-ion and ion- neutral scattering processes.

The lower trace, marked chopped laser, shows the abundance of m/z 12 1 under the same conditions of storage but with resonance ejection of precursor after 55 ms of reaction time, concurrent with laser irradiation for up to 55 ms, at the conclusion of which the cycle is recommenced; thus the thermal (or dark) and irradiation periods were of equal duration. Irradiation was carried out with a CW COz laser of 20 Wcm-' at 944 cm-'. The increasing reduction in abundance of m/z 12 1 throughout the irradiation period shows the degree of photodissociation of the proton bound dimer. The inset in the lower right hand comer of Fig. 2 is illustrative of the technique by which changes in ion abundance (diminution or enhancement) upon laser irradiation of the system is monitored at a specific reaction

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HUGHES ET AL.: 2

FIG. 2. Ion abundance versus storage time profiles of proton bound alcohol d~mer, m/z 121, derived from ion/molecule reactions in isopropanol. Toneburst is QUlSTOR resonance ejection (QRE) of m/z 61, the protonated parent alcohol precursor of ntlz 121, commencing at 55 ms of storage time. Chopped laser refers to 55 ms of storage followed by 55 ms of laser irradiation with concurrent resonance ejection of m/z 61. Continuous laser refers to irradiation throughout 100 ms of storage time. CW COz laser irradiation of 20 W cm-' at 944 cm-'. All experiments were performed at 5.3 mPa.

time; the baseline which is recorded routinely is not shown causes a falloff in the rate of formation of m / z 103; a maximum here. The observations of m/z 12 1 abundance shown in the abundance at -90 ms is observed followed by an indication in inset were obtained after 100 ms of storage time. The signal the next 20 ms of ion loss processes. The time-delayed max- intensity due to the steady state attained through the thermal reactions [I]-[4] was monitored initially and designated "off" in that the laser was not employed. During the next several ion creation, storage, reaction, and sampling cycles the laser was "on" continuously during the reaction tlme of 100 ms, and the ion abundance was reduced due to diminution of the steady state concentration on account of additional photochemical removal processes such as [5]-[7]. Successive thermal steady state abundances shown In the inset show a gradual decline in signal intensity; this effect is due to a diminution in pressure in the QUISTOR caused by heating of the device during continuous irrad~ation. The effect is more pronounced with a new small QUISTOR, r, = 6 .0 X lo-' m, as described in the accompanying paper; allowance for the heating of the device was made subsequently.

As photodissociation of the proton bound dimer, m/z 121, was effected with the laser at 944 cm- ' , an exammation was made of the possible ~nvolvement of each of reactions [5]-[7], in turn, and the results are shown in Figs. 3-5, respectively. In Fig. 3 , the ion abundance of the protonated ether, m / z 103, over the same reaction time and conditions as for Fig. 2 is shown. The upper and lower traces from the single ion monitoring experiments are similar in profile; the lower trace, marked toneburst m/z 61, is remarkably similar to the corresponding trace for m/z 121 in Fig. 2 . The resonance ejection of the precursor ion, m/z 61, Initiated at 55 ms of storage

imum is as anticipated in that the resonantly ejected nz/z 61 ion is a precursor but once removed in that m/z 12 1 is the immedi- ate precursor of m/z 103. The upper trace was obtained under the same conditions of pressure and resonance ejection of m/z 6 1 after 55 ms; laser irradiation was initiated after 55 ms (concurrent with resonance ejection). There is a small but signifi- cant difference in the upper and lower single ion monitoring traces showing that the abundance of m/z 103 is increased upon laser irradiation, as expected, and that reaction [5] is operative. The inset shown in Fig. 3 is similar to that of Fig. 2; though the signal is noisy the enhancement of the m / z 103 signal intensity upon irradiation is clearly evident.

In Fig. 4 are shown single ion monitoring traces of protonated isopropanol, m/z 61. The trace marked toneburst ( m / z 61) illustrates the steady diminution of the m/z 61 ion signal upon gentle resonance ejection; the ions may be ejected more rapidly by increasing the peak-to-peak voltage of the radio- frequency toneburst shown in Fig. I . The trace marked normal is that of the unperturbed thermal ion abundance showing a steady decline with time as m/z 61 is consumed through, for example, reaction [I]. The top trace shows an enhancement of m / z 61 upon laser irradiation after 6 0 ms of storage; the increase in m/z 61 was observed also upon continuous irradiation throughout 100 ms of storage as shown in the inset in Fig. 4 . Thus reaction [6] is observed to be operative, showing that a minimum of 11 photons (not allowing for collisional deacti-

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838 CAN. J . CHEM. VOL. 61, 1983

I c h o p p e d l a s e r I

continuous l a se r

(100 rns)

FIG. 3. Ion abundance versus storage time profile of m/z 103. Conditions as for Fig. 2.

FIG. 4. Ion abundance versus storage time profile of m/z 61. The normal trace corresponds to the ion intensity from dark or thermaI ion/molecule reactions. Conditions as for Fig. 2.

vation or radiative loss) have been absorbed by the proton neutral (C3H6) as in [4] to produce the m/z 79 ion which may bound alcohol dimer. be tentatively identified as a proton bound mixed dimer of

Reaction [7] was then examined. Chemically activated pro- isopropanol and water. The lower traces each marked toneburst ton bound alcohol dimer ions formed in [I] may lose an alkene (m/z 61) in Fig. 5 show that the m/z 79 ion abundance profile

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HUGHES ET AL.: 2 839

FIG. 5 . Ion abundance versus storage time profiles of m/z 79. Conditions as for Fig. 2.

FIG. 6. Single photon infrared absorption spectrum in the range 1000 to 900 cm-I for isopropanol, isopropan-d-01, 2-dl-isopropanol, and perdeuteroisopropanol. Vertical arrow marks laser wavelength.

matches closely that of t ~ / z 61 marked toneburst in Fig. 4; this is as expected. The toneburst or resonance ejection of m/z 61 was initiated after 55 ms of storage. The traces in the top right hand corner of Fig. 5 are amplified by a factor of 6 to show more clearly the effect of chopped laser irradiation. The upper curves marked chopped laser show an enhancement of tnlz 79 upon laser irradiation concurrent with resonance ejection of m/z 61. This effect is seen also upon continuous irradiation for 100 ms as shown in the inset in Fig. 5. Thus i t appears that reaction [7] is operative, that photodissociation of the proton bound alcohol dimer under these operating conditions follows all three reaction channels described by reactions [5]-[7].

Multiphoton absorption by gaseous ions appears to occur at laser wavelengths corresponding to absorption in the single photon absorption ir spectrum. The ir absorption spectra in the vicinity of 944 cm-' were obtained for isopropanol and its deuterated analogues, 2-dl-isopropanol, isopropan-d-01, and perdeuterated isopropanol, and are shown in Fig. 6. The spectra were obtained with a Perkin-Elmer 700 ir spectro- photometer under conditions of saturated vapour pressure of each alcohol and in a thermally equilibrated gas cell. Each of the isopropanols shows strong absorption at 944 cm-', hence each deuterated analogue was examined in turn in order to determine the relative weights for the reaction pathways corresponding to reactions [5]-[7]. The observations, obtained at 5.3 mPa, are reported in Table 1 as chntzges in ion abundance for each of the major ions, wrought initially by laser irradiation and subsequently by laser irradiation concurrent with resonance ejection of the appropriate proton (or Dt) bound alcohol dimer. The data for isopropan-d-01 are incomplete. The extent of photodissociation for each of the dimer ions formed from isopropanol, 2-dl-isopropanol and perdeuterated isopropanol is in

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840 CAN J. CHEM. VOL. 61. 1983

TABLE 1. Isopropanol and deuterated analogues: a comparison of infrared lascr induced changes in ion abundanccs

(CH,),CHOH (CH3)XDOH (CDI),CDOD (CH,),CHOD

A/" A/'' ill" A/" A/'' ill" All ' ttl/? (%) ( % I rn/z (%) (%) tt1/z (%) (%) t n / z (%)

6 1 32 - 0 62 14 8 70 42 - 0 63 + 79 28 0 80 18 0 90 I I 0 82 +

103 29 (14) 105 13 (12) 118 - 0 (40) 104 -0 12 1 (56) (100) 123 (54) (100) 138 (68) (100) 124 ( )

"Change in ion abundance from that observed after 100 111s of storaxe to that observed after 100 ms of storage with lascr irradiation durlng the final 50 nis. Parentheses indicate a decrease in ion abundance.

"As for footnote u bur with resonance ejection of the appropriate proton bound alcohol dilner concurrent with laser irradiation.

MIZ 123 MIZ 105

( Q R E MIZ 123)

FIG. 7. Single ion monitored abundances of ions derivcd from 2-tll-isopropanol, at 5 .3 mPa. L refers to 50 ms of storage followed by 50 rns of CW C0, laser irradiation of 20 W cm-' at 944 cm-I. A: protonated dimcr of 2-dl-isopropanol. t n / z 123, followed by irradiation concurrcnt with QUISTOR resonant ejection (QRE) of t n / z 62; B: protonated ether, m / i 105; C: t n l z 105, irradiated concurrently with resonant ejection of t n / z 123. Baselinc as indicated.

reverse order to that anticipated from the single photon absorption spectra shown in Fig. 6. The variation in the extent of photodissociation appears to be real in that photolysis of proton bound mixed alcohol dimers produces intermediate yields. This work will be reported in more detail when completed.

Zero change in ion abundance for m/z 118 in perdeuteroisopropanol was observed initially, and thus it appeared that [5] was not an available reaction channel in this system in contrast with isopropanol and 2-dl-isopropanol. However, irradiation of m l z 1 18 in the absence of m l z 138 brought about a decrease in ion abundance as shown in Table I . This behaviour is ex- plained in the text following.

In all of the above experiments, it has been assumed that only photon absorption by the proton bound dimer is of importance. Photon absorption and subsequent photoexcitation of neutral molecules can be ignored as the residence time of neutrals in the laser beam is extremely short (- 10 ps). Photo- excitation of protonated alcohol, m l z 61, can be ignored when tn lz 61 is resonance ejected rapidly. Photoexcitation of the remaining species of importance, rnlz 79 and tn lz 103, was explored, together with the corresponding ions derived from the deuterated analogues.

In Fig. 7 are shown single ion monitoring traces of ions derived from 2-dl-isopropanol. The two traces marked A are

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HUGHES ET AL.: 2

C.

rn/z 6 2

FIG. 8 . Single ion monitored abundances of further ions derived from 2-dl-isopropanol, at 5 .3 mPa. A: t n / z 80; B : m / z 80 with QRE of m/z 62; C: m/z 62; D: m/z 62 with concurrent QRE of m / z 123. Conditions as for Fig. 7.

for the proton bound alcohol dimer, m/z 123. It should be noted that the dimer contains two deuterium atoms, one from each alcohol molecule, and is proton, not deuterium, bound. In each case, the initial part of the trace corresponds to thermal conditions for 100 ms of storage while the parts marked L refer to 50 ms of storage followed by 50 ms of laser irradiation. In the first of the two traces marked A, some 55% dissociation of dimer, m/z 123, is observed; with resonance ejection of protonated alcohol, m/z 62, as shown in the second A trace, reactions [ l ] and [2] have been arrested and some 60% of dimer photodissociated. Trace B shows the anticipated increase in the protonated ether,. m/z 105, with laser irradiation. Isotopic scrambling is not observed in the chemically activated proton bound dimer and reaction [5] results in the loss of H,O rather than D20 or HDO from m/z 123. As m/z 123 is the precursor of m/z 105, irradiation of the ion assembly in the absence of t t ~ l z 123 (wrought by resonance ejection) should not induce any change in the prevailing m/z 105 ion abundance. However, as can be seen from trace C obtained under the above conditions, a diminution in tn/z 105 ion abundance is observed upon laser irradiation. Thus the increase in m/z 105 ion abundance in trace B is the net increase arising from the photoproduction [5] and the photodissociation process

[8] ((CH,),CD),OH' + nl"hv products m/z 105

Identification of the products of [8] was not possible, but this

process affords an excellent example of the application of ion storage mode switching in the QUlSTOR as discussed in the accompanying paper. Multiphoton dissociation of protonated diisopropyl ether has not been reported previously. However, the observation is not entirely unexpected as Beauchamp and co-workers (2b) have observed the ir laser photochemistry of protonated diethyl ether. They reported that the laser-induced process is

rather than loss of alkane to form protonated acetaldehyde. Thus while multiphoton dissociation of m/z 105 may be anticipated, and the reaction analogous to [9] for protonated diisopropyl ether may be operative, the structure of m / z 105 remains to be verified. The m/z 105 ions observed here were formed from the alcohol dimer, and we have not examined the isomer formed by protonation of diisopropyl ether.

In Fig. 8 are shown results of examination of m / z 80 (note that the proton bound dimer apparently sheds C3H5D as expected) and the protonated alcohol, m/z 62. The parts of the traces marked L refer to 50 ms of storage followed by 50 ms of laser irradiation; the unmarked sections refer to 100 ms of thermal storage. The m/z 80 ion abundance, trace A, shows the expected increase in intensity with laser irradiation [7] and, similarly, an expected increase when m/z 62 is resonantly ejected. In trace C, where m/z 62 is monitored, the expected

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842 CAN J CHEM VOL 61. 1983

( Q R E MIZ 138)

FIG. 9. Singlc ion monitored abundanccs of selected ions dcrived from pcrdcutcroisopsopanol. A: r i l / : 138, thc proton (D') bound per- dcutcrated alcohol dirner: B: r n / : 118; C: t i ? / : 118 with concurrent QRE of rn/: 138: D: r r l / z 90: E: r i l / : 70. Conditions as for Fis. 7 .

increase with laser irradiation is observed. Although the signal is very noisy in trace D in which rn/z 62 is monitored, it appears that upon resonance ejection of rn/z 123. thus re- moving the possibility of [6] occurring, laser irradiation in- duces a smali signal enhancement which may be indicative that m/: 62 is a photoproduct of [8].

Of the four isopropanol analogues investigated, the proton (D' ) bound alcohol dimer most susceptible to multiphoton dissociation is that derived from perdeuteroisopropano1. rn/z 138, as is shown in Table I and in trace A of Fig. 9 where approximately 70% of the dimer is dissociated. This evidence indicates that the majority of ions were irradiated by the narrow laser beam directed through the centre of the QUISTOR. The ion of m/z 118 (correspondng to m / z 103 protonated ether in the isopropanol system) was monitored as shown in traces B and C. In trace B, 50 ms of laser irradiation following 50 ms of thermal storage produced no obvious change in ion abundance from that observed after 100 nis of storage; lower, in trace C where the dimer, m/z 138, is ejected, laser irradiation of 50 ms duration following 50 ms of storage caused a reduction of signal intensity of approximately 42%. Thus, we con- clude that the null change in trace B is due to almost exact compensation by [5] for the relatively high degree of photodissociation wrought by [8]. Traces D and E show the expected increases in ion abundance with laser radiation of m/z 90 (corresponding to m/z 79) and m / z 70 (corresponding to m/z 61), respectively.

Structure of m/z 79; (CHJ2CHOH - H+ . OHZ The losses of the elements of C,H, and H 2 0 by [7] and [5],

respectively. from the proton bound alcohol dimer suggest that the ion formed by loss of C3H6, m/z 79, may have the structure of a proton bound niixed dinier of alcohol and water. In this event, rn/r 79 should absorb ir photons from the laser via the alcohol moiety and, eventually.-be photodissociated to regen- erate protonated alcohol. Such a postulate may appear to be supported by the possibility of a reaction sequence of [5] and [9]. However, at 5.3 mPa pressure, as shown in trace A of Fig. 10, no change in ion abundance of m/z 79 was observed when irradiated in the absence of m/z 121 which was resonance ejected. Thus it is concluded that either the m/z 79 ion species is transparent to radiation at 944 cnl-', which is unlikely as this would require a major change in hybridization, or the binding is much stronger than that of a proton bound alcohol-water dinier which is reported as 100 kJ mol-' for CH~OH,'--H?O (10) and therefore of a different nature. The proton (Dt) bound mixed dimer of isopropan-d-01 with D?O, m/z 83, was formed at a relatively ~ow-~ressu re of 0.67 mPa. Due to the consid- erably greater proton affinity of isopropan-cl-01 (assumed to be equal to that of isopropanol) than that of D20, 8 16 kJ mol-' cf. 690 kJ mol-' ( 1 I), only a trace of the alcohol was added to D?O, hence the partial pressure of the alcohol in the reaction chamber is not known. After 100 ms storage at 0.67 mPa, negligible proton (Dt) bound alcohol dimer was observed. While the signal intensity of m/z 83 was low and is shown in trace B of Fig. 10, the signal does appear to diminish with 50 ms of laser irradiation following 50 ms of storage compared to the signal intensity after 100 ms of storage. With the pressure restored to 5.3 mPa, the signal intensity of m/z 82 shows, in trace C of Fig. 10, a marked increase upon irradiation and

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B.

FIG. 10. Single ion monitored abundances of ions of the fornm i-C,H70H-H* .OH,. A: 111/z 79 ions derived from isopropanol by [7] at 5.3 mPa with QRE of protonated dimer r ~ / z 121: B: I ? I / ~ 82 ions produced by clustering D,O with ~ S O - C ~ H ~ O D : at 0.67 mPa (N.B. only a minute trace of protonated dimer. I I I / : 124. was observed at this pressure); C: 1 1 1 / : 82 ion derived friom iso-C3H70D by [7] at 5.3 mPa. lrradiation conditions as for Fig. 7.

photodissociation of the proton ( D t ) bound alcohol dimer (i-C3H70D)?DL of rn/: 124 which loses C3H6 by [7] to yield t n / z 82. Thus it is concluded. principally from the results portrayed in traces A and B of Fig. 10, that the product ion of [7] differs in structure from that of a proton bound mixed dimer of alcohol and water. It is important to note that in the clustering of D,O with i -C3H70Di . a mixed dimer of I ? ? / : 83 results, whereas alkene loss from the syn~metrical dimer, (i-C3H70D)?DL, results in an ion of t r ~ / z 82.

Let us consider a plausible structure for the proton bound isopropanol dimer t n / z 12 1 and the possible mechanisms by which isomeric forms of m / z 79 may be produced. The m / z 12 1 ion may lose C3H6 in a concerted action involving 6 atoms. to yield 1. Two different forms of rearrangement are then possible, the first rearrangement to yield 11 and 1V produces an asymmetric ion of the form R' . H 2 0 . H , 0 , whereas the alter- native rearrangement produces the symmetric ion, 111, H 2 0 . R' . OHz. While structure 111 has been proposed previously ( 2 c ) , a mechanism for its formation was not discussed.

lot? r-elaratiotz H

Proton bound alcohol dimers formed initially by [ I ] in a CH3 \o I /H /

cy3 chemically activated state, and observed at the expiry of a HCf- - - 0 - - - - H H20- - - -H~?- --OH

/ 2

chosen trapping period, may be either radiatively stabilized under collision-free conditions or radiatively and collisionally

\H CH3 CH3

stabilized at higher pressures. At an operating pressure of I1 I11

5.3 mPa the time between ion-neutral collisions is approximately 0.38 ms, which is appreciable with respect to the normal storage and irradiation periods employed here. A crude cy3

I H--

experiment within the limitations of the equipment was at- HCi - - o<- - -H-L- / \--.H+o

tempted in which proton bound alcohol dimers were irradiated CH3

I+-*- as a function of ion age, age being crudely synonymous with storage time, relaxation time, and average number of collisions IV

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844 CAN. 1. CHEM.

suffered by the ion. While primary ions are formed virtually instantaneously in 50 p s , the peak intensity of proton bound alcohol dimers is attained after 40-50 nis of storage leading to a distribution of ions at any given storage time. Nevertheless the distributions of ion ages will broaden with storage time from a narrow distribution initially for pure nascent ions. The distribution functions may be calculated from the ion abundance profiles with time, but this calculation has not been completed. Ideally, an irradiation period of several milli- seconds should be used to probe the effect of ion relaxation on dissociation pathways but the chopping facility and the tluence of the C W CO? laser dictated a minimum irradiation period of 50 ms. Thus, proton bound alcohol dimers were irradiated for a constant period of 50 ms; irradiation was commenced initially after 5 ms of storage and subsequently delayed to longer storage times. Ions were mass analyzed at the conclusion of irradiation. From the results of each mass analysis, the reduction in signal intensity of the dimer ion and t h e changes in signal intensity of m/z 103, m / z 61 , and m / z 79 due to [5]-[7], respectively, were noted. Photoinduced signal intensity changes obtained at a common amplification of detector signal indicated the degree to which each dissociative pathway was followed. In arriving at the m / z 103 signal intensity change, allowance was made for photodissociation of m / z 103 by determining the m / z signal intensity change with resonance ejection of the dimer ion. Photodissociation yields, expressed as a per- centage of the total yield of photodissociation products, are plotted versus relaxation time and sampling time as shown in Fig. 11. The experiment is crude for the reasons expressed above and the scatter of the data points is appreciable, yet the observed behaviour may indicate a reconciliation of experimental findings reported here with those of Beauchamp ( 2 c ) which are shown as filled data points in Fig. 1 1. Let us assume that, on average, (ROH)?Hi requires an induction period of 35 ms of laser irradiation under the prevailing conditions of pressure and laser power in order to acquire the energy neces- sary, but not necessarily sufficient, for dissociation. As the time between successive collisions is 0 .38 ms, the average number of collisions suffered by ions not detected (and pre- sumed to have dissociated) at the shortest sampling time will range from 100- 145 collisions, and from 210-250 collisions at the maximum sampling time. Thus the experiment encom- passes a possible range of 100-250 collisions on average and a twofold range of radiative probabilities for nascent ions activated initially with some 125 kJ mol-'. All those dissociative pathways [5]-[7] were observed.

At the shortest sampling (or relaxation) time the photodissociation yields of m / z 103 and m / z 61 were almost equal at approximately 50%. As sampling time was delayed, the photodissociation yield of m / z 103 increased at the expense of that for m / z 61; the yield of m / z 79 remained sensibly constant throughout at 4%. It is concluded that photoescalation of partially relaxed [(ROH)?Ht]* may induce preferentially the reverse of [I]; the less the nascent ion is relaxed the fewer will be the number of photons required for reversal of [I]. The more relaxed the ion becomes, the greater is the dominance of [5] as a dissociation pathway. The invariance of the photodissociation yield of m / z 79 is suggested to be associated with an isomer- ization process. The dominance of [5] as a dissociative channel in Beauchamp's work is concluded to be due to near complete radiative relaxation during the relatively long (-1 s) storage time; the pressure region employed in ref. 2 c corresponds to a collision frequency of 10 s - ' .

Relaxation Time (ms)

Sampling Time (rns)

FIG. I I . Plot of photodissociation yield versus sampling time and relaxation time for the three multiphoton dissociation products of (i-C3H,0H)ZH'. Relaxation time is defined as the period of ion storage from initiation of ion creation by electron impact to the corn- mencement of laser irradiation. At the conclusion of 50 ms of irradiation the ions were mass analyzed. Sampling time refers to the total storage time and is equal to the relaxation time plus the irradiation time which was held constant at 50 ms. Relaxation time scale is proportional to the number of collisions suffered by each ion prior to irradiation, while sampling time is proportional to the number of collisions during the entire period of ion storage. 50 ms CW COz laser irradiation of 20 W cm-' at 944 cm- '; pressure 5.3 mPa. A. rn/z 103 product of [5]; 0, m/z 61 product of [6]; x , m/z 79 product of [7]. A and 0 which correspond to tn/z 103 and m/z 6 1 , respectively, were observed by Beauchamp at 4 x lo-' mPa with a relaxation time of 500 ms and a sampling time of 950 rns.

There are several possible extensions of the present results which are either being explored in this laboratory or contem- plated for study. Improved absorptivities of gaseous ions can be obtained by monitoring dissociation as a function of wavelength. Collisional and radiative relaxation of chemically activated ions may be probed by temporal variation of laser irradiation, and complemented by probing of resonance activated, i.e. selectively heated, ions. Individual ion species may be isolated in the QUISTOR and subsequently irradiated under total ion storage conditions in order to accumulate ionic photoproducts. The time scale of slow multiphoton absorption allows for radiative losses, hence ir spontaneous emission lifetimes may be studied.

Summary and conclusions The ability to distinguish structural isomers of gaseous ions

on the basis of ir absorbance is of great interest in that it would complement the capabilities of mass spectrometry. However, it is exceedingly difficult to observe ir absorption directly, due to the low densities of gaseous ions which can be assembled, e.g. the maximum number of ions which may be trapped simulta-

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HUGHES ET AL.. Z 845

neously in the QUISTOR is -LO1) ions based on the space charge. As the extent of multiphoton excitation depends on energy fluence rather than laser intensity, slow multiphoton induced dissociation can be effected in target n~olecules or ions irradiated under near collision-free conditions. provided that excitation rates exceed the rates for spontaneous emission. Thus slow multiphoton dissociation offers an indirect method for determining ir absorption spectra of gaseous ions and can be a source of ion structural information which previously was not obtainable. In addition, ir multiphoton excitation presents a novel method for ion fragmentation which can be employed as a further distinguishing criterion. Slow ir multiphoton dissociation of polyatomic gaseous ions constitutes a convincing demonstration of the incoherence of the process.

It is concluded that ( i ) slow multiphoton dissociation may be employed to determine the dissociation pathway of lowest activation energy provided that the photon-absorbing ions are well relaxed. ( i i ) Thermal dissociation pathways may be followed by partially relaxed or unrelaxed ions upon multiphoton absorption. ( i i i ) Dissociation via the reaction channel with the lowest activation energy is always observed but not necessarily exclusively. (ill) Slow multiphoton dissociation may be employed to distinguish between isomers. ( v ) The ion formed by loss of H?O from the proton bound alcohol dimer is itself photodissociative. (1.i) While the ir absorption spectrum of gaseous ions may mimic the wavelength dependance of single photon absorption by neutrals, the order of relative absorbances by neutrals is not necessarily maintained with gaseous ions.

Acknowledgements The authors acknowledge with thanks the financial support

of Trent University, the Natural Sciences and Engineering Research Council of Canada (for operating and equipment grants), Imperial Oil Company Limited, and Queen's Univer- sity for a Graduate Student Assistantship. We acknowledge also the technical assistance of G . Wynn, C . J . S. Stuart, and W . King, and the laboratory assistance of Charlotte Kohn supported by a Canadian Industries Limited Summer Studentship. We are greatly appreciative of the cooperation of Drs.

C . W . Willis and D. Rayner of the National Research Council of Canada for the loan of the C O laser.

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3. J. G. BLACK, E. YABLONOVITCH. N. BLOE~IBERGEN, and S. M U K A ~ I E L . Phys. Rev. Lett. 38. 1131 (1977).

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61, 824 (1983).

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multiphoton dissociation of ions derived from isopropanol and deuterated analogues in a quistor with...

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