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The microwave spectrum of ethyl cyanate
JOURNAL
OF MOLECULAR
SPECTROSCOPY
138,375-382 (1989)
The Microwave Spectrum of Ethyl Cyanate TAKESHI SAKAIZUMI, HIROKI MURE, OSAMU OHASHI, AND ICHIRO YAMAGUCHI Department
of Chemistry, Faculty of Scienceand Technology, Sophia University, Kioi-cho, Chiyoda-ku. Tokyo 102, Japan
The microwave spectra of ethyl cyanate (CH,CH,-OCN) and its “N species generated by reacting O-ethyl thiocarbamate or O-ethyl thiocarbamate- 15Nwith mercury( II) oxide havebeen observed in the frequency range from 8 to 40 GHz. The rotational constants for normal and 15N species have been determined. The dipole moments (debye) obtained for normal species are fiL. = 4.70 f 0.18, pb = 0.38 -c 0.15, and pLtti = 4.72 + 0.33. The observed reaction product was concluded to be antiperipIanar( tram)-ethyl cyanate. Q 1989 Academic press, IX. 1. INTRODUCTION
Molecules with an ambidextrous cyanato (-OCN) group form rather intriguing compounds because the cyanato group is able to react at the site of either the oxygen or the nitrogen atom, forming either a R-OCN or a R-NC0 molecule. R-OCN (R = CHs and C2H5) molecules rapidly change into another isomer, R-NCO, or trimerize (1). Isocyanic acid (HNCO) (2) exists only as the R-NC0 type in the gas phase, while methyl derivatives have been found by microwave spectroscopy to exist in two forms: CH3-OCN (3) and CHs-NC0 (4). In the case of ethyl derivatives, the microwave spectra of ethyl isocyanate ( C2 HsNCO) (5), ethyl isothiocyanate ( CZH5-NCS) (6)) and ethyl isoselenocyanate ( CZHSNCSe) (7) have been reported. Their conformations have been determined to be synperiplanar (cis). The rotational conformation of ethyl thiocyanate ( C2H5-SCN) (8) has been determined to be synclinal(guuche) by microwave spectroscopy. Although ethyl cyanate ( CzHs-OCN) has been detected by IR ( 9)) NMR ( 9), and mass spectroscopy ( IO), no information about its geometrical conformation has been reported. It would be interesting to see what conformations for ethyl cyanate could exist around the C-O bond. In this study, the microwave spectra of CZH5-OC’4N and CzHs-OC”N in the ground and excited vibrational states were observed in the frequency range from 8 to 40 GHz. The rotational constants, AZ( =Z, - Zb- Zc), and dipole moments of normal and 15N species were obtained. The reaction product was determined to be antiperiplanar( truns)-ethyl cyanate from the comparison between the observed and calculated rotational constants and the rs coordinate of the nitrogen atom. 2. EXPERIMENTAL
DETAILS
Ethyl cyanate was generated by a method adapted from Jensen and Holm ( 11). Oethyl thiocarbamate ( CZHs-OC( :S)NHz) was passed through a U-tube packed with 375
0022-2852189 $3.00 Copyright 0
1989 by Academic Press, Inc.
All rights of reproduction in any form reserved.
376
SAKAIZUMI
ET AL.
mercury( II) oxide. The reaction products were led directly to an absorption cell (3m X-band waveguide). O-ethyl thiocarbamate was prepared according to the method of Davies and Maclaren ( 12) and was identified by melting point (38°C) and IR, ‘HNMR, 13C-NMR, and mass spectroscopy. Ethyl cyanate- 15N was prepared by the reaction of O-ethyl thiocarbamate“N with mercury oxide. O-ethyl thiocarbamate15N was synthesized by mixing carbon disulfide and ammonium hydroxide- “N (30 atom% 15N) in ethanol. Mercury oxide (yellow) was commercially obtained from Wako Pure Chemical Industries, Ltd. The mass spectra of the reaction products of O-ethyl thiocarbamate and mercury oxide were observed with a quadrupole mass spectrometer ( ANELVA AQA-360). The microwave spectrometer employed was a conventional lOO-kHz square- and sinusoidal-wave Stark-modulated instrument. The microwave sources used were a signal generator (HP-8672A) in the frequency range from 8 to 18 GHz and two YIGtuned GaAs oscillators (WJ 5600-301F and WJ 56 lo-302FD) from 18 to 26.5 GHz and from 26.5 to 40 GHz. 3. RESULTS
AND
DISCUSSION
When O-ethyl thiocarbamate was passed through the U-tube packed with mercury oxide, the molecular ion peak (m/z 105) of O-ethyl thiocarbamate in the mass spectrum disappeared completely and a new peak of m/z 7 1 appeared. The microwave spectrum of the reaction products observed at the Stark field of 1000 V/cm was very complicated in the frequency range from 8 to 40 GHz, but the spectrum of ethyl isocyanate was not observed. The spectrum observed at the Stark field of 40 V/cm consisted of three equidistantly spaced groups (B + C = 4900 MHz) between 26.5 and 40 GHz. The spectral lines were readily assigned to u-type R-branch transitions with J = 6 +- 5 to J = 8 f 7. The higher K-1 transitions in the K-structure could not be assigned separately because the centrifugal distortion effect is very small. The hyperfme structure due to the nitrogen atom displayed no splitting for a-type Rbranch transitions. The spectral lines of lower K_, value in the ground and excited vibrational states for normal and 15ZVspecies were assigned by their Stark behavior. The spectral lines in b-type transitions were not observed because of the weakness of the intensity. These are listed in Tables I and II. The rotational constants were obtained by the least-squares method: the results are listed in Table III. The observed and calculated transition frequencies are in good agreement, as shown in Tables I and II. The values of AZ ( = Z, - Z, - Zb) obtained for normal and “N species in the ground vibrational state, as shown in Table III, are almost the same as -6.59 uA*, observed for ethyl isocyanate (5). The value suggests that the skeleton of the heavy atoms is planar and that four hydrogen atoms are located out of the molecular skeletal plane. In order to find to which molecule the observed spectrum corresponds, the model calculation of the rotational constants were done for ethyl cyanate and its isomers, under the conditions that the main molecular ion peak of m/z 71 (C3H5NO) be obtained from the mass spectrum and that the value of AZ suggest the skeletal plane of the heavy atoms. The skeletal parameters of C-OCN and C-ONC groups in ethyl cyanate ( CH3CH2-QCN) and ethylnitrile-N-oxide (CH3CH2-ONC) were quoted from
SPECTRUM OF ETHYL CYANATE
377
TABLE I Observed Rotational Transitions (MHz) of Ethyl Cyanate and Its “N Species in the Ground Vibrational State C
C H -OC14N 2 5
Transition 3(1,3)
-
Obsd
Obsd-Calcd
2
H -OC15N 5
Obsd
Obsd-Calcd
2(1,2)
14703.51
-0.04 0.15
14494.41
0.21
3(0,3)
-
210#2)
14888.09
3(1,2)
-
2(1,1)
15075.15
0.00
14672.29
0.05
19091.23
0.05
4(1,4)
-
3(1,3)
19604.13
-0.12
4(0,4)
-
3(0,3)
19848.74
0.10
5Cl.5)
-
4(1,4)
24504.63
0.09
5(0,5)
-
4(0,4)
24807.62
-0.06
24151.98
0.06
5(1,4)
-
4(1,3)
25123.81
-0.04
24452.52
0.07
6(1,6)
-
5(1,5)
29404.38
0.06
28634.99
0.10
6(0,6)
-
5(0,5)
29764.62
0.00
28978.01
-0.11
6t2.4)
-
5(2,3)
29792.03
0.08
29003.04
0.01
6(1,5)
-
5(1,4)
30147.57
0.11
29341.96
0.07
7(1,7)
-
6(1#6)
34303.49
0.01
33405.99
0.04
7(0,7)
-
6(0,6)
34719.21
0.13
33802.06
0.01
7(2,5)
-
6(2,4)
34762.46
-0.08
33841.54
0.13
7t1.6)
-
6(1,5)
35170.31
-0.12
34230.72
-0.02
B(1.8)
-
7(1,7)
39201.86
-0.08
38176.21
-0.16
8(0,8)
-
7(0,7)
39670.51
-0.11
38623.30
-0.02
8(2,7)
-
7(2,6)
39700.57
0.04
38650.74
0.15
8(2,6)
-
7(2,5)
39735.53
-0.02
38682.53
0.01
8(1,7)
-
7(1,6)
39118.77
-0.13
those of CH,-OCN and CH3-ONC optimized by ab initio molecular orbital calculation ( IS), respectively. The parameters of ethyl fulminate ( CH3CH2-CHO) were quoted from the rS structure (14) of CHs-CNO. The structural parameters of ethyl groups in three candidates were taken from ethyl isocyanate (5). The molecular constants observed were compared with those calculated for C2H5-OCN, CZH5-ONC, and CzH5CNO, as shown in Table IV. The rotational constants observed are very close to those calculated for antiperiplanar (ap)-C2H5-OCN and ap-CzH5-ONC. However, the calculated rotational constants of ap-CzH,-OCN and ap-CzH5-ONC are quite similar and cannot be distinguished clearly from each other.
SAKAIZUMI
378
ET AL.
TABLE II Observed Rotational Transitions (MHz) of Cz H5-OC 14Nin the Excited Vibrational States
(l,O)a)
Transition
Obsd
c2,0ja'
0 -Cbl
Obsd
29630.67
(O,l)")
Obsd
0 -Cb)
0
29511.77
0.04
29949.35
0.23
29803.09
0.08
6(1,5)-5(1,4)
30213.23
0.16
30287.07
0.09
30196.10
-0.12
7(1,7)-6(1,6)
34428.92
0.12
34567.62
0.02
34338.61
0.25
6(1,6)-5(1,5) 6(0,6)-5(0,5)
Cb'
-0.29
29434.03
0.16
-
7(0,7)-6(0,6)
34819.96
-0.21
34935.14
-0.09
34763.23
-0.08
7(1,6)-6(1,5)
35246.76
-0.23
35333.31
-0.13
35227.28
0.10
8(1,8)-7(1,7)
39344.97
-0.21
39503.91
-0.18
C-O torsion, uq: methyl torsion. b, 0 - c: Obsd - Calcd.
') (v,, u~)--u,:
The model calculation for apC2H5-OCN and ap-C2H5-ONC suggests that the acoordinates of their nitrogen atoms are quite different. The Y,coordinate of the nitrogen atom was calculated by using Kraitchman’s equation ( 1.5). The observed and calculated coordinates of the nitrogen atom are shown in Table V. The rS coordinate observed is consistent with that calculated from the model of ap-C2H5-OCN. Therefore, the observed reaction product was concluded to be antiperiplanar ( turns)-ethyl cyanate. TABLE III Observed Rotational Constants (MHz) and Al (d *) of Ethyl Cyanate”) C2H5-
(O,O)
A
30055(94)
B C C)
AI
b)
(l,O)
OC
b)
27259(439)
14
N
C2H5-
(2,O)
b)
27658(314)
(O,l)
b)
29110(347)
to,01
OC
15
b)
297941110)
2543.53(l)
2547.26(3)
2551.51(2)
2548.41(2)
2474.87(l)
2419.67(l)
2430.36(2)
2442.0911)
2421.41(2)
2357.03(l)
-9.00(30)
-9.34(21)
-6.96(21)
-6.64(5)
N
-6.75(6)
') Figures in parentheses indicate the uncertainty attached to the last figures, estimated from 2.5 times the standard deviations. Conversion factor: 505 379 uA* MHz. ‘) (q,, v,)-u,: C-O torsion, u4: methyl torsion. c’ Al = I, - I, - I,.
379
SPECTRUM OF ETHYL CYANATE TABLE IV Observed and Calculated Rotational Constants (MHz) and Al (uAZ) Calcd C2H5-OCN a)
Obsd
(wld'
A
30055(94)
C2H5-ONC
(ae)
(spJe'
b)
C2H5-CNO
(SP)
29306.00
9918.92
27911.49
9684.14
24003.69
B
2543.53(l)
2477.51
4536.83
2587.01
4923.17
2329.03
C
2419.67(l)
2352.40
3240.64
2410.69
3404.51
2181.65
B+C
4963.30(2)
4829.91
7777.41
5027.70
8327.68
4510.68
-0.9910
-0.991
-0.612
-0.989
-0.516
-0.986
-6.40
-6.40
-6.40
-6.40
-6.40
K
AIf)-6.64(2)
C)
a) Ethyl
cyanate. ‘) Ethyl fulminate. cJ Ethyl &rile-N-oxide. 1: ap: antiperiplanar( truns). sp: synperiplanar( cis) .
n AI=I,-I.-Ib.
4. DIPOLE MOMENT
The dipole moments in the ground vibrational state were determined from the Stark effects of M = 0 and 1 in 3(0,3)-2(0,2), A4 = 0 in 3( 1,2)-2( l,l), and M = 1 in 4( 1,4)-3( 1,3) transitions for ap-ethyl cyanate, as shown in Table VI. The electric field strength in the absorption cell was calibrated using the dipole moment of the
TABLE V rS Coordinates (A) of Nitrogen Atom of Ethyl Cyanate Calcd Obsd C2H5-0-CN
C2H5-O-NC
lal
2.366(l)
2.371
1.306
lb1
O-39(15)
0.233
0.117
ICI
o.oa'
0.0
0.0
') Assumed
to be zero by symmetry.
SAKAIZUMI
380
ET AL.
TABLE VI
Observed Stark Coefficients” and Dipole Moments (D) of Ethyl Cyanate
Transition
3(0,3)
-
2(0,2)
M
Obsd
Calcd
0
-2.96
-2.88
1
-0.766
-0.811
3(1,2)
-
2(1,1)
0
-0.600
-0.792
4(1,4)
-
3(1,3)
1
3.91
3.90
“a Lib PC Ptota1
4.70(18) 0.38(16) O.ob' 4.72(23)
‘) [MHz/(V/cm)*] X 10-5. Figures in parentheses indicate the uncertainty attached to the last figures, estimated from 2.5 times that standard deviations. b, Assumed to be zero by symmetry.
OCS molecule, 0.7 152 1 D ( 16). The c-component of the dipole moment was assumed to be zero because of the plane of symmetry. The dipole moments obtained are pa = 4.70 k 0.18, & = 0.38 + 0.15, and ptotal = 4.72 + 0.33 D. The dipole moment of ethyl cyanate is larger than that of ethyl isocyanate (P = 2.80 D) (5) and is very similar to those of ethyl cyanide (p = 4.05 D) ( 17) and ethyl isocyanide (P = 4.0 1 D) (18). 5. VIBRATIONALLY
EXCITED
STATES
The spectral lines in the excited vibrational states (v,, v,) were easily assigned by their Stark behaviors (Table II). The values (-9.00 and -9.34 uA2) of AI obtained for the ( 1,O) and (2,0) states as listed in Table III, are much larger than that in the ground vibrational state. This suggests that the sets of ( 1,O) and (2,0) states should be assigned to the out-of-plane vibrational mode ( 19). The vibrational frequencies of the ( l,O), (2,0), and (0,l) states were found to be 120 f 30, 250 t 40, and 300 * 50 cm-‘, respectively, from the relative intensity measurements. The vibrational mode of v, was assigned to the C-O torsion. The vibrational mode of v, cannot be distinguished to be in the in-plane mode or the out-of-plane mode from the value of AI observed. But the mode of v, may be assigned to the methyl torsion because the rotational constants of vq = 1 are close to those of the ground vibrational state and the vibrational frequency is found to be 300 f 50 cm-’ (8). The u-type R-branch transitions in the v, = 1 state displayed no splitting.
SPECTRUM
OF ETHYL
CYANATE
381
TABLE VII Rotational Isomers of CH3CH2-NCX and CH3CH2-XCN Observed by Microwave Spectroscopy x=s
x=0 synperiplanar
a)
antiperiplanarc) (JCrans)
Ref. Ref. This Ref.
synperiplanarb) (cis)
(cis)
a) b, ‘) d,
(X = 0 and S)
synclinald) (gauche)
(5). (6). work. (8).
6. CONFORMATION
The rotational isomers of R-XCN and R-NCX (R = CH3CH2 and X = 0 and S) identified by microwave spectroscopy are summarized in Table VII. In the R-NCX type, ethyl isocyanate (X = 0) and ethyl isothiocyanate (X = S) take the synperiplanar( cis) conformation as one of two isomers, in which the methyl group eclipses the N = C = X group for the C-N bond. On the other hand, ethyl thiocyanate (X = S) in the R-XCN type takes the synclinal(gauche) conformation (8). Although it was expected that ethyl cyanate (X = 0) would take the same conformation as ethyl thiocyanate, the observed spectrum was assigned to the antiperiplanar( tvuns) conformation as one of two isomers. We searched for the spectral lines due to the synclinal conformation but the weakness of the lines prevented assignment. This suggests that the antiperiplanar conformation is more stable than the synclinal conformation in ethyl cyanate. It was concluded that CH3CH2-NCX type molecules take the same conformation on the replacement sulfur or oxygen atom but CH3CH2-XCN type molecules take dilferent conformations. REXEIVED:
July 10, 1989 REFERENCES
I. K. A. JENSEN, M. DUE, AND A. HOLM, Acza Chem. Stand. 19.438-442 ( 1965). _7. K. YAMADA, J. Mol. Spectrosc. 79, 323-344 ( 1980). 3. T. SAKAIZUMI, H. MURE, 0. OHASHI, AND I. YAMAGUCHI, J. Mol. Spectrosc., in press. 4. J. KOPUT, J. Mol. Spectrosc. 106, 12-21 (1984); J. Mol. Spectrosc. 115, 131-146 (1986). 5. T. SAKAIZUMI, 0. YAMADA, K. USHIDA, 0. OHASHI, AND I. YAMAGUCHI, Bull. Chem. Sot. Japan 49,
290%2912(1976). 6. T.
SAKAIZUMI, 0. OHASHI, AND I. YAMAGUCHI,
Bull. Chem. Sot. Japan 49, 948-953 ( 1976)
382
SAKAIZUMI
ET AL.
7. T. SAKAIZUMI, K. IIDA, 0. OHASHI, AND I. YAMAGUCHI, Bull. Chem. Sot. Japan 59, 1991-1995 (1986). 8. A. BJORSETH AND K. M. MARSTOKK,J. Moi. Struct. 11, 15-23 ( 1972). 9. N. GROVING AND A. HOLM, Acta Chem. Stand. 19,443-450 ( 1965). 10. K. A. JENSEN,A. HOLM, AND C. WENTRUP,Acta Chem. Stand. 20,2107-2122 ( 1966). II. K. A. JENSEN AND A. HOLM, Acta Chem. Stand. l&2417-2418 (1964). 12. W. DAVIESAND J. A. MACLAREN,J. Chem. Sot. 1434-1437 (1951). 13. D. POPPINGERAND L. RADOM, J. Amer. Chem. Sot. 100,3674-3685 ( 1978). 14. P. B. BLACKBURN,R. D. BROWN, F. R. BURN, J. G. CROFTS,AND 1. R. GILLARD, Chem. Phys. Lett. 7, 102-104 (1970). IS. J. KRAITCHMAN,Amer. J. Phys. 21, 17-24 (1953). 16. J. S. MUENTER, J. Chem. Phys. 4&4544-4547 ( 1968). 17. H. M. HEISE,H. LUTZ, AND H. DREIZLER,Z. Naturforsch. A 29, 1345-1355 ( 1974). 18. R. J. ANDERSONAND W. D. GWINN, J. Chem. Phys. 49,3988-3991 ( 1968). 19. D. R. HERSCHBACH AND V. W. LAURIE, J. Chem. Phyx 40,3142-3153 ( 1964).
OF MOLECULAR
SPECTROSCOPY
138,375-382 (1989)
The Microwave Spectrum of Ethyl Cyanate TAKESHI SAKAIZUMI, HIROKI MURE, OSAMU OHASHI, AND ICHIRO YAMAGUCHI Department
of Chemistry, Faculty of Scienceand Technology, Sophia University, Kioi-cho, Chiyoda-ku. Tokyo 102, Japan
The microwave spectra of ethyl cyanate (CH,CH,-OCN) and its “N species generated by reacting O-ethyl thiocarbamate or O-ethyl thiocarbamate- 15Nwith mercury( II) oxide havebeen observed in the frequency range from 8 to 40 GHz. The rotational constants for normal and 15N species have been determined. The dipole moments (debye) obtained for normal species are fiL. = 4.70 f 0.18, pb = 0.38 -c 0.15, and pLtti = 4.72 + 0.33. The observed reaction product was concluded to be antiperipIanar( tram)-ethyl cyanate. Q 1989 Academic press, IX. 1. INTRODUCTION
Molecules with an ambidextrous cyanato (-OCN) group form rather intriguing compounds because the cyanato group is able to react at the site of either the oxygen or the nitrogen atom, forming either a R-OCN or a R-NC0 molecule. R-OCN (R = CHs and C2H5) molecules rapidly change into another isomer, R-NCO, or trimerize (1). Isocyanic acid (HNCO) (2) exists only as the R-NC0 type in the gas phase, while methyl derivatives have been found by microwave spectroscopy to exist in two forms: CH3-OCN (3) and CHs-NC0 (4). In the case of ethyl derivatives, the microwave spectra of ethyl isocyanate ( C2 HsNCO) (5), ethyl isothiocyanate ( CZH5-NCS) (6)) and ethyl isoselenocyanate ( CZHSNCSe) (7) have been reported. Their conformations have been determined to be synperiplanar (cis). The rotational conformation of ethyl thiocyanate ( C2H5-SCN) (8) has been determined to be synclinal(guuche) by microwave spectroscopy. Although ethyl cyanate ( CzHs-OCN) has been detected by IR ( 9)) NMR ( 9), and mass spectroscopy ( IO), no information about its geometrical conformation has been reported. It would be interesting to see what conformations for ethyl cyanate could exist around the C-O bond. In this study, the microwave spectra of CZH5-OC’4N and CzHs-OC”N in the ground and excited vibrational states were observed in the frequency range from 8 to 40 GHz. The rotational constants, AZ( =Z, - Zb- Zc), and dipole moments of normal and 15N species were obtained. The reaction product was determined to be antiperiplanar( truns)-ethyl cyanate from the comparison between the observed and calculated rotational constants and the rs coordinate of the nitrogen atom. 2. EXPERIMENTAL
DETAILS
Ethyl cyanate was generated by a method adapted from Jensen and Holm ( 11). Oethyl thiocarbamate ( CZHs-OC( :S)NHz) was passed through a U-tube packed with 375
0022-2852189 $3.00 Copyright 0
1989 by Academic Press, Inc.
All rights of reproduction in any form reserved.
376
SAKAIZUMI
ET AL.
mercury( II) oxide. The reaction products were led directly to an absorption cell (3m X-band waveguide). O-ethyl thiocarbamate was prepared according to the method of Davies and Maclaren ( 12) and was identified by melting point (38°C) and IR, ‘HNMR, 13C-NMR, and mass spectroscopy. Ethyl cyanate- 15N was prepared by the reaction of O-ethyl thiocarbamate“N with mercury oxide. O-ethyl thiocarbamate15N was synthesized by mixing carbon disulfide and ammonium hydroxide- “N (30 atom% 15N) in ethanol. Mercury oxide (yellow) was commercially obtained from Wako Pure Chemical Industries, Ltd. The mass spectra of the reaction products of O-ethyl thiocarbamate and mercury oxide were observed with a quadrupole mass spectrometer ( ANELVA AQA-360). The microwave spectrometer employed was a conventional lOO-kHz square- and sinusoidal-wave Stark-modulated instrument. The microwave sources used were a signal generator (HP-8672A) in the frequency range from 8 to 18 GHz and two YIGtuned GaAs oscillators (WJ 5600-301F and WJ 56 lo-302FD) from 18 to 26.5 GHz and from 26.5 to 40 GHz. 3. RESULTS
AND
DISCUSSION
When O-ethyl thiocarbamate was passed through the U-tube packed with mercury oxide, the molecular ion peak (m/z 105) of O-ethyl thiocarbamate in the mass spectrum disappeared completely and a new peak of m/z 7 1 appeared. The microwave spectrum of the reaction products observed at the Stark field of 1000 V/cm was very complicated in the frequency range from 8 to 40 GHz, but the spectrum of ethyl isocyanate was not observed. The spectrum observed at the Stark field of 40 V/cm consisted of three equidistantly spaced groups (B + C = 4900 MHz) between 26.5 and 40 GHz. The spectral lines were readily assigned to u-type R-branch transitions with J = 6 +- 5 to J = 8 f 7. The higher K-1 transitions in the K-structure could not be assigned separately because the centrifugal distortion effect is very small. The hyperfme structure due to the nitrogen atom displayed no splitting for a-type Rbranch transitions. The spectral lines of lower K_, value in the ground and excited vibrational states for normal and 15ZVspecies were assigned by their Stark behavior. The spectral lines in b-type transitions were not observed because of the weakness of the intensity. These are listed in Tables I and II. The rotational constants were obtained by the least-squares method: the results are listed in Table III. The observed and calculated transition frequencies are in good agreement, as shown in Tables I and II. The values of AZ ( = Z, - Z, - Zb) obtained for normal and “N species in the ground vibrational state, as shown in Table III, are almost the same as -6.59 uA*, observed for ethyl isocyanate (5). The value suggests that the skeleton of the heavy atoms is planar and that four hydrogen atoms are located out of the molecular skeletal plane. In order to find to which molecule the observed spectrum corresponds, the model calculation of the rotational constants were done for ethyl cyanate and its isomers, under the conditions that the main molecular ion peak of m/z 71 (C3H5NO) be obtained from the mass spectrum and that the value of AZ suggest the skeletal plane of the heavy atoms. The skeletal parameters of C-OCN and C-ONC groups in ethyl cyanate ( CH3CH2-QCN) and ethylnitrile-N-oxide (CH3CH2-ONC) were quoted from
SPECTRUM OF ETHYL CYANATE
377
TABLE I Observed Rotational Transitions (MHz) of Ethyl Cyanate and Its “N Species in the Ground Vibrational State C
C H -OC14N 2 5
Transition 3(1,3)
-
Obsd
Obsd-Calcd
2
H -OC15N 5
Obsd
Obsd-Calcd
2(1,2)
14703.51
-0.04 0.15
14494.41
0.21
3(0,3)
-
210#2)
14888.09
3(1,2)
-
2(1,1)
15075.15
0.00
14672.29
0.05
19091.23
0.05
4(1,4)
-
3(1,3)
19604.13
-0.12
4(0,4)
-
3(0,3)
19848.74
0.10
5Cl.5)
-
4(1,4)
24504.63
0.09
5(0,5)
-
4(0,4)
24807.62
-0.06
24151.98
0.06
5(1,4)
-
4(1,3)
25123.81
-0.04
24452.52
0.07
6(1,6)
-
5(1,5)
29404.38
0.06
28634.99
0.10
6(0,6)
-
5(0,5)
29764.62
0.00
28978.01
-0.11
6t2.4)
-
5(2,3)
29792.03
0.08
29003.04
0.01
6(1,5)
-
5(1,4)
30147.57
0.11
29341.96
0.07
7(1,7)
-
6(1#6)
34303.49
0.01
33405.99
0.04
7(0,7)
-
6(0,6)
34719.21
0.13
33802.06
0.01
7(2,5)
-
6(2,4)
34762.46
-0.08
33841.54
0.13
7t1.6)
-
6(1,5)
35170.31
-0.12
34230.72
-0.02
B(1.8)
-
7(1,7)
39201.86
-0.08
38176.21
-0.16
8(0,8)
-
7(0,7)
39670.51
-0.11
38623.30
-0.02
8(2,7)
-
7(2,6)
39700.57
0.04
38650.74
0.15
8(2,6)
-
7(2,5)
39735.53
-0.02
38682.53
0.01
8(1,7)
-
7(1,6)
39118.77
-0.13
those of CH,-OCN and CH3-ONC optimized by ab initio molecular orbital calculation ( IS), respectively. The parameters of ethyl fulminate ( CH3CH2-CHO) were quoted from the rS structure (14) of CHs-CNO. The structural parameters of ethyl groups in three candidates were taken from ethyl isocyanate (5). The molecular constants observed were compared with those calculated for C2H5-OCN, CZH5-ONC, and CzH5CNO, as shown in Table IV. The rotational constants observed are very close to those calculated for antiperiplanar (ap)-C2H5-OCN and ap-CzH5-ONC. However, the calculated rotational constants of ap-CzH,-OCN and ap-CzH5-ONC are quite similar and cannot be distinguished clearly from each other.
SAKAIZUMI
378
ET AL.
TABLE II Observed Rotational Transitions (MHz) of Cz H5-OC 14Nin the Excited Vibrational States
(l,O)a)
Transition
Obsd
c2,0ja'
0 -Cbl
Obsd
29630.67
(O,l)")
Obsd
0 -Cb)
0
29511.77
0.04
29949.35
0.23
29803.09
0.08
6(1,5)-5(1,4)
30213.23
0.16
30287.07
0.09
30196.10
-0.12
7(1,7)-6(1,6)
34428.92
0.12
34567.62
0.02
34338.61
0.25
6(1,6)-5(1,5) 6(0,6)-5(0,5)
Cb'
-0.29
29434.03
0.16
-
7(0,7)-6(0,6)
34819.96
-0.21
34935.14
-0.09
34763.23
-0.08
7(1,6)-6(1,5)
35246.76
-0.23
35333.31
-0.13
35227.28
0.10
8(1,8)-7(1,7)
39344.97
-0.21
39503.91
-0.18
C-O torsion, uq: methyl torsion. b, 0 - c: Obsd - Calcd.
') (v,, u~)--u,:
The model calculation for apC2H5-OCN and ap-C2H5-ONC suggests that the acoordinates of their nitrogen atoms are quite different. The Y,coordinate of the nitrogen atom was calculated by using Kraitchman’s equation ( 1.5). The observed and calculated coordinates of the nitrogen atom are shown in Table V. The rS coordinate observed is consistent with that calculated from the model of ap-C2H5-OCN. Therefore, the observed reaction product was concluded to be antiperiplanar ( turns)-ethyl cyanate. TABLE III Observed Rotational Constants (MHz) and Al (d *) of Ethyl Cyanate”) C2H5-
(O,O)
A
30055(94)
B C C)
AI
b)
(l,O)
OC
b)
27259(439)
14
N
C2H5-
(2,O)
b)
27658(314)
(O,l)
b)
29110(347)
to,01
OC
15
b)
297941110)
2543.53(l)
2547.26(3)
2551.51(2)
2548.41(2)
2474.87(l)
2419.67(l)
2430.36(2)
2442.0911)
2421.41(2)
2357.03(l)
-9.00(30)
-9.34(21)
-6.96(21)
-6.64(5)
N
-6.75(6)
') Figures in parentheses indicate the uncertainty attached to the last figures, estimated from 2.5 times the standard deviations. Conversion factor: 505 379 uA* MHz. ‘) (q,, v,)-u,: C-O torsion, u4: methyl torsion. c’ Al = I, - I, - I,.
379
SPECTRUM OF ETHYL CYANATE TABLE IV Observed and Calculated Rotational Constants (MHz) and Al (uAZ) Calcd C2H5-OCN a)
Obsd
(wld'
A
30055(94)
C2H5-ONC
(ae)
(spJe'
b)
C2H5-CNO
(SP)
29306.00
9918.92
27911.49
9684.14
24003.69
B
2543.53(l)
2477.51
4536.83
2587.01
4923.17
2329.03
C
2419.67(l)
2352.40
3240.64
2410.69
3404.51
2181.65
B+C
4963.30(2)
4829.91
7777.41
5027.70
8327.68
4510.68
-0.9910
-0.991
-0.612
-0.989
-0.516
-0.986
-6.40
-6.40
-6.40
-6.40
-6.40
K
AIf)-6.64(2)
C)
a) Ethyl
cyanate. ‘) Ethyl fulminate. cJ Ethyl &rile-N-oxide. 1: ap: antiperiplanar( truns). sp: synperiplanar( cis) .
n AI=I,-I.-Ib.
4. DIPOLE MOMENT
The dipole moments in the ground vibrational state were determined from the Stark effects of M = 0 and 1 in 3(0,3)-2(0,2), A4 = 0 in 3( 1,2)-2( l,l), and M = 1 in 4( 1,4)-3( 1,3) transitions for ap-ethyl cyanate, as shown in Table VI. The electric field strength in the absorption cell was calibrated using the dipole moment of the
TABLE V rS Coordinates (A) of Nitrogen Atom of Ethyl Cyanate Calcd Obsd C2H5-0-CN
C2H5-O-NC
lal
2.366(l)
2.371
1.306
lb1
O-39(15)
0.233
0.117
ICI
o.oa'
0.0
0.0
') Assumed
to be zero by symmetry.
SAKAIZUMI
380
ET AL.
TABLE VI
Observed Stark Coefficients” and Dipole Moments (D) of Ethyl Cyanate
Transition
3(0,3)
-
2(0,2)
M
Obsd
Calcd
0
-2.96
-2.88
1
-0.766
-0.811
3(1,2)
-
2(1,1)
0
-0.600
-0.792
4(1,4)
-
3(1,3)
1
3.91
3.90
“a Lib PC Ptota1
4.70(18) 0.38(16) O.ob' 4.72(23)
‘) [MHz/(V/cm)*] X 10-5. Figures in parentheses indicate the uncertainty attached to the last figures, estimated from 2.5 times that standard deviations. b, Assumed to be zero by symmetry.
OCS molecule, 0.7 152 1 D ( 16). The c-component of the dipole moment was assumed to be zero because of the plane of symmetry. The dipole moments obtained are pa = 4.70 k 0.18, & = 0.38 + 0.15, and ptotal = 4.72 + 0.33 D. The dipole moment of ethyl cyanate is larger than that of ethyl isocyanate (P = 2.80 D) (5) and is very similar to those of ethyl cyanide (p = 4.05 D) ( 17) and ethyl isocyanide (P = 4.0 1 D) (18). 5. VIBRATIONALLY
EXCITED
STATES
The spectral lines in the excited vibrational states (v,, v,) were easily assigned by their Stark behaviors (Table II). The values (-9.00 and -9.34 uA2) of AI obtained for the ( 1,O) and (2,0) states as listed in Table III, are much larger than that in the ground vibrational state. This suggests that the sets of ( 1,O) and (2,0) states should be assigned to the out-of-plane vibrational mode ( 19). The vibrational frequencies of the ( l,O), (2,0), and (0,l) states were found to be 120 f 30, 250 t 40, and 300 * 50 cm-‘, respectively, from the relative intensity measurements. The vibrational mode of v, was assigned to the C-O torsion. The vibrational mode of v, cannot be distinguished to be in the in-plane mode or the out-of-plane mode from the value of AI observed. But the mode of v, may be assigned to the methyl torsion because the rotational constants of vq = 1 are close to those of the ground vibrational state and the vibrational frequency is found to be 300 f 50 cm-’ (8). The u-type R-branch transitions in the v, = 1 state displayed no splitting.
SPECTRUM
OF ETHYL
CYANATE
381
TABLE VII Rotational Isomers of CH3CH2-NCX and CH3CH2-XCN Observed by Microwave Spectroscopy x=s
x=0 synperiplanar
a)
antiperiplanarc) (JCrans)
Ref. Ref. This Ref.
synperiplanarb) (cis)
(cis)
a) b, ‘) d,
(X = 0 and S)
synclinald) (gauche)
(5). (6). work. (8).
6. CONFORMATION
The rotational isomers of R-XCN and R-NCX (R = CH3CH2 and X = 0 and S) identified by microwave spectroscopy are summarized in Table VII. In the R-NCX type, ethyl isocyanate (X = 0) and ethyl isothiocyanate (X = S) take the synperiplanar( cis) conformation as one of two isomers, in which the methyl group eclipses the N = C = X group for the C-N bond. On the other hand, ethyl thiocyanate (X = S) in the R-XCN type takes the synclinal(gauche) conformation (8). Although it was expected that ethyl cyanate (X = 0) would take the same conformation as ethyl thiocyanate, the observed spectrum was assigned to the antiperiplanar( tvuns) conformation as one of two isomers. We searched for the spectral lines due to the synclinal conformation but the weakness of the lines prevented assignment. This suggests that the antiperiplanar conformation is more stable than the synclinal conformation in ethyl cyanate. It was concluded that CH3CH2-NCX type molecules take the same conformation on the replacement sulfur or oxygen atom but CH3CH2-XCN type molecules take dilferent conformations. REXEIVED:
July 10, 1989 REFERENCES
I. K. A. JENSEN, M. DUE, AND A. HOLM, Acza Chem. Stand. 19.438-442 ( 1965). _7. K. YAMADA, J. Mol. Spectrosc. 79, 323-344 ( 1980). 3. T. SAKAIZUMI, H. MURE, 0. OHASHI, AND I. YAMAGUCHI, J. Mol. Spectrosc., in press. 4. J. KOPUT, J. Mol. Spectrosc. 106, 12-21 (1984); J. Mol. Spectrosc. 115, 131-146 (1986). 5. T. SAKAIZUMI, 0. YAMADA, K. USHIDA, 0. OHASHI, AND I. YAMAGUCHI, Bull. Chem. Sot. Japan 49,
290%2912(1976). 6. T.
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ET AL.
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