The equilibrium structure of methyl pseudohalides: an ab initio study

Volume 189 (1992), number 3

CHEMICAL PHYSICS LETTERS

7 February 1992

The equilibrium structure of methyl pseudohalides: an ab initio study Tibor Pasinszki, Tamas Veszpremi Inorganic Chemistry Department, The Technical University ofBudapest, 1521 Budapest, Hungary

and Miklb

FehCr

Institut ftir Physikalische Chemie, Universitiit Base/, Klingelbergstrasse 80. CH-4056 Basel, Switzerland

Received 10 October 199 1; in final form 11 November 199 1

Ab initio calculations have been carried out on the molecules CH,NCX and CH,XCN (X=0, S and Se). It was found that these molecules all have a bent equilibrium structure. The relatively more rigid CH,XCN molecules can be sufficiently well described by small basis set Hartree-Fock calculations. The accurate description of the more flexible CHSNCX molecules, however, requires that at least partial allowance is made for the correlation energy. According to these calculations, CH,NCO, CHSNCS and CH$SeCN are the more stable isomers.

1. Introduction As pseudohalides find an ever increasing application in polymer chemistry as important precursors [ I], there is increasing interest in characterizing their structure by experimental and theoretical means. One of the most studied questions is how the vibrations and rotations affect the observed structure and how sensitive different spectroscopic methods are to detect structural changes. The different CH3NCX and CHsXCN (X=0, S, Se) compounds have diverse structures. Although some of these compounds have been studied by different spectroscopic methods in the past, a systematic study of these compounds has never been carried out to date. One of the aims of this work was to “characterize” the similarities and differences between these two isomer groups. Different spectroscopic studies have shown that alkyl cyanates, thiocyanates and selenocyanates have a bent frame. Microwave spectroscopy of methyl and ethyl cyanate [2,3], thiocyanate [4,5] and methyl selenocyanate [6] showed that the CXCN chain is unambiguously bent and the bond angle is rather small at the 0, S, and Se atom. The molecular frame

of these molecules is very rigid, the barrier to linearity in CH$CN for example is about 6300 cm-‘, as determined by MW spectroscopy [ 7 1. Infrared and Raman measurements also predict a similar strongly bent structure [8-lo]. Alkyl isocyanates, isothiocyanates and isoselenocyanates on the other hand, are much more flexible than their isomeric counterparts. Their structure has been under investigation by different spectroscopic and diffraction methods for the last few years and it became clear that in order to describe the experimental observations, the low-energy large-amplitude motions of the pseudohalide group have to be taken into account. The satisfactory analysis of the microwave spectrum of methyl isocyanate [ 111, isothiocyanate [ 121 and isoselenocyanate [ 131 was only made possible in the 1980’s using the so-called “quasi-symmetric top model” [ 141 which explicitly takes these large-amplitude motions into account. The C-N-C angle at the equilibrium geometry is 140.0” for CH,NCO, 150.9” for CH,NCS and 161.7” for CH,NCSe, the potential barrier to linearity is 928 cm-‘, 193 cm-’ and 25 cm-’ [ 1l-131. This potential barrier is so low for CH,NCSe that all vibra-

0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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Volume 189 (1992).

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CHEMICAL

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tional levels lie above this, hence this molecule is effectively linear [ 13 1. As a result of low energy large amplitude motions, the bond angle at the nitrogen atom in alkyl isocyanates and isothiocyanates is observed to be small in electron diffraction studies (“shrinkage effect”) [ 1% 171, although these molecules have a bent structure when these large amplitude motions are considered [ 16,17 1. Results from vibrational spectroscopy also provide some support for a bent C-N-C-O chain [ 18-201. Although the vibrational spectra of several different alkyl isoselenocyanates [ 2 1] and isothiocyanates [ 22-251 had been previously analyzed considering a rigidly bent C-N-C-X chain, it was first pointed out during the analysis of the vibrational spectrum of CH3NCS [26,27] that the spectrum can be better analyzed considering a pseudolinear chain. It was, however, impossible to find different conformers in the IR spectrum of ethyl and isopropyl isothiocyanates [ 27,281 which may perhaps be also explained by the fact that the vibrational spectrum of these molecules is not very much dependent on the angle at the nitrogen atom. There have been several ab initio calculations carried out on HNCO and HOCN with different basis sets and at different levels of theory [ 29-331. The HF calculations predict a bent structure for both molecules [ 291. In HNCO, the bond angle at the nitrogen atom is 10 ’ smaller with an STO-3G basis and 34’ larger with a 4-3 1G basis than the experimental value of 124’ [ 341. In calculations with different basis sets it was found that polarization functions need to be included in the basis in order to reproduce the experimental structure [ 30,3 11. On allowing for the correlation energy with second or fourth order perturbation theory or configuration interaction [ 3 l331, the experimental geometry can be well reproduced. On the basis of these calculations, the HNCO isomer is the more stable by about 100 kJ/mol. Calculations performed on the different isomers of HNCS similarly predicted the isothiocyanate to be the most stable structure [ 35,361. Although the hydrogen and halogen substituted pseudohalides have been relatively well studied by ab initio methods, there is little known about their alkyl substituted derivatives and the relative stability of their different isomers. From the methyl derivatives, only CH,NCO and CHJOCN have been 246

LETTERS

7 February

1992

studied, with an STO-3G basis [37] and with a 631G” basis with second order Mgller-Plesset perturbation theory (MP2) [ 311. These latter calculations predict that both of these molecules are bent and the isocyanate is more stable by 113 kJ/mol than the cyanate. The structure of thio- and seleno-cyanates and their isomers have not been studied by ab initio methods. The aim of this work was to perform calculations on CH,XCN and CH3NCX (where X = 0, S, and Se) in order to study their geometric structure and relative stability of the isomers. The dependence of the calculated structure on the basis and the level of theory was also investigated.

2. Calculations All the calculations in this work were carried out using the CADPAC quantum chemistry package [ 38 1. The geometry of each molecule was first optimized with different basis sets using the gradient method [ 391. The following basis sets were utilized in the calculations: 4-3 1G, 4-3 1G”, 6-3 1G, 6-3 1G (* ) and 6-3 1G”. On the selenium atom a [ 5s, 4p, 2d] contracted basis set was used [40]. The correlation energy was partially allowed for in some calculations by second-order Msller-Plesset perturbation theory.

3. Results and discussion The calculated equilibrium structures of methyl cyanate, thiocyanate and selenocyanate are given in table 1, the numbering of the atoms is shown in fig. 1. The calculated structure of these molecules are generally in good agreement with the measured microwave values. The C,-X-C2 angle is small and strongly decreases in the series CH,OCN, CH$CN and CH$eCN. The X-C2 bond length is smaller than X-C, and the difference between the two decreases in the series 0 > S > Se. There is relatively good agreement between the calculations using small basis sets, the experimental microwave values and for CH,OCN also with the values from an earlier MP2 calculation [ 3 11. This shows that the use of polarization functions and the inclusion of correlation energy hardly have any ef-

Volume

189 (1992),

Table 1 The calculated

number

CHEMICAL

3

and experimental

structure

of CH,OCN,

PHYSICS

CH,SCN

7 February

LETTERS

and CH,SeCN

1992

‘)

CHaOCN 4-31G

c,-x x-c*

1.465 1.293 1.144 1.072

C,-N

G-H, ‘G-Hz X&N GXG XGH, XV-b H,G& Hz’3I-L

1.077

experimental (MW) =’ 1.464 1.283 1.162 1.073 1.083 177.0 113.3 105.4

1.452 1.302 1.184

1.466 1.296 1.148 1.073 1.078 178.5 119.9 104.4 109.4 111.2 111.2 .206.644058

178.6 120.0 104.3 109.4 111.2 111.1 -206.433649

total energy

6-31G” MP2 b,

6-31G

178.4 113.5

110.6 107.3

CHpSCN 4-31G

q-x

6-31G

1.899 1.738 1.144 1.077 1.075 178.9 99.1 105.1 109.0 110.9 111.7 - 528.764087

x-c* C,-N

G-H, G-Hz XCzN GXG XGH, XCJ-b H,GHz WX% total energy

1.886 1.752 1.148 I .077 I .076 179.1 99.2 105.3 109.3 110.7 111.4 529.3 I8576

6-31G(*)

experimental (MW) *’

1.821 1.700 1.49 1.080 I .078 178.8 99.8 106.2 110.8 109.4 110.4 - 529.359018

I .824 I .684 1.170 1.073 1.086 179.8 99.0 105.4 109.7 110.3 ill.3

CH$eCN 4-31G(*)

c,-x x-c2 C2-N

G-H, G-Hz XCIN WG XGH, XCJ% H,GHz

H2GHz total energy

6-31G(‘)

1.922 1.831 1.144 1.078 1.073 179.6 98.0 104.8 108.2 110.6 113.9 -2527.906925

‘) Bond lengths in A, bond angles in deg., energies in au. b)Seeref. [31]. “‘Seeref. [2]. d)Seeref. [4]. “‘Seeref.

I .920 I .832 1.149 1.079 1.074 179.8 97.9 105.7 108.3 110.3 113.7 -2528.047971

experimental (MW) =’ 1.954 1.836 1.162 1.073 1.083 179.3 96.0 105.4 109.3 110.6 111.4

[6].

247

Volume 189 (1992),

number

3

a.

Fig. 1. The numbering of the atoms CHSNCX (X=0, S or Se).

CHEMICAL

PHYSICS

N=C=X

and (b)

feet because of the relatively rigid frame of these molecules. This rigid structure is also supported by the relatively large calculated potential energy barrier to linearity for CH,OCN, CH3SCN and CH,SeCN of 7036, 30975 and 33924 cm-‘, as obtained in this work ( 6-3 1G (*) /HF) . The calculated and experimental geometries of CHsNCO, CH3NCS and CH,NCSe are given in table 2, the numbering of the atoms is shown in fig. 1. As can be seen from table 2, the calculated structure, especially the Ci-N-C2 angle, depends very strongly on the basis set and the level of theory used. In case of CHxNCO, the use of polarization functions is required, in case of the other two molecules, partial allowance for the correlation energy also has to be made in order to reproduce the experimental bent structure. This is in agreement with the fact that the barrier to linearity is very small in these compounds, as discussed in the introduction. This barrier also significantly decreases in the series NC0 > NCS > NCSe, which also means that electron correlation is more important in the description of the potential energy surface for these isomers. In Hartree-Fock calculations, not only the calculated bond angles are in error but also some bond lengths, e.g. the N-C2 distance is substantially shorter and the C2-X bond is much larger than the experimental value. The C,-0 bond in CH3NC0 is longer than the N-C2 bond, although the opposite would be expected. When correlation energy is partially allowed for by the MP2 method, the calculated bond lengths and bond angles are much nearer to the experimental values. In the MP2 calculations, the C,N-C2 angle increases in the series NCO
7 February

1992

C-X angle from linearity is slightly larger than for the corresponding isomers but is always smaller than 10”. The N-C2 distance decreases slightly in the OS-Se series. The close relationship between the geometric and electronic structure of these molecules can be explained using the following scheme:

b.

in (a) CH,XCN

LETTERS

X=C=N /

/ HJC

H3C (a)

H&-N

(cl

= C-X

X-C=N H:C

(b)

(d)

Structures (a) and (c) look similar, (b) and (d) are different. According to the calculations, the structure of the molecules CH3NCX changes gradually from (a) to (b), the structure of the CH,XCN compounds from (c) to (d) in the series X=0, S and Se. The same tendency can be deduced qualitatively from the electronic structure of these molecules, as known from photoelectron spectroscopy [ 4 1,42 1. The experimental ionization energies of CH,NC, CH3NC0 and CH,NCSe and their correlation is shown in fig. 2. It can be seen that the interaction between the CH3NC entity and the selenium atom is small and as a reSUlt, the xNc iOniZatiOn energy iS hardly different between CH3NC and CH,NCSe. The other n-type orbital (x2) in CH,NCSe is mostly localized on the selenium atom and has a strongly nonbonding character. This makes structure (b) more likely. Alternatively, the n2 ionization energy in CH3NC0 is much smaller and the xl much bigger than 5cNc.This indicates that the interaction between the two orbitals is considerable and thus structure (a) is more likely. The calculations support this, as the s-type molecular orbitals are more localized on the N-C2 fragment in CH,NCSe than in CH,NCO and thus the electronic structure of CH3NCSe is more similar to CH,NC than to CH3NC0. This results in the decrease of the N-C* bond length and the increase of the C-N-C angle. Indeed, MP2 calculations predict a corresponding increase of the C-N-

Volume

189 (1992),

Table 2 The calculated

number

CHEMICAL

3

and experimental

structure

of CH,NCO,

PHYSICS

7 February

LETTERS

I992

CH3NCS and CHpNCSe ‘)

CH,NCO

C,-N N-C2

c,-x C,-H XCzN CiNCz NC,H H&Hz total energy

4-3lG

6-31G

4-3lG”

6-31G”

4-3 I G” MP2

1.419 1.161 1.188 1.080 180.0 180.0 110.0 108.9 -206.471100

1.422 1.163 1.192 1.081 180.0 180.0 110.0 108.9 - .206.681347

I .433 1.176 1.156 1.082 175.3 147.7 110.1 108.4 - -206.598058

1.437 1.179 1.158 1.082 175.2 143.6 110.0 108.4 -206.795772

6-31Gw MP2 b’

1.435 1.209 1.188

1.440 1.215 1.190

1.086 173.1 139.7 110.2 108.3 - -207.223696

172.3 137.6

experimental (MW) c’ 1.4507 1.214 1.166 1.101 170.3 135.6 106.8

CHxNCS

C,-N N-C2

c*-x C-H X&N GNC2

NC,H H&HZ total energy

4-31G

6-31G

1.419 1.155 1.646 1.079 180.0 180.0 109.8 109.2 - - 528.780852

1.423 1.155 1.670 1.080 180.0 180.0 109.6 109.3 - - 529.338876

4-3 lG#

6-3lG”

1.418 1.152 1.591 1.082 179.6 177.1 109.8 109.1

4-31G” MP2

1.421 1.150 1.607 I .082 179.5 177.8 109.5 109.4

- .528.909695

- .529.4321 II

-

experimental (MW) *’

1.427 1.206 1.562 1.087 174.1 146.1 110.1 108.4 529.471435

1.4288 1.207 1.5665 1.100 169.3 147.68 109.08

CHaNCSe 4-31G(*)

C,-N N-C2 c2-x C,-H X&N GNC2

NC,H HzCiHz total energy

1.420 1.154 1.767 1.079 180.0 180.0 109.7 109.3 2527.873055

6-31G(*)

1.421 1.157 1.767 1.080 180.0 180.0 109.7 109.2 -2528.010031

‘i Bond lengths in A, bond angles in deg., energies in au. “Seeref. [31]. “‘Seeref. [Ill. d’Seeref. [12]. e)Seeref.

C angle of 20.0” (experimental value 26.1’ [ 111) and a simultaneous decrease in the N-C2 bond length of 0.0 16 A (experimental value 0.0 19 8, [ 111). The strong effect of the N-C* distance on the C-N-C angle can also be exemplified by the results of STO-3G calculations, where an unexpectedly long N-C2 bond

4-3 1GMP2

experimental (MW) =’

1.415 1.193 1.666 1.087 179.0 159.7 109.9 108.7 -2528.544377

1.4323 1.195 1.717 1.100 180.0 161.7 109.5

[13].

length (1.235 A) and a non-linear C-N-C angle ( 124.5 ’ ) were obtained in this work. In the CHsXCN series, the x-type orbitals are localized on the NC2 fragment and the effect of this on the geometry is exactly opposite: the C-X-C angle decreases in the series OCN > SCN > SeCN. 249

Volume 189 (1992), number 3

CHEMICAL PHYSICS LETTERS

7 February 1992

References CH,NCO ___.I.

[ 11Ullmann’s encyclopedia of industrial chemistry, 5th Ed., Vol. Al4 (VerlagChemie, Weinheim, 1989) pp. 61 l-625. [ 2 ] T. Sakaizumi, H. Mure, 0. Ohashi and I. Yamaguchi, J. Mol.

CH,NC Cl$NCSe

1x2

Spectry. 140 (1990) 62.

[ 31 T. Sakaizumi, H. Mure, 0. Ohashi and I. Yamaguchi, J. Mol. Y

II

Fig. 2. Correlation of experimental CH,NCO, CH,NC and CH,NCSe.

13

15

ionization

I.E.

(cV)

energies in

The calculated relative stabilities (i.e. the difference in total energy), of different CH3NCX compounds with respect to the corresponding isomers were calculated at the Hartree-Fock level using a 63 lG(*) basis set. It was found that CH,NCO and CH3NCS are more stable isomers than CH30CN and CH,SCN by 98 and 23 kJ/mol, respectively. In the corresponding selenium compound, however, CH,SeCN is the more stable isomer by 100 kJ/mol. This is in good agreement with experimental observations, as CHjOCN readily isomerizes to CHJNCO [ 431, CH$CN isomerizes at heating only [ 441 whereas CH,SeCN does not isomerize [ 45 1.

4. Conclusion The structure of the molecules CHsXCN (X = 0, S and Se) can be well described using small basis sets but without including polarization functions or the effects of electron correlation. This is largely due to the rigid structure of these molecules. In order to describe the structure of the CHJNCX (X = 0, S and Se) however, the use of polarization functions is needed and at least partial allowance has to be made for the correlation energy. The most sensitive point in the geometry determination is the N-C, distance, which also has a major influence on the C-N-C angle. The chemical consequence of the differing geometrical structure of the isomer-pairs is their differing relative stability, which is well reproduced in the calculations.

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CHEMICAL PHYSICS LE-l-IERS

[ 301 C. Glidewell and C. Thomson, Chem. Phys. Letters 86 (1982) 340. [ 311 H.-G. Mack and H. Oberhammer, Chem. Phys. Letters 157 ( 1989) 436. [ 321 C. Glidewell and C. Thomson, J. Mol. Struct. THEGCHEM 104 (1983) 287. [ 331 J.H. Teles, G. Maier, B.A. Hess, L.J. Schaad, M. Winnewiser and B.P. Winnewiser, Chem. Ber. 122 (1989) 753. [34] L. Fusina and I.M. Mills, J: Mol. Spectry. 86 (1981) 488. [ 351 B. Bak, J.J. Christiansen, O.J. Nielsen and H. Svanholt, Acta Chem. Stand. A 31 (1977) 666. [36] H. Leung, R.J. Suffolk and J.D. Watts, Chem. Phys. 109 (1986) 289. [ 371 D. Poppinger and L. Radom, J. Am. Chem. Sot. 100 ( 1978) 3674.

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[38] R.D. Amos and J.E. Rice, CADPAC: The Cambridge Analytic Derivatives Package, issue 4.0, (Cambridge, 1987). [ 391 P. Pulay, Mol. Phys. 17 (1969) 197. [40] J.M. Lehn, G. Wipff and J. Demuynch, Helv. Chim. Acta 60 (1977) 1239. [41] H. Bock, S. Augen, P. Rosmus, B. Solouki and E. Weissflog, Chem. Ber. I17 (1984) 184. [42] T. Pasinszki, J. Rtiy and T. Veszpremi, J. Electron Spectry., in press. [43] K.A. Jensen, M. Due and A. Holm, Acta Chem. Stand. 19 (1965) 438. [44] P.A.S. Smith and D.W. Emerson, J. Am. Chem. Sot. 82 ( 1960) 3076. [45] W.J. Franklin and R.L. Werner, Tetrahedron Letters 34 (1965) 3003.

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