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Microwave spectra, nuclear quadrupole coupling constants, dipole moments, and rotational isomers of propargylimine
JOllRYAl
OF MOI.ECLILAR
SPECTROSCOPY
111,
83-92 ( 1985)
Microwave Spectra, Nuclear Quadrupole Coupling Constants, Dipole Moments, and Rotational Isomers of Propargylimine MASAAKI
SUGIE. HARUTOSHI
rVa~ro~~ul C/~rrn~~~ul Luhortuor~~
TAKEO. AND CHI MATSUMURA
for Ir~du\rr~~, ~hr&~.
~~vdmha. Iharaki
305.
Jupan
The microwave spectra of the short-lived molecule. propargy!imine (CH=CCH=NH). produced by the pyrolysis of dipropargylamine. have been observed. The spectra are analyzed with the aid of an ah initio MO calculation, and the rotational constants for the two isomers (Z- and E-propargylimines) have been determined as :I = 54640.328 MHz, B = 4862.4191 MHz. and C = 4458.1986 MHz. and A = 63099.320 MHz, B = 4766.6104 MHz, and C = 4425.5144 MHz, respectively. The dipole moments, the quadrupole coupling constants, and the relative populations between the two isomers have also been obtained. cc’198’ Academic PWSS.Inc.
INTRODUCTION
Imines and enamines have attracted much attention in the field of astronomy and biology in recent years. However, because most of them cannot exist as stable molecules, only a small number of them have been detected spectroscopically in the gas phase. The detection of methylenimine (CH2=NH) was first made by Johnson and Lovas (I) using microwave spectroscopic techniques. Since then several unstable imines have been detected in the pyrolysis products of amines by means of microwave spectroscopy. Recently, Hamada et (I/. investigated the same imines by infrared spectroscopy and succeeded in observing the spectra of methylenimine (2). ethylidenimine (CH,CH=NH) (3). and allylimine (CH2=CHCH=NH) (4) by the pyrolysis of dimethylamine ((CH,),NH), diethylamine ((C*H&NH), and diallylamine ((CH2=CHCH&NH), respectively. They found that alkylidenimines were effectively produced by the pyrolysis of corresponding dialkylamines. They extended this method to propargylimine (CH=CCH=NH) (5) and successfully detected it by the pyrolysis of dipropargylamine ((CH=C’CH,),NH) for the first time. The pyrolysis products of dipropargylamine were also investigated by microwave spectroscopy in this laboratory, and the existence of propargylimine was confirmed. Preliminary results of the microwave study have been jointly reported with the results of infrared spectroscopy. In the above study only the Z isomer(truns) was analyzed by both techniques. while the existence of the E isomer(cis) was estimated only from an unassigned infrared band. In the present study the microwave spectrum of E-propargylimine was successfully analyzed and the precise molecular constants have been determined for both isomers. For the analysis of spectra of short-lived species, the precise estimation of their molecular constants is indispensable, because the measured spectra naturally show 83
0022-2852185 $3.00 C‘opynght 8 1985 by Academic Press. Inc. All nghts of reproduction
in any form reserved
84
SUGIE, TAKEO,
AND
MATSUMURA
a complicated mixture of many lines due to the precursor and decomposed products. For this purpose an ab initio MO calculation is a powerful tool. Many ab initio MO calculations have been carried out with sufficiently large basis sets to estimate the geometries of small molecules, and it has been shown that such treatments generally lead to accurate structural predictions, although they show small systematic deviations from the experimental data. The theoretical treatment has been increasingly used to investigate structures of molecules for which experimental data are insufficient. In this work an ab initio MO calculation was used not only to give the initial prediction of molecular geometry, but also to estimate other molecular properties such as dipole moments and nuclear quadrupole coupling constants which were important in the analysis of microwave spectra. EXPERIMENTAL
DETAILS
Dipropargylamine was obtained from a commercial source and used without further purification. The purity of the sample was checked by observing its infrared spectrum. The detailed procedure has been described in the previous paper (5). The pyrolysis system used was the same as described in this paper. Although the lifetime of propargylimine in a glass cell used for the infrared spectroscopy was so long that the decrease of the spectral line was not noticed after 20 min, its lifetime in a copper-waveguide cell for the microwave spectroscopy was about 15 min. This necessitated the use of a flow-through method. Propargylimine was also formed by pyrolyzing propargylamine (CHECCH~NH~), although a much stronger spectrum of allylimine was observed in this system. Since the pyrolysis of dipropargylamine gave propargylimine much more efficiently under our experimental conditions, it was used as a precursor throughout the present work. Microwave spectra were obtained with a conventional lOO-kHz Stark modulated spectrometer. Microwave sources were an HP 8672A frequency synthesizer for 826.5 GHz, an HP 8690B sweep oscillator for 26.5-50 GHz, and OKI klystrons (50V 10, 60V 10, and 70VllA) for 50-70 GHz. The sweep oscillator and klystrons were phase-locked to the frequency synthesizer, and the scanning of frequency was made by a computer control system using an HP 9835A desktop computer and an HP-IB interface. The output signal of the spectrometer was also processed by the desktop computer through an A/D converter, and thereby the center frequencies of the spectral lines were automatically obtained. Most spectra were observed at dry ice temperature, and measurements of the relative intensity between the isomers were made at room temperature. Ab inicio MO CALCULATIONS Ab initio MO calculations were performed to estimate the geometrical structures, dipole moments, nuclear quadrupole coupling constants, and energy difference between the two isomers. The program HONDOG was applied, which was originally written by Dupuis and King (6) and modified at the Institute for Molecular Science, Okazaki. Calculations were carried out on the FACOM M380 computer system in Tsukuba Research Center.
MICROWAVE
SPECTRA
OF PROPARGYLIMINE
TABLE Calculated
Molecular
Structures
I
and Rotational
Z-propargylimine
Constants
1.1845
1.i838
c-c
1.4480
1.4444
i
1.2504
1.2506
C-H1
a
1.0558
1.0557
C-H2
a
1.0780
1.0827
1.0060
1.0046
N-H LC-c-c
b
LC-c =N
179.33
176.
125.80
121.42
76’
/C=C-Hl
a,b
179.49
179.06
&-C-H2
a
115.82
114.60
111.97
110.94
A
56884
65775
B
4924
4833
C
4531
4502
LC=N-H
a.
See
Fig.1
for
hydrogen b.
The in C-C
the
numbering
of
MHz
the
atoms.
C-C the
of Propargylimines
E-propargylimine
c-c C=N
85
and anti
bonds,
C-H1
bonds
position
are to
slightly
the
C=N and
respectively.
A 4-31G* basis set was used throughout the present work. The fully optimized geometries for Z- and E-propargylimines are given in Table I, and the location of the atoms in the principal axis system is shown in Fig. 1. The dipole moments in the equilibrium structure were also calculated as follows: pcl = 2.39 D and & = 0.26 D for the Z isomer and pn = 0.23 D and & = 2.13 D for the E isomer. The total energies of Z and E isomers are - 169.534473 and - 169.532820 au, respectively. From the energy difference of 0.001653 au (1.037 kcal/mol), the relative population of the E Isomer to the Z isomer is expected to be about l/6 at room temperature. The nuclear quadrupole coupling constants were also calculated (7). The result will be shown later for comparison with the experimental values.
FIG. I. Z- and E-propargylimines
with their principal
axes and dipole moments.
86
SUGIE, TAKEO. AND MATSUMURA TABLE II Transition Frequencies of Z-Propargylimine” (in MHz) transition
obs.
Ab
O-branch
a-tYPe
31454.56 36673.08 42281.55 48275.30 27465.76 31887.51 36725.30 41983.32 47662.91
0.09 0.02 0.07 -0.03 -0.02 0.01 -0.01 0.04 -0.02
transition
5 5 5 5 5 5 5 5 5
0, 1, 1, 2, 2, 3, 3, 4, 4,
b-type
a-type
R-branch
lo,
2 2 *
0, 1.
1.1-
3 0, 3 1, ;;, 41:44 1, 4 2, 4 2, 4 3, 4 3,
a. b. c.
12 2 -
3 3 243 3 2 2 1-
-
0 0, lo, 11,l
0 1
11, 2 0, 2 1, 21,1 30,3 31 3 1; 3 2, 3 2,1 3 3, 3 3,
0 2 2
: 2 1 0
9320.61 18638.70 18237.50 19045.74 27951.81 27354.56 28566.80 37257.42 36469.75 38085.92 37283.32= 37307.6gc 37294.18= 37294.18=
The transition frequencies A = obs. - talc. The estimated uncertainties hyperfine splittings, while
0.00 -0.02 0.02 0.07 -0.02 -0.03 -0.02 -0.03 -0.02 -0.02 0.04 -0.08 0.07 0.02
are
corrected
of these those of
5 5 4 4 3 3 2 2 1-
-
4 4 4 4 4 4 4 4 4
0, 1, 1, 2, 2, 3, 3,1 4, 4,
Ab
1 0
46553.09 45582.37 47602.36 46600.71 46649.60 46619.54= 46619.54C 46623.74= 46623.74’=
-0.01 -0.02 0.03 0.03 -0.03 0.04 -0.15 0.04 0.04
1 2 3 4
50178.04 50584.97 51199.89 52028.41
0.06 0.03 -0.04 -0.01
29790.46= 40479.04= 51281.22 44467.85 39393.02 34774.92 34322.33= 37998.32C
0.05 0.02 -0.03 -0.06 -0.08 0.02 0.06 -0.06
4 4 3 3 2 2
Q-branch
11, 0 2 1,13 1, 2 4 1, 3 b-type * 9 10 13 14 l5 10 l7
obs.
lo, 2 0, 3 0, 4 0,
R-branch 0, 8 0, 9 0,lO 2,ll 2,12 2.13 2, 9 1,16
for transitions the others
-
7 1, 7 8 1, 8 9 1, 9 14 1,14 15 1,15 16 1,16 11 1,lO 16 2,15
1413 quadrupole are 0.2 are less
splittings. MHz due to the than 0.1 MHz.
unresolved
ANALYSIS OF THE SPECTRUM
From the calculated geometry and dipole components of Z-propargylimine, strong a-type J = 4 - 3 and J = 5 + 4 transitions were expected in the regions 36-38 and 45-48 GHz, respectively. Thus, an initial spectral search was made in the region 30-50 GHz, and a number of a-type transitions with resolved Stark lobes were identified. After the A rotational constant was refined by measuring all the atype R-branch transitions up to J = 5 - 4, some of the a-type Q-branch transitions were carefully searched for and identified. Low-J transitions showed hyperfine structures due to the electric quadrupole moment of the nitrogen atom, and they were used for the determination of the nuclear quadrupole coupling constants. A series of low-J b-type Q-branch transitions were expected above 50 GHz. They were identified on the basis of the Stark effect and partially resolved hyperfme structure. With the rotational constants refined, some of the b-type R-branch transitions were located in the final stage. As described in the previous section the population of E-propargylimine was predicted to be l/6 of Z-propargylimine. In fact, many intense lines remained unidentified after the spectral lines due to Z-propargylimine, acrylonitrile, methylacetylene, cyanoacetylene, and ammonia were eliminated. Furthermore, some of them exhibited partially resolved hyperfme structures with characteristic patterns.
87
MICROWAVE SPECTRA OF PROPARGYLIMINE TABLE 111 Transition Frequencies of E-Propargylimine” (in MHz)
transition
obs.b
transition
R-branch 4 0,430,3 4 1, 4 -- 3 1, 3 4 1, 331,2 50,54 0,4 5 1, 541,4 52,34 2,2 53,34 3,2 53,24 3,l 5 4, 2 -- 4 4, 1 5 4,14 4, 0
36753.06 36084.07 37447.97 45929.97 45102.08 45990.57 45974.74 45974.74 45980.67 45380.67
-0.12 -0.01 0.09 -0.05 -0.02 -0.10 0.01 -0.08 0.07 0.07
b-type Q-branch 11, olo,1 21.12 0, 2 31, 2 - 3 0, 3
58667.84 59010.76 59527.90
0.07 0.07 0.07
b-type R-branch ll,10 0.0 ll,12 0,2 21,230.3 31, 34 0, 4 41,4 - 5 0.5 70,7 - 6 1, 6 *o,a71,7 90,981, 8 10 0,lO - 9 1, 9
67518.92 39944.04 30417.43 20728.72 10882.79 9255.56 13532.56 29936.00 40455.52
0.02 -0.04 -0.06 -0.05 -0.04 0.07 0.08 0.06 0.02
a-type
a.
b. c.
The The due a =
12 13 14 18 19 20 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 24 25 26 27 28 29
2,ll 2.12 2,13 1,17 1,18 1,19 2,15 2,16 2,17 2,18 2,19 2,20 2,21 2,22 2,23 2.24 2,25 2,26 2,27 2,28 2,29 3,22 3.23 3.24 3.24 3,25 3.26
-
13 14 l5 17 18 19 l* 19 2o 21 22 23 24 25 26 27 28 29 30 31 32 25 26 27 28 29 3o
1,12 1,13 1,14 2,16 2.17 2,18 1,18 1,lv 1,20 1,21 1,22 1.23 1,24 1,25 1,26 1.27 1.28 1,29 1,30 1,31 1,32 2,23 2,24 2,25 2,27 2,28 2.29
obs.b
40760.70 29286.44 17664.03 18056.25 30233.25 42537.28 45604.60 41180.32 37167.75 33585.41 30450.88 27780.42 25588.78 23889.00 22692.24 22007.78 21842.71 22202.01 23088.97 24504.59 26448.09 45362.65 33633.70 21682.56 47555.55 40351.48 33379.37
-0.12 -0.09 -0.05 -0.03 -0.10 -0.12 0.04 0.07 0.07 0.05 0.03 0.02 0.00 -0.03 -0.08 -0.06 -0.02 -0.09 -0.03 0.04 0.10 0.12 -0.01 -0.13 0.05 0.00 -0.04
transition frequencies are corrected for 14N quadrupole splittings. uncertainties of the observed frequencies are estimated to be 0.2 MHz to partially resolved hyperfine splittings. obs. - talc.
The spectrum predicted on the basis of the calculated parameters including the nuclear quadrupole coupling constants showed that the b-type R-branch transitions should appear as doublets with the following characteristic patterns; the b-type Rbranch transitions make a few series of doublets due to hyperfine structure, one component being twice as intense as the other. It depends on the transition series which component is stronger. With the aid of this prediction, two series of high-1 b-type R-branch transitions with K_, = 2 - 1 were initially identified. Once the series of R-branch transitions were assigned, the other b-type R- and Q-branch transitions were easily identified, and several a-type R-branch transitions were also found in spite of their very weak intensities. The observed and calculated transition frequencies of Z- and E-propargylimines are listed in Tables II and III, respectively. The rotational and five centrifugal distortion constants determined by a least-squares fit to the following Watson’s Hamiltonian (8) H = U’c, -
dJ’(J +
l)* - d&J
+ l)(Pz)
- dK(Pf)
- dMYW,,J(J+ 1) - d,,U@‘~), are given in Table IV.
88
SUGIE. TAKEO, AND MATSUMURA TABLE IV Observed Rotational and Centrifugal Distortion Constants of Propargylimines” (in MHz and u A*) Z-propargylimine
A
E-propargylimine
54640.228(83)
63099.320(79)
S
4862.4191(59)
4766.6104(72)
C
4458.1986(52)
4425.5144(64)
A
0.17454(19)
0.16259(23)
dJ
-0.016713(82)
-0.014455(30)
dJK
-1.580(33)
dK
-9.50(37)
-1.692(11) -11.35(13)
dWJ
0.000004032(17)
0.000003499(12)
dwK
0.0002740(73)
0.0002999(23)
a.
Numbers in parentheses represent 3 times the standard deviations.
DIPOLE MOMENT
The Stark effect was measured with voltages from 300 to 1800 V/cm on the transitions 10~,-00,0,20,2-10,1, 21,2-l,,,, and 2 1,1-1I,0 for Z-propargylimine and 11,0lo,, , 2,,,-20,2, and 31.2-30,3for E-propargylimine. The electric field strength in the cell was calibrated using the Stark effect of the OCS 2 - 1 transition and its dipole moment of 0.7 152 1 D (9). Since planarity of the molecules has been established by TABLE V Observed and Calculated Dipole Moment of Propargylimines” (in D) Z-propargylimine obs.
E-propargylimine
CalC.
obs.
CalC.
0.23
11.3
2.14(2)
2.39
0.25(l)
!'b
0.2(3)
0.26
1.88(2)
2.13
"c
0.0 b
0.00
0.0 b
0.00
2.40
1.90(2)
2.14
utotal 2.15(5)
a.
Numbers
in parentheses represent 3 times
the standard deviations. b.
The component uc was assumed to be zero in the analysis of the Stark effect.
MICROWAVE SPECTRA OF PROPARGYLIMINE
89
TABLE VI Nuclear Quadrupole Coupling Constants of Propargylimines” (in MHz) Z-propargylimine
Xa.3
E-propargylimine
ca1c.
obs.
-4.1(2)
ohs.
-3.9
ca1c.
0.113)
0.8
'(bb
1.4(4)
0.6
-3.8(2)
-4.1
XC,
2.7(3)
3.3
3.7(4)
3.2
a.
Numbers the
in
parentheses
standard
represent
3 times
deviations.
the inertia defects, the dipole moments were calculated by a least-squares method assuming pC = 0 for both isomers. The dipole moments obtained are listed in Table V along with the predicted values. NUCLEAR QUADRUPOLE
COUPLING CONSTANTS
Well-resolved hyperfine structures due to the electric quadrupole moment of the nitrogen atom were observed in the low-J u-type R-branch transitions of Zpropargylimine. From the hyperfine structures of 10,,-00,0, 2,,,-1 ,J, and 2,~-1 ,,r, the 14N quadrupole coupling constants of Z-propargylimine were determined as listed in Table VI. On the other hand, only partially resolved hyperiine structures were observed for E-propargylimine. Most of the b-type R-branch transitions exhibit doublets. The spacing of the doublet ranges from 1.32 to 1.41 MHz for the transitions (J + l),,J +- J2,J_1, and from 2.09 to 3.08 MHz for the transitions (.I + l)l,J+l (J)2,J_2. The transitions (J + l)O,J+, - (JhJ also exhibit splittings of about 1.3 MHz. The 14N quadrupole coupling constants were determined by a least-squares fit of the splittings of the doublets. The determined values are given in Table VI.
TABLE VII Observed and Calculated Rotational Constants of Propargylimines (in MHz) Z-propargylimine
E-propargylimine
obs.
CalC.
C./O.
obs.
talc.
C./O.
A
54640
56084
1.041
B
4862
4924
1.013
63099
65775
1.042
4767
4833
C
4458
4531
1.016
1.014
4426
4502
1.017
90
SUGIE. TAKEO, AND MATSUMURA TABLE VIII Observed Molecular Constants of Propargylimines and the Related Molecules” (in MHz and D) proparqylimine
ethylidenimine
HCX-CH=NH
z
E
54640.228
A
(83)
B
4862.4191(59)
c
4458.1986
b
allylimine
CH3-CH=NH
63099.320
(52)
z
(79)
E
49960.6
(117)
’
“2C=CH-CM”
z
43759.52(54)
53120.528(80)
4766.6104(72)
9828.21(15)
9778.4950
4425.5144(64)
8650.29(15)
8701.3167(166)
E
(106)
4564.581 4134.423(90)
45773.627(54) (117)
4560.924(18) 4148.249(9)
2.14(2)
0.25(l)
2.408(14)
0.834(23)
2.39(3)
1.1313)
0.2(3)
1.88(2)
0.88116)
1.882(5)
0.77(6)
1.66(3)
UC
0.0
0.0
0.0
0.0
0.0
0.0
KM
-4.1(2)
“a Lib
Xbb XC,
0.1(3)
0.70
-3.51(20)
(45)
1.4(4)
-3.8(21
1.01(61)
-3.90(24)
2.7(3)
3.7(4)
2.50(51)
3.20(37)
a.
Numbers
b.
Ref.
C.
rlefs.
in
parentheses
represent
3 times
the
standard
-3.010(78) 0.041(204) 2.969(81)
0.759(90) -3.866(129) 3.1071183)
deviatians.
(9). (10)
and
(11).
RELATIVE INTENSITIES
Because each spectral line of propargylimine has a different line shape and a different line width due to unresolved hyperfine structure, the peak intensity is not suitable for the determination of relative intensity. Therefore, the relative spectral intensities of the two isomers were determined by measuring the areas of spectral lines with a planimeter. The reliability of this method was estimated to be higher than 20% from the measured relative intensities of four different transitions including a and b types in the E conformer. The disturbances of Stark lobes were minimized by the application of high Stark voltage and the exclusion of transitions whose intensities strongly depended on the Stark voltage. The measured transitions are 30,3-20,2, 3,.3-21.2, 3,,2-21.1, 40,4-30.3, 41.3-3,,2, and 5~-4i,, for Z-propargylimine, and 5~41.4, ~o,Y-~I,~, 90.9-%,8, 100.~0-91.9, 131.~~122~1~ and 141,13- 132,12 for E-propargylimine. The energy difference (EE - E,) was determined to be 0.78 + 0.20 kcal/mol, assuming that the molecule was in equilibrium at room temperature. This value is in fair agreement with the calculated value of 1.037 kcal/mol. DISCUSSION
The present work shows that propargylimine is the main pyrolysis product of dipropargylamine, and the Z isomer is more stable than the E isomer as in ethylidenimine (10) and allylimine (II). It is concluded that the two isomers
MICROWAVE
SPECTRA OF PROPARGYLIMINE
91
have planar structures since they exhibit small positive inertia defects as shown in Table IV. The calculated rotational constants show slight systematic deviation from the experimental values. So far, it has been shown that the bond length calculated with the neglect of electron correlation is smaller than the true value, and the same tendency is seen in the present case. However, the ratios of the calculated rotational constants to the observed ones are similar in both isomers as shown in Table VII. The calculated dipole moments are about 10% larger than the observed values. The calculated quadrupole coupling constants also agree with the observed values within 20%, except for the case that the constants are of small values (xhh of Zpropargyhmine and xnrrof E-propargylimine). It has been clarified that the calculated molecular properties by the use of 4-3 1G* basis set are reliable enough to be used for the analysis of microwave spectrum of propargylimine. Although no isotopic species were observed in the present study, it is evident that two observed species are Z- and E-propargylimine. First. propargylimine is estimated to be the main product of the pyrolysis of dipropargylamine from similar reactions mentioned in the introduction. Second, the observed rotational constants and other molecular properties agree well with the corresponding calculated values, not only with the rotational constants but also with the dipole moments and nuclear quadrupole coupling constants. Third, the molecular constants obtained for this molecule are quite similar to those of ethylidenimine (10) and allylimine (II. Z_?) as shown in Table VIII. As an example, quadrupole coupling constants for these molecules listed in the table resemble each other quite well, indicating that a similar electronic configuration around the N atoms of these molecules exists. Finally, it is suggested that this molecule isomerizes easily to acrylonitrile (CH?=CHCN) at elevated temperatures, considering that the stronger absorption lines of acrylonitrile were observed at a higher pyrolysis temperature. Recently, we made a database of the rotational constants in the computer file, which have so far been obtained by microwave spectroscopy and appeared in Ref. (13). In the early stage of this study the rotational constants determined for the unknown molecule were compared with the stored data and it was confirmed that no rotational constants existing in the database coincide with them. It is interesting to mention that the molecule with the most similar rotational constants as those of the observed species is propynal (propargyl aldehyde), which is isoelectronic with propargylimine. RECEIVED: November 5, 1984 REFERENCES 1. D. R.JOHNSONAND F. J. LOVAS, Chem. Phy.7. Lett. 15, 65-68(1972). 2.Y. HAMADA, K. HASHIGUCHI, M. TSUBOI, Y. KOGA. AND S. KONDO, J. Mol. Spectrosc. 105, 70-80 (1984). 3.K. HASHIGUCHI, Y. HAMADA, M. TSUBOI. Y. KOGA. AND S. KONDO, J. Mol. Spectrosc. 105, 81-92
(1984).
92
SUGIE, TAKEO. AND MATSUMURA
4. Y. HAMADA, M. TSUBOI, T. MATSUZAWA, K. YAMANOUCHI, K. KUCHITSU, Y. KOGA, AND S. KONDO, .I. Mol. Spectrosc. 105, 453-464 ( 1984). 5. Y. HAMADA, M. TSUBOI,H. TAKEO, AND C. MATSUMURA,J. Mol. Spectrosc. 106, 175-185 (1984). 6. M. DUPUISAND H. F. KING, J. Chem. Phys. 68, 3998-4004 (1978). 7. K. YAMANOUCHI, K. KUCHITSU, M. SUGIE, H. TAKEO, C. MATSUMURA, S. KATO, AND K. MOROKUMA, to be published. 8. J. K. G. WATSON, .I. Chem. Phys. 46, 1935-l 949 ( 1967). 9. J. S. MUENTER,J. Chem. Phys. 48,4544-4547 (1968). 10. F. J. LOVAS, R. D. SUENRAM,D. R. JOHNSON, F. 0. CLARK, AND E. TIEMANN, J. Chem. Phys. 72,
4964-4972 (1980). Il. R. E. PENN, J. Mol. Spectrosc. 69, 373-382 (1978). 12. R. D. BROWN, P. D. GODFREY, AND D. A. WINKLER, Chem. Phys. 59, 243-247 (1981). 13. J. DEMAISON, W. HOTTNER, B. STARCK, I. BUCK, R. TISCHER, AND M. WINNEWISSER,“Molecular Constants from Microwave, Molecular Beam, and Electron Spin Resonance Spectroscopy” (K.-H. Hellwege and A. M. Hellwege, Eds.). Springer-Verlag. Berlin, 1974.
OF MOI.ECLILAR
SPECTROSCOPY
111,
83-92 ( 1985)
Microwave Spectra, Nuclear Quadrupole Coupling Constants, Dipole Moments, and Rotational Isomers of Propargylimine MASAAKI
SUGIE. HARUTOSHI
rVa~ro~~ul C/~rrn~~~ul Luhortuor~~
TAKEO. AND CHI MATSUMURA
for Ir~du\rr~~, ~hr&~.
~~vdmha. Iharaki
305.
Jupan
The microwave spectra of the short-lived molecule. propargy!imine (CH=CCH=NH). produced by the pyrolysis of dipropargylamine. have been observed. The spectra are analyzed with the aid of an ah initio MO calculation, and the rotational constants for the two isomers (Z- and E-propargylimines) have been determined as :I = 54640.328 MHz, B = 4862.4191 MHz. and C = 4458.1986 MHz. and A = 63099.320 MHz, B = 4766.6104 MHz, and C = 4425.5144 MHz, respectively. The dipole moments, the quadrupole coupling constants, and the relative populations between the two isomers have also been obtained. cc’198’ Academic PWSS.Inc.
INTRODUCTION
Imines and enamines have attracted much attention in the field of astronomy and biology in recent years. However, because most of them cannot exist as stable molecules, only a small number of them have been detected spectroscopically in the gas phase. The detection of methylenimine (CH2=NH) was first made by Johnson and Lovas (I) using microwave spectroscopic techniques. Since then several unstable imines have been detected in the pyrolysis products of amines by means of microwave spectroscopy. Recently, Hamada et (I/. investigated the same imines by infrared spectroscopy and succeeded in observing the spectra of methylenimine (2). ethylidenimine (CH,CH=NH) (3). and allylimine (CH2=CHCH=NH) (4) by the pyrolysis of dimethylamine ((CH,),NH), diethylamine ((C*H&NH), and diallylamine ((CH2=CHCH&NH), respectively. They found that alkylidenimines were effectively produced by the pyrolysis of corresponding dialkylamines. They extended this method to propargylimine (CH=CCH=NH) (5) and successfully detected it by the pyrolysis of dipropargylamine ((CH=C’CH,),NH) for the first time. The pyrolysis products of dipropargylamine were also investigated by microwave spectroscopy in this laboratory, and the existence of propargylimine was confirmed. Preliminary results of the microwave study have been jointly reported with the results of infrared spectroscopy. In the above study only the Z isomer(truns) was analyzed by both techniques. while the existence of the E isomer(cis) was estimated only from an unassigned infrared band. In the present study the microwave spectrum of E-propargylimine was successfully analyzed and the precise molecular constants have been determined for both isomers. For the analysis of spectra of short-lived species, the precise estimation of their molecular constants is indispensable, because the measured spectra naturally show 83
0022-2852185 $3.00 C‘opynght 8 1985 by Academic Press. Inc. All nghts of reproduction
in any form reserved
84
SUGIE, TAKEO,
AND
MATSUMURA
a complicated mixture of many lines due to the precursor and decomposed products. For this purpose an ab initio MO calculation is a powerful tool. Many ab initio MO calculations have been carried out with sufficiently large basis sets to estimate the geometries of small molecules, and it has been shown that such treatments generally lead to accurate structural predictions, although they show small systematic deviations from the experimental data. The theoretical treatment has been increasingly used to investigate structures of molecules for which experimental data are insufficient. In this work an ab initio MO calculation was used not only to give the initial prediction of molecular geometry, but also to estimate other molecular properties such as dipole moments and nuclear quadrupole coupling constants which were important in the analysis of microwave spectra. EXPERIMENTAL
DETAILS
Dipropargylamine was obtained from a commercial source and used without further purification. The purity of the sample was checked by observing its infrared spectrum. The detailed procedure has been described in the previous paper (5). The pyrolysis system used was the same as described in this paper. Although the lifetime of propargylimine in a glass cell used for the infrared spectroscopy was so long that the decrease of the spectral line was not noticed after 20 min, its lifetime in a copper-waveguide cell for the microwave spectroscopy was about 15 min. This necessitated the use of a flow-through method. Propargylimine was also formed by pyrolyzing propargylamine (CHECCH~NH~), although a much stronger spectrum of allylimine was observed in this system. Since the pyrolysis of dipropargylamine gave propargylimine much more efficiently under our experimental conditions, it was used as a precursor throughout the present work. Microwave spectra were obtained with a conventional lOO-kHz Stark modulated spectrometer. Microwave sources were an HP 8672A frequency synthesizer for 826.5 GHz, an HP 8690B sweep oscillator for 26.5-50 GHz, and OKI klystrons (50V 10, 60V 10, and 70VllA) for 50-70 GHz. The sweep oscillator and klystrons were phase-locked to the frequency synthesizer, and the scanning of frequency was made by a computer control system using an HP 9835A desktop computer and an HP-IB interface. The output signal of the spectrometer was also processed by the desktop computer through an A/D converter, and thereby the center frequencies of the spectral lines were automatically obtained. Most spectra were observed at dry ice temperature, and measurements of the relative intensity between the isomers were made at room temperature. Ab inicio MO CALCULATIONS Ab initio MO calculations were performed to estimate the geometrical structures, dipole moments, nuclear quadrupole coupling constants, and energy difference between the two isomers. The program HONDOG was applied, which was originally written by Dupuis and King (6) and modified at the Institute for Molecular Science, Okazaki. Calculations were carried out on the FACOM M380 computer system in Tsukuba Research Center.
MICROWAVE
SPECTRA
OF PROPARGYLIMINE
TABLE Calculated
Molecular
Structures
I
and Rotational
Z-propargylimine
Constants
1.1845
1.i838
c-c
1.4480
1.4444
i
1.2504
1.2506
C-H1
a
1.0558
1.0557
C-H2
a
1.0780
1.0827
1.0060
1.0046
N-H LC-c-c
b
LC-c =N
179.33
176.
125.80
121.42
76’
/C=C-Hl
a,b
179.49
179.06
&-C-H2
a
115.82
114.60
111.97
110.94
A
56884
65775
B
4924
4833
C
4531
4502
LC=N-H
a.
See
Fig.1
for
hydrogen b.
The in C-C
the
numbering
of
MHz
the
atoms.
C-C the
of Propargylimines
E-propargylimine
c-c C=N
85
and anti
bonds,
C-H1
bonds
position
are to
slightly
the
C=N and
respectively.
A 4-31G* basis set was used throughout the present work. The fully optimized geometries for Z- and E-propargylimines are given in Table I, and the location of the atoms in the principal axis system is shown in Fig. 1. The dipole moments in the equilibrium structure were also calculated as follows: pcl = 2.39 D and & = 0.26 D for the Z isomer and pn = 0.23 D and & = 2.13 D for the E isomer. The total energies of Z and E isomers are - 169.534473 and - 169.532820 au, respectively. From the energy difference of 0.001653 au (1.037 kcal/mol), the relative population of the E Isomer to the Z isomer is expected to be about l/6 at room temperature. The nuclear quadrupole coupling constants were also calculated (7). The result will be shown later for comparison with the experimental values.
FIG. I. Z- and E-propargylimines
with their principal
axes and dipole moments.
86
SUGIE, TAKEO. AND MATSUMURA TABLE II Transition Frequencies of Z-Propargylimine” (in MHz) transition
obs.
Ab
O-branch
a-tYPe
31454.56 36673.08 42281.55 48275.30 27465.76 31887.51 36725.30 41983.32 47662.91
0.09 0.02 0.07 -0.03 -0.02 0.01 -0.01 0.04 -0.02
transition
5 5 5 5 5 5 5 5 5
0, 1, 1, 2, 2, 3, 3, 4, 4,
b-type
a-type
R-branch
lo,
2 2 *
0, 1.
1.1-
3 0, 3 1, ;;, 41:44 1, 4 2, 4 2, 4 3, 4 3,
a. b. c.
12 2 -
3 3 243 3 2 2 1-
-
0 0, lo, 11,l
0 1
11, 2 0, 2 1, 21,1 30,3 31 3 1; 3 2, 3 2,1 3 3, 3 3,
0 2 2
: 2 1 0
9320.61 18638.70 18237.50 19045.74 27951.81 27354.56 28566.80 37257.42 36469.75 38085.92 37283.32= 37307.6gc 37294.18= 37294.18=
The transition frequencies A = obs. - talc. The estimated uncertainties hyperfine splittings, while
0.00 -0.02 0.02 0.07 -0.02 -0.03 -0.02 -0.03 -0.02 -0.02 0.04 -0.08 0.07 0.02
are
corrected
of these those of
5 5 4 4 3 3 2 2 1-
-
4 4 4 4 4 4 4 4 4
0, 1, 1, 2, 2, 3, 3,1 4, 4,
Ab
1 0
46553.09 45582.37 47602.36 46600.71 46649.60 46619.54= 46619.54C 46623.74= 46623.74’=
-0.01 -0.02 0.03 0.03 -0.03 0.04 -0.15 0.04 0.04
1 2 3 4
50178.04 50584.97 51199.89 52028.41
0.06 0.03 -0.04 -0.01
29790.46= 40479.04= 51281.22 44467.85 39393.02 34774.92 34322.33= 37998.32C
0.05 0.02 -0.03 -0.06 -0.08 0.02 0.06 -0.06
4 4 3 3 2 2
Q-branch
11, 0 2 1,13 1, 2 4 1, 3 b-type * 9 10 13 14 l5 10 l7
obs.
lo, 2 0, 3 0, 4 0,
R-branch 0, 8 0, 9 0,lO 2,ll 2,12 2.13 2, 9 1,16
for transitions the others
-
7 1, 7 8 1, 8 9 1, 9 14 1,14 15 1,15 16 1,16 11 1,lO 16 2,15
1413 quadrupole are 0.2 are less
splittings. MHz due to the than 0.1 MHz.
unresolved
ANALYSIS OF THE SPECTRUM
From the calculated geometry and dipole components of Z-propargylimine, strong a-type J = 4 - 3 and J = 5 + 4 transitions were expected in the regions 36-38 and 45-48 GHz, respectively. Thus, an initial spectral search was made in the region 30-50 GHz, and a number of a-type transitions with resolved Stark lobes were identified. After the A rotational constant was refined by measuring all the atype R-branch transitions up to J = 5 - 4, some of the a-type Q-branch transitions were carefully searched for and identified. Low-J transitions showed hyperfine structures due to the electric quadrupole moment of the nitrogen atom, and they were used for the determination of the nuclear quadrupole coupling constants. A series of low-J b-type Q-branch transitions were expected above 50 GHz. They were identified on the basis of the Stark effect and partially resolved hyperfme structure. With the rotational constants refined, some of the b-type R-branch transitions were located in the final stage. As described in the previous section the population of E-propargylimine was predicted to be l/6 of Z-propargylimine. In fact, many intense lines remained unidentified after the spectral lines due to Z-propargylimine, acrylonitrile, methylacetylene, cyanoacetylene, and ammonia were eliminated. Furthermore, some of them exhibited partially resolved hyperfme structures with characteristic patterns.
87
MICROWAVE SPECTRA OF PROPARGYLIMINE TABLE 111 Transition Frequencies of E-Propargylimine” (in MHz)
transition
obs.b
transition
R-branch 4 0,430,3 4 1, 4 -- 3 1, 3 4 1, 331,2 50,54 0,4 5 1, 541,4 52,34 2,2 53,34 3,2 53,24 3,l 5 4, 2 -- 4 4, 1 5 4,14 4, 0
36753.06 36084.07 37447.97 45929.97 45102.08 45990.57 45974.74 45974.74 45980.67 45380.67
-0.12 -0.01 0.09 -0.05 -0.02 -0.10 0.01 -0.08 0.07 0.07
b-type Q-branch 11, olo,1 21.12 0, 2 31, 2 - 3 0, 3
58667.84 59010.76 59527.90
0.07 0.07 0.07
b-type R-branch ll,10 0.0 ll,12 0,2 21,230.3 31, 34 0, 4 41,4 - 5 0.5 70,7 - 6 1, 6 *o,a71,7 90,981, 8 10 0,lO - 9 1, 9
67518.92 39944.04 30417.43 20728.72 10882.79 9255.56 13532.56 29936.00 40455.52
0.02 -0.04 -0.06 -0.05 -0.04 0.07 0.08 0.06 0.02
a-type
a.
b. c.
The The due a =
12 13 14 18 19 20 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 24 25 26 27 28 29
2,ll 2.12 2,13 1,17 1,18 1,19 2,15 2,16 2,17 2,18 2,19 2,20 2,21 2,22 2,23 2.24 2,25 2,26 2,27 2,28 2,29 3,22 3.23 3.24 3.24 3,25 3.26
-
13 14 l5 17 18 19 l* 19 2o 21 22 23 24 25 26 27 28 29 30 31 32 25 26 27 28 29 3o
1,12 1,13 1,14 2,16 2.17 2,18 1,18 1,lv 1,20 1,21 1,22 1.23 1,24 1,25 1,26 1.27 1.28 1,29 1,30 1,31 1,32 2,23 2,24 2,25 2,27 2,28 2.29
obs.b
40760.70 29286.44 17664.03 18056.25 30233.25 42537.28 45604.60 41180.32 37167.75 33585.41 30450.88 27780.42 25588.78 23889.00 22692.24 22007.78 21842.71 22202.01 23088.97 24504.59 26448.09 45362.65 33633.70 21682.56 47555.55 40351.48 33379.37
-0.12 -0.09 -0.05 -0.03 -0.10 -0.12 0.04 0.07 0.07 0.05 0.03 0.02 0.00 -0.03 -0.08 -0.06 -0.02 -0.09 -0.03 0.04 0.10 0.12 -0.01 -0.13 0.05 0.00 -0.04
transition frequencies are corrected for 14N quadrupole splittings. uncertainties of the observed frequencies are estimated to be 0.2 MHz to partially resolved hyperfine splittings. obs. - talc.
The spectrum predicted on the basis of the calculated parameters including the nuclear quadrupole coupling constants showed that the b-type R-branch transitions should appear as doublets with the following characteristic patterns; the b-type Rbranch transitions make a few series of doublets due to hyperfine structure, one component being twice as intense as the other. It depends on the transition series which component is stronger. With the aid of this prediction, two series of high-1 b-type R-branch transitions with K_, = 2 - 1 were initially identified. Once the series of R-branch transitions were assigned, the other b-type R- and Q-branch transitions were easily identified, and several a-type R-branch transitions were also found in spite of their very weak intensities. The observed and calculated transition frequencies of Z- and E-propargylimines are listed in Tables II and III, respectively. The rotational and five centrifugal distortion constants determined by a least-squares fit to the following Watson’s Hamiltonian (8) H = U’c, -
dJ’(J +
l)* - d&J
+ l)(Pz)
- dK(Pf)
- dMYW,,J(J+ 1) - d,,U@‘~), are given in Table IV.
88
SUGIE. TAKEO, AND MATSUMURA TABLE IV Observed Rotational and Centrifugal Distortion Constants of Propargylimines” (in MHz and u A*) Z-propargylimine
A
E-propargylimine
54640.228(83)
63099.320(79)
S
4862.4191(59)
4766.6104(72)
C
4458.1986(52)
4425.5144(64)
A
0.17454(19)
0.16259(23)
dJ
-0.016713(82)
-0.014455(30)
dJK
-1.580(33)
dK
-9.50(37)
-1.692(11) -11.35(13)
dWJ
0.000004032(17)
0.000003499(12)
dwK
0.0002740(73)
0.0002999(23)
a.
Numbers in parentheses represent 3 times the standard deviations.
DIPOLE MOMENT
The Stark effect was measured with voltages from 300 to 1800 V/cm on the transitions 10~,-00,0,20,2-10,1, 21,2-l,,,, and 2 1,1-1I,0 for Z-propargylimine and 11,0lo,, , 2,,,-20,2, and 31.2-30,3for E-propargylimine. The electric field strength in the cell was calibrated using the Stark effect of the OCS 2 - 1 transition and its dipole moment of 0.7 152 1 D (9). Since planarity of the molecules has been established by TABLE V Observed and Calculated Dipole Moment of Propargylimines” (in D) Z-propargylimine obs.
E-propargylimine
CalC.
obs.
CalC.
0.23
11.3
2.14(2)
2.39
0.25(l)
!'b
0.2(3)
0.26
1.88(2)
2.13
"c
0.0 b
0.00
0.0 b
0.00
2.40
1.90(2)
2.14
utotal 2.15(5)
a.
Numbers
in parentheses represent 3 times
the standard deviations. b.
The component uc was assumed to be zero in the analysis of the Stark effect.
MICROWAVE SPECTRA OF PROPARGYLIMINE
89
TABLE VI Nuclear Quadrupole Coupling Constants of Propargylimines” (in MHz) Z-propargylimine
Xa.3
E-propargylimine
ca1c.
obs.
-4.1(2)
ohs.
-3.9
ca1c.
0.113)
0.8
'(bb
1.4(4)
0.6
-3.8(2)
-4.1
XC,
2.7(3)
3.3
3.7(4)
3.2
a.
Numbers the
in
parentheses
standard
represent
3 times
deviations.
the inertia defects, the dipole moments were calculated by a least-squares method assuming pC = 0 for both isomers. The dipole moments obtained are listed in Table V along with the predicted values. NUCLEAR QUADRUPOLE
COUPLING CONSTANTS
Well-resolved hyperfine structures due to the electric quadrupole moment of the nitrogen atom were observed in the low-J u-type R-branch transitions of Zpropargylimine. From the hyperfine structures of 10,,-00,0, 2,,,-1 ,J, and 2,~-1 ,,r, the 14N quadrupole coupling constants of Z-propargylimine were determined as listed in Table VI. On the other hand, only partially resolved hyperiine structures were observed for E-propargylimine. Most of the b-type R-branch transitions exhibit doublets. The spacing of the doublet ranges from 1.32 to 1.41 MHz for the transitions (J + l),,J +- J2,J_1, and from 2.09 to 3.08 MHz for the transitions (.I + l)l,J+l (J)2,J_2. The transitions (J + l)O,J+, - (JhJ also exhibit splittings of about 1.3 MHz. The 14N quadrupole coupling constants were determined by a least-squares fit of the splittings of the doublets. The determined values are given in Table VI.
TABLE VII Observed and Calculated Rotational Constants of Propargylimines (in MHz) Z-propargylimine
E-propargylimine
obs.
CalC.
C./O.
obs.
talc.
C./O.
A
54640
56084
1.041
B
4862
4924
1.013
63099
65775
1.042
4767
4833
C
4458
4531
1.016
1.014
4426
4502
1.017
90
SUGIE. TAKEO, AND MATSUMURA TABLE VIII Observed Molecular Constants of Propargylimines and the Related Molecules” (in MHz and D) proparqylimine
ethylidenimine
HCX-CH=NH
z
E
54640.228
A
(83)
B
4862.4191(59)
c
4458.1986
b
allylimine
CH3-CH=NH
63099.320
(52)
z
(79)
E
49960.6
(117)
’
“2C=CH-CM”
z
43759.52(54)
53120.528(80)
4766.6104(72)
9828.21(15)
9778.4950
4425.5144(64)
8650.29(15)
8701.3167(166)
E
(106)
4564.581 4134.423(90)
45773.627(54) (117)
4560.924(18) 4148.249(9)
2.14(2)
0.25(l)
2.408(14)
0.834(23)
2.39(3)
1.1313)
0.2(3)
1.88(2)
0.88116)
1.882(5)
0.77(6)
1.66(3)
UC
0.0
0.0
0.0
0.0
0.0
0.0
KM
-4.1(2)
“a Lib
Xbb XC,
0.1(3)
0.70
-3.51(20)
(45)
1.4(4)
-3.8(21
1.01(61)
-3.90(24)
2.7(3)
3.7(4)
2.50(51)
3.20(37)
a.
Numbers
b.
Ref.
C.
rlefs.
in
parentheses
represent
3 times
the
standard
-3.010(78) 0.041(204) 2.969(81)
0.759(90) -3.866(129) 3.1071183)
deviatians.
(9). (10)
and
(11).
RELATIVE INTENSITIES
Because each spectral line of propargylimine has a different line shape and a different line width due to unresolved hyperfine structure, the peak intensity is not suitable for the determination of relative intensity. Therefore, the relative spectral intensities of the two isomers were determined by measuring the areas of spectral lines with a planimeter. The reliability of this method was estimated to be higher than 20% from the measured relative intensities of four different transitions including a and b types in the E conformer. The disturbances of Stark lobes were minimized by the application of high Stark voltage and the exclusion of transitions whose intensities strongly depended on the Stark voltage. The measured transitions are 30,3-20,2, 3,.3-21.2, 3,,2-21.1, 40,4-30.3, 41.3-3,,2, and 5~-4i,, for Z-propargylimine, and 5~41.4, ~o,Y-~I,~, 90.9-%,8, 100.~0-91.9, 131.~~122~1~ and 141,13- 132,12 for E-propargylimine. The energy difference (EE - E,) was determined to be 0.78 + 0.20 kcal/mol, assuming that the molecule was in equilibrium at room temperature. This value is in fair agreement with the calculated value of 1.037 kcal/mol. DISCUSSION
The present work shows that propargylimine is the main pyrolysis product of dipropargylamine, and the Z isomer is more stable than the E isomer as in ethylidenimine (10) and allylimine (II). It is concluded that the two isomers
MICROWAVE
SPECTRA OF PROPARGYLIMINE
91
have planar structures since they exhibit small positive inertia defects as shown in Table IV. The calculated rotational constants show slight systematic deviation from the experimental values. So far, it has been shown that the bond length calculated with the neglect of electron correlation is smaller than the true value, and the same tendency is seen in the present case. However, the ratios of the calculated rotational constants to the observed ones are similar in both isomers as shown in Table VII. The calculated dipole moments are about 10% larger than the observed values. The calculated quadrupole coupling constants also agree with the observed values within 20%, except for the case that the constants are of small values (xhh of Zpropargyhmine and xnrrof E-propargylimine). It has been clarified that the calculated molecular properties by the use of 4-3 1G* basis set are reliable enough to be used for the analysis of microwave spectrum of propargylimine. Although no isotopic species were observed in the present study, it is evident that two observed species are Z- and E-propargylimine. First. propargylimine is estimated to be the main product of the pyrolysis of dipropargylamine from similar reactions mentioned in the introduction. Second, the observed rotational constants and other molecular properties agree well with the corresponding calculated values, not only with the rotational constants but also with the dipole moments and nuclear quadrupole coupling constants. Third, the molecular constants obtained for this molecule are quite similar to those of ethylidenimine (10) and allylimine (II. Z_?) as shown in Table VIII. As an example, quadrupole coupling constants for these molecules listed in the table resemble each other quite well, indicating that a similar electronic configuration around the N atoms of these molecules exists. Finally, it is suggested that this molecule isomerizes easily to acrylonitrile (CH?=CHCN) at elevated temperatures, considering that the stronger absorption lines of acrylonitrile were observed at a higher pyrolysis temperature. Recently, we made a database of the rotational constants in the computer file, which have so far been obtained by microwave spectroscopy and appeared in Ref. (13). In the early stage of this study the rotational constants determined for the unknown molecule were compared with the stored data and it was confirmed that no rotational constants existing in the database coincide with them. It is interesting to mention that the molecule with the most similar rotational constants as those of the observed species is propynal (propargyl aldehyde), which is isoelectronic with propargylimine. RECEIVED: November 5, 1984 REFERENCES 1. D. R.JOHNSONAND F. J. LOVAS, Chem. Phy.7. Lett. 15, 65-68(1972). 2.Y. HAMADA, K. HASHIGUCHI, M. TSUBOI, Y. KOGA. AND S. KONDO, J. Mol. Spectrosc. 105, 70-80 (1984). 3.K. HASHIGUCHI, Y. HAMADA, M. TSUBOI. Y. KOGA. AND S. KONDO, J. Mol. Spectrosc. 105, 81-92
(1984).
92
SUGIE, TAKEO. AND MATSUMURA
4. Y. HAMADA, M. TSUBOI, T. MATSUZAWA, K. YAMANOUCHI, K. KUCHITSU, Y. KOGA, AND S. KONDO, .I. Mol. Spectrosc. 105, 453-464 ( 1984). 5. Y. HAMADA, M. TSUBOI,H. TAKEO, AND C. MATSUMURA,J. Mol. Spectrosc. 106, 175-185 (1984). 6. M. DUPUISAND H. F. KING, J. Chem. Phys. 68, 3998-4004 (1978). 7. K. YAMANOUCHI, K. KUCHITSU, M. SUGIE, H. TAKEO, C. MATSUMURA, S. KATO, AND K. MOROKUMA, to be published. 8. J. K. G. WATSON, .I. Chem. Phys. 46, 1935-l 949 ( 1967). 9. J. S. MUENTER,J. Chem. Phys. 48,4544-4547 (1968). 10. F. J. LOVAS, R. D. SUENRAM,D. R. JOHNSON, F. 0. CLARK, AND E. TIEMANN, J. Chem. Phys. 72,
4964-4972 (1980). Il. R. E. PENN, J. Mol. Spectrosc. 69, 373-382 (1978). 12. R. D. BROWN, P. D. GODFREY, AND D. A. WINKLER, Chem. Phys. 59, 243-247 (1981). 13. J. DEMAISON, W. HOTTNER, B. STARCK, I. BUCK, R. TISCHER, AND M. WINNEWISSER,“Molecular Constants from Microwave, Molecular Beam, and Electron Spin Resonance Spectroscopy” (K.-H. Hellwege and A. M. Hellwege, Eds.). Springer-Verlag. Berlin, 1974.