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Conformation and structure of acetylisocyanate, CH3C(O) NCO, from electron diffraction and microwave data and ab initio calculations
359
Journal of Molecular Structure, 265 (1992) 359-366 Elsevier Science Publishers B.V.. Amsterdam
Conformation and structure of acetylisocyanate, CH3C (0 )NCO, from electron diffraction and microwave data and ab initio calculations Hans-Georg Mack and Heinz Oberhammer’ Institut fiir Physikalische (Germany)
und Theoretische
Chemie, Universitat
Tiibingen, W-7400 Tiibingen
Carlos 0. Della Vbdova Facultad de Ciencias Exactas, Universidad National de La P&a, Departamento Quimica Inorganica, 1900 La Plats (Argentina)
de Quimica,
(Received 9 September 1991)
Abstract The geometric structure of acetylisocyanate has been determined from electron diffraction intensities and rotational constants. In accordance with previous vibrational studies, only the cis conformation (cis position of the vicinal C=O and N=C double bonds) is observed. Ab initio calculations predict the preference of the cis form by 3.8 kcal mol-’ (HF/6-31G*) and 2.4 kcal mol-’ (MP2/6-31G*) respectively. The following geometric parameters were derived (ra diztances and L, angles, error limits are -20 values): (C=O),_=1.179(3) 4, dCO= C=O (carboonyl) - C=O (isocyan$e) =0.040 A o (from ab initio), C=N = 1.199 (7) A, N-C=l.413(7) A, C-C=l.499(11), A, C-H=1.090 A (from ab initio), C-N=C=128.2(13)“, N-C-C=111.5(4)“, C-C=O=124.0(23)“, N-C=O=124.5(23)“, N=C=O=173.0 (from ab initio ) , HCH = 107.2 (22) ‘.
INTRODUCTION
In a previous study we reported the gas-phase structure and conformational composition of chlorocarbonyl isocyanate, ClC (0)NCO [ 11.This compound exists in the gas phase as a mixture of planar trans and cis conformers with Cl
;4
,B
f0
0
-
k--N* Cl'
0 trans
cis
‘Author to whom correspondence should be addressed.
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
360
the trans position of the two double bonds being preferred by dG= 0.7 (3) kcal mol-’ relative to the cis position. For acetylisocyanate the strongest transitions in the microwave spectrum (MW) correspond to a planar cis conformer [ 21, but the presence of a second form cannot be excluded on the basis of the MW spectrum. Since the intensities of rotational transitions depend strongly on the dipole moment, the trans rotamer could exist in an appreciable amount if its dipole moment is much smaller than that of the cis conformer. However, the gas-phase IR spectrum of CH,C (0) NC0 shows an unsplit C=O band, indicating the presence of only one conformer, and its B-type contour is in accordance with the MW result [ 21. Thus, these spectroscopic data indicate that acetylisocyanate exists in the gas phase solely as the cis conformer. The preference of the cis form is also concluded from the MW spectra for vinylazide, H2C=CHNz [3] and for methyl azidoformate, CHBOC(0)N3 [4]. To our knowledge no complete geometric structures of compounds for which the cis position of the two vicinal double bonds is the only observed conformation, have been reported in the literature. Since the three rotational constants derived from the MW spectrum of acetylisocyanate provide only very limited structural information for this form, we performed a structure determination using joint analysis of electron diffraction (ED) intensities and MW rotational constants. This experimental study is supplemented by ab initio calculations at the HF/6-31G* and MP2/6-31G* level. These calculations were performed with the GAUSSIAN 86 program system [ 51.
EXPERIMENTAL SECTION
Acetylisocyanate was prepared by reaction of CH,C (0)Cl and AgNCO using the method proposed by Landsberg and Iqbal [ 21. The reaction product was distilled and fractionated at reduced pressure and finally purified by gas chromatography. The purity of the compound was confirmed by IR (vapour), Raman (liquid), ‘H NMR, 13C NMR and chemical analyses. It is worth noting that the IR spectra of the vapour phase were identical to those obtained from the codistillation fraction [ 61. The electron-diffraction intensities were recorded with the Balzers Gasdiffractograph KD-G2 [7] at two camera (nozzle-to-plate) distances (25 and 50 cm) with an accelerating voltage of about 60 kV. The electron wavelength was calibrated with ZnO diffraction patterns. The sample reservoir was kept at - 8” C and the inlet system and nozzle were maintained at room temperature. The camera pressure did not exceed 10m5 mbar during the experiment. The exposure times were 6-7 and 18-24 s for the long and short camera distance, respectively. Two plates of each camera distance were analyzed by the usual procedures [ 81. Numerical values for the total scattering intensities in the s-
361
1
I
0
5
I
1
10
15
I
20
I
25
I
30
I
35
s/A-’ Fig. 1. Experimental differences.
(dots)
and calculated
(full line) molecular scattering
intensities
and
ranges 2-18 A-’ and 8-35 A-’ in steps of ds=O.2 hi-’ are available as Supplementary Data”. The averaged molecular intensities are presented in Fig. 1. STRUCTURE
ANALYSIS
Model calculations demonstrate that the radial distribution function (Fig. 2) in the range above 2.5 A is quite sensitive to the conformation of acetylisocyanate. The experimental curve can be reproduced only by the cis form and contributions of the trans structure greater than 10% can be excluded on the basis of the ED intensities. A joint analysis of ED and MW data requires the calculation of vibrational corrections dr = r, - r, and LIBi = B 5 - B L from a harmonic force field. Unfortunately, the low frequency vibrations ( < 400 cm -’ ) , which are most important in these calculations, have not been measured experimentally. Therefore, an ab initio force field (HF/6-31G*) was derived (Table 1). Three vibrations are predicted to have frequencies below 400 cm-‘, which correspond to the torsion around the N-C single bond (97 cm- ’ ), to the C-N=C bending vibration (126 cm-‘) and to the CH3 torsion (140 cm-l). The concept of perpendicular amplitudes causes unreasonably large contributions of the two torsional vibrations to the harmonic corrections dr for interatomic distances, which do not depend on the torsional motion (e.g. dr= 0.144 for C-H, drz0.023 for C2=03 or dr=0.021 for Nl *- aC2; for atom “Deposited with the B.L.L.D. as Supplementary Publication number SVP 26440 (2 pages).
362
Fig. 2. Experimental radial distribution curve, difference curve and molecular model. TABLE 1 D&or@ valence force constants (HF/6-31G’) mdyn A). For atom numbering see Fig. 2
for the acetylisocyanate skeleton (mdyn A-l and
Bond
Force constant
Bond
Force constant
C2=03 c4=05 Nl=C2 Nl-C4 C4-C6 L C4-Nl=C2
19.96 16.40 14.92 5.91 5.27 0.42
L Nl-C4-C6 L Nl-C4=05 L Nl=C2=03 ~(C6C4Nlc2) oop(C4=05)” 0op(C2=03)~
2.16 2.71 0.90 0.05 0.61 0.71
“C=O out of C6C4Nl plane. %2=03 out of C2NlC4 plane.
numbering see Fig. 2). Therefore, the contributions of the torsional motions were neglected for torsion-independent interatomic distances. The harmonic corrections dr are given in Table 3 together with the vibrational amplitudes. The corrections for the rotational constants are evident from the values in Table 4. The uncertainties in the B f constants are estimated to be 15% of the corrections LIBi. A preliminary molecular model was refined by simultaneous fitting of the molecular intensities and rotational constants. The intensities were modified
TABLE 2 Geometric parameters of CH& (0)NCO For atom numbering see Fig. 2
(joint analysis ED and MW and ab initio calculations).
Parameter
ED+MW”
HF/6-31G’
MP2/6-31G*
(C=O),,
1.179(3) 0.04ob 1.159(6) 1.199(6) 1.199(7) 1.413(7) 1.499(11) 1.090b 128.2(13) 111.5(4) 124.0(23) 124.5(23) 173.0b 107.2(22)
1.162 0.047 1.139 1.186 1.211 1.411 1.501 1.083 126.5 112.5 125.0 122.5 174.3 109.3
1.197 0.037 1.179 1.216 1.231 1.427 1.499 1.091 128.5 111.7 125.5 122.8 172.8 109.3
AC0 = (C4=05) - (C2=03) C2=03’ c4=05= Nl=C2 Nl-C4 C4-C6 C-H C4-Nl=C2 Nl-C4-C6 C6-C4=05 Nl-C4=05’ Nl=C2=03 H-C-H
“Distances r, in A, angles L a in degrees, with 2a error limits. bFrom ab initio calculations, see text. ‘Dependent parameter.
TABLE 3 Interatomic distances, vibrational amplitudes and harmonic corrections (without non-bonded distances involving hydrogen atoms) of CH,C(O)NCO Parameter
Distance
QED)
l(spectr.)
Ar=r,-r,
C-H C2=03 c4=05 Nl=C2 Nl-C4 C4-C6 N1...05 C2*..C4 N1...03 05**.C6 Nla**C6 C2**.05 o3..*c4 C2*..C6 03*..05 03*..C6
1.09 1.16 1.20 1.21 1.41 1.50 2.32 2.35 2.35 2.39 2.41 2.84 3.46 3.57 3.73 4.73
0.078”
0.078 0.037 0.037 0.039 0.051 0.050 0.064 0.104 0.044 0.064 0.080 0.170 0.133 0.065 0.247 0.091
0.012 0.007 0.005 0.005 0.003 0.005 0.004 -0.003 0.010 0.006 0.003 -0.008 -0.002 0.004 -0.011 -0.001
“Not refined.
0.037”
1
0.058( 16)
0.070( 6) I
>
0.11 ( 3) 0.11 ( 3) 0.21 ( 8) 0.08 ( 3)
364 TABLE 4 Rotational constants (GHz) of CH, C (0)NCO
A
B6(exp)”
B6 (exp)
B; (talc)
10.773160(25)
10.78854 (230) 2.22190 ( 12) 1.86298( 5)
10.78920 2.22186 1.86303
B
2.222693( 5)
c
1.862934( 5)
“Ref. 2.
with a diagonal weight matrix and scattering amplitudes and phases reported by Haase [9] were used. The relative weight of ED and MW data was adjusted until the rotational constants were reproduced within their estimated uncertainties. Assuming planarity of the heavy atom skeleton and C,, symmetry for the CH, group, eleven geometric parameters are required to describe the geometric structure of this compound. Even a joint analysis does not allow unique determination of all geometric parameters. The C-H and the double-bond lengths C2=03, C4=05 and Nl=C2 are especially highly correlated, and it is not possible to fit these bond lengths simultaneously. For this reason, (C=O),, and Nl=C2 were refined, and C-H and the difference dCO= (C4=05) - (C2=03 ) were constrained to values which are intermediate between the two ab initio results (HF and MP2). An estimated uncertainty of _+0.01 A for dC0 is taken into account in the error limits of all other geometric parameters. Vibrational amplitudes for closely spaced distances were collected in groups and badly determined amplitudes were fixed at their spectroscopic values. The final results of the joint analysis are collected in Table 2 together with the ab initio results. DISCUSSION
The electron diffraction experiment confirms the interpretation of the microwave and IR spectra with respect to the cis conformation of acetylisocyanate. From the IR spectrum, which is the most sensitive method for observing a possible trans form, we estimate a contribution of less than 3% of this second conformer. According to the ab initio calculations the C=O vibration of this higher energy form would occur at a higher frequency (d Y= 29 cm-‘) relative to the C=O vibration of the cis conformer. In this region no absorption is observed in the experimental gas-phase spectra. Thus, we estimate an energy difference between the two conformers of dE=,?&,,-&,> 2.0 kcal mol-‘. Ab initio calculations predict dE values of 2.4 kcal mol-l (MP2/6-31G*) and 3.8 kcal mol-’ (HF/6-31G*), respectively. Both values are in accordance with the experiments. Acetylisocyanate is the only carbonyl isocyanate of the type XC( 0)NCO whose conformational properties in the gas-phase have been
365
studied, for which only the cis form is observed. In the compounds with X = F, Cl or Br mixtures of cis and trans conformers were found, with the amount of the trans structure increasing in the sequence X=F (30 (8) % ) [lo], Cl (75(8)%) [l] andBr (92(5)%) [ll].Weareunabletogiveastraightforward explanation of this influence of the substituents X on the conformational properties of these compounds. Structural parameters in the three carbonyl isocyanates with X=CH3, Cl and F do not differ drastically. The Nl-C4 single bond in acetylisocyanate is slightly longer (1.413( 7) A) relative to those in the halogen derivatives (1.384(6) A for X=Cl and 1.386(8) A for X=F). This value is much shorter than the N-C bond length (1.460 A) estimated in the MW study [2]. The bond angle at nitrogen in isocyanates RNCO is known to depend strongly on the electronegativity of the substituent R, increasing from 118.2 (9) ’ for R = Cl [ 12,131 to 159.6( 2) ’ for R= SiH, [ 141. No such strong dependence on X is observed for the XC(O)NCO compounds. The C2=Nl-C4 angle in CH$ (0)NCO is only slightly larger (128.2 (13) ’ from the joint analysis and 126.1’ from the MW study [2] ) than that in the cis form of FC (0)NCO (124.3 (15) ’ ) [lo]. The corresponding angle in the trans conformer of ClC(O)NCO (127.1(16)“) [l] is intermediate. It is interesting to note, however, that according to the ab initio calculations this CNC angle is always smaller by about 8-10” in those conformations, which have the vicinal double bonds cis to each other, compared with those in the trans forms. ACKNOWLEDGEMENTS
H.G.M. and H.O. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, C.O.D.V. gratefully acknowledges Prof. Dr. Mult. A. Haas (Ruhr-Universitit Bochum, Germany) for his stimulating contributions and helpful development of this work, and Mr. G. Bollmann for valuable help with the experimental work. C.O.D.V. also thanks the Consejo National de Investigaciones Cientificas y TQcnicas (Conicet ), Facultad de Ciencias Exactas (UNLP) and Fundacion Antorchas, Republica Argentina, for financial support.
REFERENCES 1 2 3 4
H.-G. Mack, H. Oberhammer end CO. Della V&ova, J. Mol. Struct., 200 (1989) 277. B.M. Landsbergand K. Iqbal, J. Chem. Sot. Faraday Trans. 2,76 (1980) 1208. R.G. Ford, J. Mol. Speztrosc., 65 (1977) 273. R.K. Kakar, CR. Quode, W. Lwowski and R.E. Wilde, J. Chem. Phye., 72 (1980) 4123.
5
6 7 8 9 10 11 12 13 14
GAUSSIAN 86, M.J. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachari, C.F. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, L.R. Kahn, D.F. DeFrees, R. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fleuder and J.A. Pople, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. G.H. Cady and D.P. Siegwarth, Anal. Chem., 31 (1959) 619. H. Oberhammer, Molecular Structure by Diffraction Methods, Vol. 4, The Chemical Society, London, 1976, p. 24. H. Oberhammer, W. Gombler and H. Willner, J. Mol. Struct., 70 (1981) 273. J. Haase, Naturforsch., Teil A, 25 (1970) 936. H.-G. Mack, H. Oberhammer and C.O. Della Vt%lova,to be published. C.O. Della VBdova, Ph.D. Thesis, University of Bochum, 1990. H. Oberhammer, Z. Naturforsch., Teil A, 26 (1971) 280. W.H. Hocking and M.C.L. Gerry, J. Mol. Spectrosc., 42 (1972) 547. M. Kreglewski and P. Jensen, J. Mol. Spectrosc., 103 (1984) 312.
Journal of Molecular Structure, 265 (1992) 359-366 Elsevier Science Publishers B.V.. Amsterdam
Conformation and structure of acetylisocyanate, CH3C (0 )NCO, from electron diffraction and microwave data and ab initio calculations Hans-Georg Mack and Heinz Oberhammer’ Institut fiir Physikalische (Germany)
und Theoretische
Chemie, Universitat
Tiibingen, W-7400 Tiibingen
Carlos 0. Della Vbdova Facultad de Ciencias Exactas, Universidad National de La P&a, Departamento Quimica Inorganica, 1900 La Plats (Argentina)
de Quimica,
(Received 9 September 1991)
Abstract The geometric structure of acetylisocyanate has been determined from electron diffraction intensities and rotational constants. In accordance with previous vibrational studies, only the cis conformation (cis position of the vicinal C=O and N=C double bonds) is observed. Ab initio calculations predict the preference of the cis form by 3.8 kcal mol-’ (HF/6-31G*) and 2.4 kcal mol-’ (MP2/6-31G*) respectively. The following geometric parameters were derived (ra diztances and L, angles, error limits are -20 values): (C=O),_=1.179(3) 4, dCO= C=O (carboonyl) - C=O (isocyan$e) =0.040 A o (from ab initio), C=N = 1.199 (7) A, N-C=l.413(7) A, C-C=l.499(11), A, C-H=1.090 A (from ab initio), C-N=C=128.2(13)“, N-C-C=111.5(4)“, C-C=O=124.0(23)“, N-C=O=124.5(23)“, N=C=O=173.0 (from ab initio ) , HCH = 107.2 (22) ‘.
INTRODUCTION
In a previous study we reported the gas-phase structure and conformational composition of chlorocarbonyl isocyanate, ClC (0)NCO [ 11.This compound exists in the gas phase as a mixture of planar trans and cis conformers with Cl
;4
,B
f0
0
-
k--N* Cl'
0 trans
cis
‘Author to whom correspondence should be addressed.
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
360
the trans position of the two double bonds being preferred by dG= 0.7 (3) kcal mol-’ relative to the cis position. For acetylisocyanate the strongest transitions in the microwave spectrum (MW) correspond to a planar cis conformer [ 21, but the presence of a second form cannot be excluded on the basis of the MW spectrum. Since the intensities of rotational transitions depend strongly on the dipole moment, the trans rotamer could exist in an appreciable amount if its dipole moment is much smaller than that of the cis conformer. However, the gas-phase IR spectrum of CH,C (0) NC0 shows an unsplit C=O band, indicating the presence of only one conformer, and its B-type contour is in accordance with the MW result [ 21. Thus, these spectroscopic data indicate that acetylisocyanate exists in the gas phase solely as the cis conformer. The preference of the cis form is also concluded from the MW spectra for vinylazide, H2C=CHNz [3] and for methyl azidoformate, CHBOC(0)N3 [4]. To our knowledge no complete geometric structures of compounds for which the cis position of the two vicinal double bonds is the only observed conformation, have been reported in the literature. Since the three rotational constants derived from the MW spectrum of acetylisocyanate provide only very limited structural information for this form, we performed a structure determination using joint analysis of electron diffraction (ED) intensities and MW rotational constants. This experimental study is supplemented by ab initio calculations at the HF/6-31G* and MP2/6-31G* level. These calculations were performed with the GAUSSIAN 86 program system [ 51.
EXPERIMENTAL SECTION
Acetylisocyanate was prepared by reaction of CH,C (0)Cl and AgNCO using the method proposed by Landsberg and Iqbal [ 21. The reaction product was distilled and fractionated at reduced pressure and finally purified by gas chromatography. The purity of the compound was confirmed by IR (vapour), Raman (liquid), ‘H NMR, 13C NMR and chemical analyses. It is worth noting that the IR spectra of the vapour phase were identical to those obtained from the codistillation fraction [ 61. The electron-diffraction intensities were recorded with the Balzers Gasdiffractograph KD-G2 [7] at two camera (nozzle-to-plate) distances (25 and 50 cm) with an accelerating voltage of about 60 kV. The electron wavelength was calibrated with ZnO diffraction patterns. The sample reservoir was kept at - 8” C and the inlet system and nozzle were maintained at room temperature. The camera pressure did not exceed 10m5 mbar during the experiment. The exposure times were 6-7 and 18-24 s for the long and short camera distance, respectively. Two plates of each camera distance were analyzed by the usual procedures [ 81. Numerical values for the total scattering intensities in the s-
361
1
I
0
5
I
1
10
15
I
20
I
25
I
30
I
35
s/A-’ Fig. 1. Experimental differences.
(dots)
and calculated
(full line) molecular scattering
intensities
and
ranges 2-18 A-’ and 8-35 A-’ in steps of ds=O.2 hi-’ are available as Supplementary Data”. The averaged molecular intensities are presented in Fig. 1. STRUCTURE
ANALYSIS
Model calculations demonstrate that the radial distribution function (Fig. 2) in the range above 2.5 A is quite sensitive to the conformation of acetylisocyanate. The experimental curve can be reproduced only by the cis form and contributions of the trans structure greater than 10% can be excluded on the basis of the ED intensities. A joint analysis of ED and MW data requires the calculation of vibrational corrections dr = r, - r, and LIBi = B 5 - B L from a harmonic force field. Unfortunately, the low frequency vibrations ( < 400 cm -’ ) , which are most important in these calculations, have not been measured experimentally. Therefore, an ab initio force field (HF/6-31G*) was derived (Table 1). Three vibrations are predicted to have frequencies below 400 cm-‘, which correspond to the torsion around the N-C single bond (97 cm- ’ ), to the C-N=C bending vibration (126 cm-‘) and to the CH3 torsion (140 cm-l). The concept of perpendicular amplitudes causes unreasonably large contributions of the two torsional vibrations to the harmonic corrections dr for interatomic distances, which do not depend on the torsional motion (e.g. dr= 0.144 for C-H, drz0.023 for C2=03 or dr=0.021 for Nl *- aC2; for atom “Deposited with the B.L.L.D. as Supplementary Publication number SVP 26440 (2 pages).
362
Fig. 2. Experimental radial distribution curve, difference curve and molecular model. TABLE 1 D&or@ valence force constants (HF/6-31G’) mdyn A). For atom numbering see Fig. 2
for the acetylisocyanate skeleton (mdyn A-l and
Bond
Force constant
Bond
Force constant
C2=03 c4=05 Nl=C2 Nl-C4 C4-C6 L C4-Nl=C2
19.96 16.40 14.92 5.91 5.27 0.42
L Nl-C4-C6 L Nl-C4=05 L Nl=C2=03 ~(C6C4Nlc2) oop(C4=05)” 0op(C2=03)~
2.16 2.71 0.90 0.05 0.61 0.71
“C=O out of C6C4Nl plane. %2=03 out of C2NlC4 plane.
numbering see Fig. 2). Therefore, the contributions of the torsional motions were neglected for torsion-independent interatomic distances. The harmonic corrections dr are given in Table 3 together with the vibrational amplitudes. The corrections for the rotational constants are evident from the values in Table 4. The uncertainties in the B f constants are estimated to be 15% of the corrections LIBi. A preliminary molecular model was refined by simultaneous fitting of the molecular intensities and rotational constants. The intensities were modified
TABLE 2 Geometric parameters of CH& (0)NCO For atom numbering see Fig. 2
(joint analysis ED and MW and ab initio calculations).
Parameter
ED+MW”
HF/6-31G’
MP2/6-31G*
(C=O),,
1.179(3) 0.04ob 1.159(6) 1.199(6) 1.199(7) 1.413(7) 1.499(11) 1.090b 128.2(13) 111.5(4) 124.0(23) 124.5(23) 173.0b 107.2(22)
1.162 0.047 1.139 1.186 1.211 1.411 1.501 1.083 126.5 112.5 125.0 122.5 174.3 109.3
1.197 0.037 1.179 1.216 1.231 1.427 1.499 1.091 128.5 111.7 125.5 122.8 172.8 109.3
AC0 = (C4=05) - (C2=03) C2=03’ c4=05= Nl=C2 Nl-C4 C4-C6 C-H C4-Nl=C2 Nl-C4-C6 C6-C4=05 Nl-C4=05’ Nl=C2=03 H-C-H
“Distances r, in A, angles L a in degrees, with 2a error limits. bFrom ab initio calculations, see text. ‘Dependent parameter.
TABLE 3 Interatomic distances, vibrational amplitudes and harmonic corrections (without non-bonded distances involving hydrogen atoms) of CH,C(O)NCO Parameter
Distance
QED)
l(spectr.)
Ar=r,-r,
C-H C2=03 c4=05 Nl=C2 Nl-C4 C4-C6 N1...05 C2*..C4 N1...03 05**.C6 Nla**C6 C2**.05 o3..*c4 C2*..C6 03*..05 03*..C6
1.09 1.16 1.20 1.21 1.41 1.50 2.32 2.35 2.35 2.39 2.41 2.84 3.46 3.57 3.73 4.73
0.078”
0.078 0.037 0.037 0.039 0.051 0.050 0.064 0.104 0.044 0.064 0.080 0.170 0.133 0.065 0.247 0.091
0.012 0.007 0.005 0.005 0.003 0.005 0.004 -0.003 0.010 0.006 0.003 -0.008 -0.002 0.004 -0.011 -0.001
“Not refined.
0.037”
1
0.058( 16)
0.070( 6) I
>
0.11 ( 3) 0.11 ( 3) 0.21 ( 8) 0.08 ( 3)
364 TABLE 4 Rotational constants (GHz) of CH, C (0)NCO
A
B6(exp)”
B6 (exp)
B; (talc)
10.773160(25)
10.78854 (230) 2.22190 ( 12) 1.86298( 5)
10.78920 2.22186 1.86303
B
2.222693( 5)
c
1.862934( 5)
“Ref. 2.
with a diagonal weight matrix and scattering amplitudes and phases reported by Haase [9] were used. The relative weight of ED and MW data was adjusted until the rotational constants were reproduced within their estimated uncertainties. Assuming planarity of the heavy atom skeleton and C,, symmetry for the CH, group, eleven geometric parameters are required to describe the geometric structure of this compound. Even a joint analysis does not allow unique determination of all geometric parameters. The C-H and the double-bond lengths C2=03, C4=05 and Nl=C2 are especially highly correlated, and it is not possible to fit these bond lengths simultaneously. For this reason, (C=O),, and Nl=C2 were refined, and C-H and the difference dCO= (C4=05) - (C2=03 ) were constrained to values which are intermediate between the two ab initio results (HF and MP2). An estimated uncertainty of _+0.01 A for dC0 is taken into account in the error limits of all other geometric parameters. Vibrational amplitudes for closely spaced distances were collected in groups and badly determined amplitudes were fixed at their spectroscopic values. The final results of the joint analysis are collected in Table 2 together with the ab initio results. DISCUSSION
The electron diffraction experiment confirms the interpretation of the microwave and IR spectra with respect to the cis conformation of acetylisocyanate. From the IR spectrum, which is the most sensitive method for observing a possible trans form, we estimate a contribution of less than 3% of this second conformer. According to the ab initio calculations the C=O vibration of this higher energy form would occur at a higher frequency (d Y= 29 cm-‘) relative to the C=O vibration of the cis conformer. In this region no absorption is observed in the experimental gas-phase spectra. Thus, we estimate an energy difference between the two conformers of dE=,?&,,-&,> 2.0 kcal mol-‘. Ab initio calculations predict dE values of 2.4 kcal mol-l (MP2/6-31G*) and 3.8 kcal mol-’ (HF/6-31G*), respectively. Both values are in accordance with the experiments. Acetylisocyanate is the only carbonyl isocyanate of the type XC( 0)NCO whose conformational properties in the gas-phase have been
365
studied, for which only the cis form is observed. In the compounds with X = F, Cl or Br mixtures of cis and trans conformers were found, with the amount of the trans structure increasing in the sequence X=F (30 (8) % ) [lo], Cl (75(8)%) [l] andBr (92(5)%) [ll].Weareunabletogiveastraightforward explanation of this influence of the substituents X on the conformational properties of these compounds. Structural parameters in the three carbonyl isocyanates with X=CH3, Cl and F do not differ drastically. The Nl-C4 single bond in acetylisocyanate is slightly longer (1.413( 7) A) relative to those in the halogen derivatives (1.384(6) A for X=Cl and 1.386(8) A for X=F). This value is much shorter than the N-C bond length (1.460 A) estimated in the MW study [2]. The bond angle at nitrogen in isocyanates RNCO is known to depend strongly on the electronegativity of the substituent R, increasing from 118.2 (9) ’ for R = Cl [ 12,131 to 159.6( 2) ’ for R= SiH, [ 141. No such strong dependence on X is observed for the XC(O)NCO compounds. The C2=Nl-C4 angle in CH$ (0)NCO is only slightly larger (128.2 (13) ’ from the joint analysis and 126.1’ from the MW study [2] ) than that in the cis form of FC (0)NCO (124.3 (15) ’ ) [lo]. The corresponding angle in the trans conformer of ClC(O)NCO (127.1(16)“) [l] is intermediate. It is interesting to note, however, that according to the ab initio calculations this CNC angle is always smaller by about 8-10” in those conformations, which have the vicinal double bonds cis to each other, compared with those in the trans forms. ACKNOWLEDGEMENTS
H.G.M. and H.O. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, C.O.D.V. gratefully acknowledges Prof. Dr. Mult. A. Haas (Ruhr-Universitit Bochum, Germany) for his stimulating contributions and helpful development of this work, and Mr. G. Bollmann for valuable help with the experimental work. C.O.D.V. also thanks the Consejo National de Investigaciones Cientificas y TQcnicas (Conicet ), Facultad de Ciencias Exactas (UNLP) and Fundacion Antorchas, Republica Argentina, for financial support.
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