E L S E V I E R Journal of Molecular Structure 413-414 (1997) 387 394

Journal of MOLECULAR STRUCTURE

Microwave spectrum of 3-methyl-2H-azirine

Masaaki Sugie a'*, Harutoshi Takeo h, Chi Matsumura a2

~'Natimml hlstitute of Materials and Chemical Research. 7M&uba, Ibaraki 305. Japan bNatiomH Institute,for Advanced Interdis'ciplina O' Research. T,~ukub~t. lbaraki 305. Japan

Received 8 January 1996; revised 14 January 1996; accepted 15 January 1996

Abstract

The microwave spectrum of 3-methyl-2H-azirine produced by pyrolyzing N-chloro-2-methylaziridine was observed. This new, unstable molecule was identified based on a comparison of observed molecular constants and those determined by an ab initio MO calculation. The rotational constants are A = 22338.04(4), B = 6618.622(9), and C = 5464.442(7) MHz. The dipole moments obtained are/,t~ = 1.90(5) and tz b = 1.86(5) D from the analysis of the Stark effect. The potential barrier to internal rotation of the methyl group was determined by the principal axis method (PAM) to be V3 = 1315(10) cal tool ~. ~ 1997 Elsevier Science B.V.

Kevwords: Microwave spectroscopy: Thermal decomposition: 3-Methyl-2H-azirine- Ab initio MO calculation

1. Introduction

In studying the pyrolysis of 2-methylaziridine ([I], Fig. 1) in the gas phase using microwave spectro- scopy, we discovered a new, transient molecule, N- methylvinylamine [Ill [1]. Amatatsu et al. [2] also observed the infrared spectra of this molecule using the same pyrolysis system. Time-dependence studies

of both microwave and infrared spectra led us to con- clude that pyrolysis first yielded N-methylvinylamine as an intermediate that readily changed to its tauto-

met, N-methylethylidenimine [III], through the rearrangement of hydrogen atoms. This observation indicates that the highly strained three-membered ring molecule tends to cleave at a relatively low

temperature, and the resultant unstable amine is read- ily stabilized by hydrogen rearrangement.

From the above investigations, we expected that

pyrolyzing N-chloro-2-methylaziridine [IV] would yield N-methyI-N-chlorovinylamine [V l as a transient molecule, and the present investigation was under- taken. Although N-methyl-N-chlorovinylamine was not found, 3-methyl-2H-azirine [VIII] having a three-membered ring skeleton was identified as a

new, transient molecule. In this paper, we report the microwave spectrum and identification of 3-methyl- 2H-azirine through ab initio MO calculation.

2. Experiments

* Corresponding author. b Dedicaled to Professor Kozo Kuchitsu on the occasion of his

70th birthday. -" Prescm address: Ebara Research Co,, Ltd., 4-2-1 Honfujisawa,

Fujisawa 251. Japan.

The sample of N-chloro-2-methylaziridine used as a precursor was produced in a flow-through system by passing 2-methylaziridine through a U tube con- taining N-chlorosuccinimide. The reaction product,

0022-2860/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0022-2860(97)00159-2

388 M. Sugie et a/./Journal o f Molecular Structure 413-414 (1997) 387-394

H N

lincH3 [I]

H H CH3 \ / /

N ~ C / C~H H3 C// %CH 2 HaC

[II l [III]

CI N / \ [IV] CH3

N C CH 2 / H3C

I [VI]

I #cH N~CH

-HC1 N [VII]

tVlIIl / \

I ~CH3 [x]

N / / \

~CH 3 [IX]

c,\ /. N %CH2 H3C

[V] -HCl

Fig. 1. Possible decomposition mechanism [br 2-methylaziridine and N-chloro-2-methylaziridine.

containing 2-methylaziridine and its chloride, was pyrolyzed in a 4-mm-i.d. quartz tube heated to about 600°C and introduced into a 3-m X-band waveguide cell. Fast flow was required because the lifetime of 3- methyl-2H-azirine is about 1 min in the brass wave- guide cell.

Microwave spectra were obtained with a conven- tional 100-kHz Stark modulation spectrometer con- trolled by an NEC PC9801 personal computer in a frequency range of 8 - 5 0 GHz. The microwave sources were an HP 8672A synthesized signal genera- tor for 8 -18 GHz, a frequency doubler driven by the synthesizer for 18-27 GHz, and an HP 8690B sweep oscillator with two plug-in RF units phase-locked to the synthesizer for 2 7 - 5 0 GHz. The frequency was scanned by the personal computer through a GP-IB interface. The spectrometer's output signal was fed to the computer through an AID converter to obtain the

center frequencies of spectral lines through least- squares fitting around peak positions to a quadratic curve. The uncertainties of the transition frequencies thus obtained appeared to be less than 0.1 MHz. Some transitions exhibited broad spectral features due to unresolved internal rotation splitting and/or nuclear quadrupole interaction. The uncertainties of such tran- sitions were, from the deviations of O - C values, estimated to be a maximum of 0.2 MHz. Spectra were observed with the cell at about - 20°C, with the in-cell sample pressure kept at 1-2 Pa.

3. Spectrum analysis

In survey spectra for the 35-45-GHz frequency region, spectral fines from an unknown species were found in addition to spectra for acrylonitrile, methyl

M. S teie et al./Journal of Molecular Structure 413 414 (1997) 387-394

T a b l e I

O b s e r v e d t r ans i t ion f r e q u e n c i e s o f 3 - m e t h y l - 2 H - a z i r i n e

3 8 9

T r a n s i t i o n Obs . O - C T r a n s i t i o n Obs . ¢) - ("

a - t ype R - b r a n c h 5 _,.~ 5 , ~ 4 2 5 8 7 . 9 2 0.110

I ,~ 0 . 1 2 0 8 2 . 9 9 - 0 .07 6_,.4 6 , ~ 4 1 3 7 2 . 8 8 I).115

2,.2 1~ , 2 4 1 0 4 . 7 6 - 0 .06 7,.~ 71~, 4 0 7 6 5 . 6 5 0.')7

212 I1~ 23012. t14 0 .16 82 8 1 - 4 1 0 0 1 . 8 6 0.')1

" 1 t,~ 2 5 3 2 0 . 0 7 - O. 12 97 ~ 9 t ,, 4 2 2 7 6 . 4 4 t)., II ~ t l - ,

3~ ~ 2 . : ! 3 6 0 0 5 . 1 0 - 0 .04 l 0 : s l()l ,~ 4 4 7 4 6 . 9 0 0., )5

31 ~ 2 I:~ 3 4 4 8 0 . 6 0 0 .18 I I 2., 11 I I{, 4 8 5 3 6 . 0 8 ¢1.'!1

3 , , 2 e, 36248 .81 0.11/) b - t y p e R - b r a n c h

_780 . . . . 0 . ) 0 3 ,1 2~c~ 3 6 4 9 2 . 7 8 0 .13 I i. I 0 , > "~ -~ :;'~

4, 4 3 . ~ 4 7 7 2 9 . 2 8 - 0 . 1 6 21., I .~ 38731 .1 ( t - 0. s

41. 4 3 t~ 4 5 9 0 6 . 9 1 0 .04 3~.~ 2, : 2 1 3 7 8 . 7 6 ().05

4:.~ 3 , , 4 8 2 8 3 . 8 4 - I).02 4oa 3h ~ 3 4 6 2 7 . 7 6 0.¢17

4~.~ 3~1 4 8 8 8 6 . 0 6 - 0 .07 5o~ 4 ~ 4 7 9 5 9 . 3 2 -- 0.116

a - t ype Q - b r a n c h 514 4 -~ 19772 .87 - 0 .07

8 :.~, 827 I I 139.43 0 .02 6 is 5 : , 349115.31 - 0 .05

10_,s 102 , 2 3 0 0 4 . 3 4 - 0 .04 8 _,~ 7 :a 1208()..:;7 (). 14

12_,,, 122.,~ 3 9 5 4 3 . 1 8 (/ .04 9:.~ 8 ~ 2 2 0 8 5 . 0 4 0 .08

13 _,.i, 13 :.1., 4 9 3 4 4 . 9 8 - 0 .03 8_, ~ 7 ~ 2 3 6 3 9 . 0 6 0.( )7

1 "~, 12 :.,o 8 6 3 3 . 0 4 0.01 9 , ~ 8 :~, 3 9 4 5 7 . 4 3 0.01

13~.m 13~lL 12985 .22 0 .03 1 1 2 , 10 ~ 3 9 1 0 2 . 4 8 - 0 . 1 0

14~ ~ 14~ p_ 1 8 6 1 7 . 6 8 - 0.01 I '~_,.~ ~ 1 I ~s 4 5 6 1 0 . 2 2 ().()9

15 ~ I: 15~1: 2 5 5 8 1 . 8 4 - 0 .03 10~? 9 4 I 1131 .54 . /~

16~ I~ 16~ 14 3 3 8 6 1 . 0 8 0.01 1 I ~ 104 , 19820 .12 {).()l

17~.u 1 7 ~ > 4 3 3 8 2 . 1 7 - 0 . 0 5 12~{, 114 , 3 1 5 1 7 . 6 2 0.09

174.~ 174 ~a 9(197.13 - 0 .05 13ai r 1 2 , s 4 2 7 8 1 . 0 2 - 0(18

1 8 4 ~ 184 is 13404 .27 - 0 .06 I I ,, 11) 4 ~ 2 5 4 2 3 . 9 6 (}()7

20 a.~,, 204 ~7 2 6 0 5 9 . 7 8 - 0.01 12 ~ I I 4,', 4 0 4 9 8 . 5 4 0 04

' 1 21 3 4 5 4 4 . 1 4 - 0 .08 1 3 4 ~ 12~: 1 3 1 5 4 . 2 6 ¢1111

224.t~ 224 ~, 4 4 4 3 5 . 3 3 0 .00 144.~ 3 , ~ 2 5 7 5 7 . 2 6 (/ I I

I. . _ ()() 225.~r ~-5"~'~ ,s 8 8 7 1 . 6 2 - 0 . 0 5 154~,. _ 14~ ,, 38:;1_ ~,~

2 3 s , , 23~ > 12944 .65 - 0.01 1441 13~,, 2 8 0 3 9 . 4 6 I) I

245 ~,~ 24~2 , 1 8 3 1 3 . 4 9 0 .05 15411 14~ ,. 4 2 1 4 9 . 0 2 009

265:~ 26s22 3 3 5 0 4 . 4 9 I).(15 18s~4 17,, ~ 4 3 6 9 8 . 3 8 - 0.(1

27 ~ , : 27 ~2 ~ 4 3 4 2 9 . 0 9 0 .03 16 ~ ~ 15 ,~ ,,. 18388 .92 (I 12

b - t y p e Q - b r a n c h 1 8 s ~ 17,. ~ 4 5 1 5 9 . 5 6 O. 12

2 ~, 2 o , 1 8 0 8 8 . 8 2 0 .02 19t,.i 4 187.~ 2 2 4 6 8 . 8 6 O. I

3 i ' 3 ,~ 2 0 0 2 4 . 5 0 0 .04 2 0 , ~ 1% I 3 5 4 1 4 . 8 7 - 0.1)

4 ~.~ 4 o 4 2 2 8 0 6 . 1 6 0 .02 19< ~ 18 v ~ 22629 .4 t , ~ 0.()9

5 ~ 5 ~ 2 6 5 7 7 . 3 8 0 .08 2(1¢,.~4 1 9 ~ 3 5 7 0 8 . 7 2 0 .02

6 ~ 6 ¢, 3 1 4 6 7 . 0 6 0 .07 21 v~5 2 0 ~ 14076 .08 - 0.(I

7 k. 7 v 3 7 5 4 9 . 6 4 0.01 2 1 7 ~4 2(1~ 14104 .10 0.(/7

8 ~. 8 ~.s 4 4 8 1 1 . 6 3 - 0 .05 22 v ~ 21 .s.~, 2 6 9 9 9 . 0 0 0. I

2 , , 2~.1 4 7 2 1 7 o , 0 .07 23716 ~'~ 40014 .1 "~ - O I

3~, 3 ~: 4 5 7 6 9 . 0 6 0 .05 2 8 , ,, " q 4 4 3 8 0 . 7 2 - l).0

4 2 ' 4 ~.~ 44144 .06 0.05 26 s ~ 25 ~ ~- 44413 .14 (I. 16

c y a n i d e , m e t h y l e n i m i n e , h y d r o g e n c y a n i d e , a n d

a m m o n i a . A s e r i e s o f s t r o n g d o u b l e t s , w h o s e

o r i g i n a p p e a r e d t o b e a t r a n s i e n t m o l e c u l e , w a s

t h e m o s t t y p i c a l p a t t e r n . T h e s e w e r e a s s i g n e d t o b -

t y p e Q - b r a n c h t r a n s i t i o n s (J2.a 2 * " J i . J - l ) . T h e a - t y p e

t r a n s i t i o n s a t 3 4 - 3 8 G H z w e r e a l s o r e a d i l y i d e n t i f i e d

b y t h e i r S t a r k p a t t e r n s a s J = 3 *-- 2 t r a n s i t i o n s . A f t e r

a - t y p e R - b r a n c h a n d b - t y p e Q - b r a n c h t r a n s i t i o n s w e r e

i d e n t i f i e d , o t h e r t r a n s i t i o n s w e r e r e a d i l y f o u n d

a n d a s s i g n e d . O b s e r v e d a n d c a l c u l a t e d t r a n s i t i o n

390 M. Sugie et al./Journal of Molecular Structure 413-414 (1997) 387-394

Table 2 Observed and calculated molecular constants

Obs. Calc."

A 22338.04(4) 22096 MHz B 6618.622(9) 6622 C 5464.442(7) 5453 Aj 0.00191 (6) 0.00172 AjK 0.0076(5) 0.0083 A K 0.1506(5) 0.0686 8j 0.000443(6) 0.000397 8 x 0.0031 (4) 0.0031 HK 0.000167(3) A - 6.4963(2) - 6.51 u,~ 2 /z,, 1.90(5) 2.11 D #b 1.86(5) 2.20

~Calculated from the optimized geometry and force field deter- mined using the MP2/6-31G** method.

frequencies belonging to the A-species are given in Table 1. Rotational and centrifugal distortion con- stants determined by least-squares fitting are given in Table 2. Because observed transition frequencies with large K_I values could not be reproduced by using only p4-type distortion constants, the sextic dis- tortion constant H~ was included in the analysis.

The Stark effect was measured using voltages from 200to 1200V cm i at 10.~-00.0, 20,2-10,1, and 2 j.j-20.2 transitions. The in-cell electric field strength was cali- brated using the Stark effect of the OCS 2 *- 1 transi- tion and its dipole moment of 0.71521 D [3]. Dipole moments calculated by the least-squares method assuming #~ = 0 are given in Table 2.

If the ring cleavage is the main route of N-chloro-2- methylaziridine pyrolysis as in the case of 2-methyl- aziridine, N-methyl-N-chlorovinylamine would be produced as an intermediate. The spectrum of the unstable species observed in the present investigation, however, showed no hyperfine structures ascribable to a chlorine nucleus. This eliminated the possibility that the ring cleavage reaction was the main route of the N-chloro-2-methylaziridine pyrolysis reaction. This suggests that dehydrochlorination occurred in the reaction. If dehydrochlorination occurs after ring cleavage, N-methylketenimine [VII or azabutadiene [VII1 would be produced. Rotational constants obtained for the molecule found in the present inves- tigation could not be fitted to values calculated using the assumed geometries of the above molecules. Other reaction paths involving dehydrochlorination without

ring cleavage were therefore considered. The products of these reaction paths are 3-methyl-2H-azirine [VIII] and 2-methyl-2H-azirine [IX]. If additional hydrogen transfer is considered, 2-methyl-1H-azirine [X] is also a possible reaction product. To calculate the geome- trical structures and dipole moments of these candi- dates, the ab initio MO calculation described in the following section was conducted.

4. Ab initio MO calculation for candidate molecules

The GAUSSIAN 94 computer program [4] was applied and calculations conducted on an IBM RS/ 6000 workstation. The fully optimized geometry was calculated using the MP2 method on the 6-31G** basis set and vibrational analysis was conducted on the opti- mized geometry. Optimized geometry for three candi- dates are shown in Table 3. Rotational constants, dipole moments, and quadrupole coupling constants calcu- lated from the optimized geometries are given in Table 4, which also gives the total energy and the barrier to internal rotation estimated by the MP2/6- 3 IG** method. In this calculation, local C3,. symmetry of the methyl group was assumed: the three CH dis- tances and three CCH angles were kept equal. V3 was determined as the difference of total energies between the optimized geometry and transition state for inter- nal rotation.

A comparison of rotational constants and dipole moments between observed and assumed molecules showed that the most probable candidate for the observed spectrum is 3-methyl-2H-azirine. Centrifu- gal distortion constants were calculated from the force field for this molecule and compared with observed values to obtain further confirmation. Calculated values agree well with corresponding observed values except for AK (Table 2). The disagreement of AK and the requirement of HK suggest the existence of some intramolecular interaction, but we could not clarify it further. We concluded from the above comparisons that the new molecule is 3-methyl-2H-azirine. This is discussed further in the sections that follow.

From the calculated quadrupole coupling constants of 3-methyl-2H-azirine, hyperfine splittings are expected to be smaller than 0.2 MHz for most of the observed transitions and we could not resolve the

M. Sugie et al./Jounml o/'Molecular Structure 413 414 (19~7) 387 .t94 391

Table 3 Optimized geometry for three methyl azirines (in A, and degree)

N(1)

H(5)

Bond length Bond angle

H(7)

ciJ I 1~1(9)

H(8)

Dihedral angle

C(2)N(1) C(3)C(2) C(4)C(3) H(5)C(2) H(6)C(2) H(7)C(4) H(8)C(4) H(9)C(4)

1.564

1.446

1.477

1.083

1.083

1.088

1.089

1.089

C(3)C(2)N(1) 49.9

C(4)C(3)C(2) 149.3

H(5)C(2)N(1) 115.7

H(6)C(2)N(I) 115.7

H(7)C(4)C(3) 110.2

H(8)C(4)C(3) 110.0

H(9)C(4)C(3) 110.0

C(4)C(3)C(2)N(1 ) 180.0

H(5)C(2)N(I)C(3) 109.9

H(6)C(2)N(I)C(3) -109.9

H(7)C(4)C(3)N(1 ) 0.0

H(8)C(4)C(3)N(I) 120.8

H(9)C(4)C(3)N(I) -120.8

Bond length

H(9) /H(8)

N(1) C(4)

/ \ H(5},,,., C(2) C(3) H(7) \

H(6)

Bond angle Dihedral angle

C(2)N(1) C(3)C(2) C(4)C(3) H(5)C(2) H(6)C(3) H(7)C(4) H(8)C(4) H(9)C(4)

1.276

1.445

1.504

1.079

1.086

1.091

1.090

1.089

C(3)C(2)N(1 ) 69.8 C(4)C(3)C(2) 122.9 H(5)C(2)N(1) 138.7 H(6)C(3)C(2) 118.6 H(7)C(4)C(3) 110.9 H(8)C(4)C(3) 110.8 H(9)C(4)C(3) 110.3

C(4)C(3)C(2)N(1) 100.1

H(5)C(2)N(I)C(3) 178.8

H(6)C(3)C(2)N(1 ) -98.2

H(7)C(4)C(3)N(1) 157.8

H(8)C(4)C(3)N(I) -82.2 H(9)C(4)C(3)N(I) 37.7

392

Tab le 3 C o n t i n u e d

Bond length

M. Sugie et al./Journal of Molecular Structure 413-414 (1997) 387-394

H(5)

N(1)

H(61""I - - C ( 4 ) - - H ( 8 )

/ H(7)

Bond angle Dihedral angle

C(2)N(1) 1.530 C(3)C(2) 1.288 C(4)C(3) 1.468 H(5)N(1) 1.028 H(6)C(2) 1.072 H(7)C(4) 1.088 H(8)C(4) 1.090 H(9)C(4) 1.090

C(3)C(2)N(1 ) 64.5 C(4)C(3)C(2) 158.0 C(4)C(3)C(2)N(1) -167.5 H(5)N( 1 )C(2) 1 0 6 . 3 H(5)N( 1 )C(2)C(3) -98.2 H(6)C(2)C(3) 1 5 6 . 3 H(6)C(2)C(3)N(1) 162.6 H(7)C(4)C(3) 1 1 0 . 8 H(7)C(4)C(3)C(2) -6.3 H(8)C(4)C(3) 1 0 9 . 3 H(8)C(4)C(3)C(2) 114.2 H(9)C(4)C(3) 1 1 0 . 8 H(9)C(4)C(3)C(2) -127.3

Table 4

M o l e c u l a r cons tan t s ca lcu la ted for three me thy l az i r ines

H N N N

/ \_ //\

E -171.4775 -171.4692 -171.4145 au

A 22096 22158 21785 MHz

B 6622 6615 6554

C 5453 6131 5296

A -6.51 -16.78 -4.88 uA 2

~a 2.11 0.78 1.54 D

gb 2.20 2.42 1.46

gc 0.00 0.17 1.33

~aa 0.41 0,57 -0.76 MHz

Zbb 0.24 -0,65 0.83

Xcc -0.65 0.08 -0.07

V3 1194 1416 1640 cal/mol

Tab l e 5

Spl i t t ing o f t rans i t ions due to internal ro ta t ion (in M H z )

Trans i t ion A v ( A - E)ob, Av(A - E),~.~I,:

123.~ 12~ro - 16.67 - 16.65

133.m-13~]1 - 19.50 - 19.66

1 4 ~ . 1 r - 1 4 ~ 12 - 2 3 . 6 8 - 2 3 . 8 4

204.]~,-204.t7 - 42 .04 - 41 .79

21.2-111 - 1.71 - 1.92

2 I . I - 11,0 1.70 1.96

2 ~.l - 2o.2 13.90 13.88

31,_ 3o.3 14.19 14.20

41 ~-41/.~ 14.03 14.01

5 e~-51,4 35 .70 35 .80

62,4-61. 5 36.68 36.78

7 2 ~ - 7 I.~ 35.39 35 .50

82~-81.7 32.75 35.89

92.7-9t s 29.28 29.35

102.s- 10 i.,~ 25.12 25.10

1 M-Oo.o 17.70 17.78

2 1 2 - I < I 15.78 17.86

30.3-21,2 - 15.56 - 15.54

82.7-73.4 -- 15.25 -- 14.88

112.1o- 101j - 49 .56 - 49 .34

M. Sugie et al.Zhmrnal qf MolecuhH Structure 413 414 (1997) 3b¢7 -394 393

Table 6 Molecular parameters representing internal rotation"

V~(obs.) 1315(10) cal moli V~(calc.) 1194 /,~ 3.20 uA: X~, 0.9963 Xh 0.0859 X~ 0.0

"1,,. ?x.. Xh. and ~ were tixed at an assumed value.

Table 7 The barrier to internal rotation (in cal tool ~)

V3(obs ) V3(calc

CHaCH2CH3

CHaCH=CH2

b 3325 (20) 3475

c 1996.7 (14) 1966

hyperfine splittings as mentioned in the preceding section. 2860 (50) d 2895

5. Internal rotation

Most of the observed transitions exhibited doublets due to the internal rotation of the methyl group (Table 5). The potential barrier was obtained using a computer program written by Naito et al. 15] based on PAM. To analyze splitting, geometrical parameters were fixed at calculated values and only V3 was deter- mined as a variable parameter. The V 3 value predicted by ab initio MO calculation is consistent with that observed (Table 6).

6. Discussion

/k ~ C H 3

H N

/ \ ~ C H 3

N

/ \ ~ C H 3

1381(2) e 1229

2608 (26) f 2704

1315(10) 1194

The above conclusion that the new molecule is 3- methyl-2H-azirine is further supported by the follow- ing considerations:

( 1 ) The value of A( = It - lb -- I,), -- 6.4963(2) uA -~, implies that this molecule has an almost planar frame and that only hydrogen atoms of two methyl groups or methyl and methylene groups are located out of the plane, since one methyl or methylene group decreases A by about 3.2 uA ".

(2) The potential barrier of internal rotation is rela- tively low, given that splitting due to internal rotation was observed in the ground vibrational state. This implies that the methyl group is adjacent to the double bond. Generally speaking, the barrier height of inter- nal rotation decreases if the methyl group is attached to an sp: carbon atom. Observed and calculated V~ values fl)r related molecules are given in Table 7. Calculation was done using the MP2/6-31G** method in the present study as described in the preceding

:'Calculated using MP2/6-31G in the presenl stud\. b[6l.~17l.Jl~].'191. I lOl.

section. The V~ terms of propene and I-methylcyclo- propene are smaller than those of propane and methyl- cyclopropane by 1330 and 1480 cal m o l i respectively. The present observed value is 1290 cal tool ~ smaller than 2-methylaziridine and is nearly equal to that of 1-methylcyclopropene. The estimated value based on MP2/6-31G** is also consistent with that observed.

The elimination of hydrogen chloride from the pre- cursor may cause the production of 2-methyl-2H- azirine and 2-methyl-IH-azir ine. However, these molecules are more unstable than 3-methyl-2H- azirine by about 0.0083 and 0.0630 a.u., according to ab initio calculation, and we have not yet flmnd these azirines among reaction products.

394 M. Sugie et al./Journal of Moleeular Structure 413-414 (1997) 387-394

We found another unstable molecule, N-methylke- tenimine ([VI], Fig. 1), under slightly different experimental conditions (sample pressure and tem- perature). Amatatsu et al. [11] observed the infrared spectrum of this molecule, although they did not find 3-methyl-2H-azirine. The analysis of the microwave spectra of N-methylketenimine will be given in a forthcoming report.

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