A study of the stable conformations of methylvinyl sulfoxide and sulfone by ab initio calculations

Journal of Molecular Structure (Theochem), 276 (1992) 167-173 Elsevier Science Publishers B.V., Amsterdam

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A study of the stable conformations of methylvinyl sulfoxide and sulfone by ab initio calculations Matti Hotokka” and Reijo Kimmelmab BDepartment of Physical Chemistry, Abe Akademi, SF-20500 Turku (Finland) bDepartment of Chemistry and Biochemistry, University of Turku, SF-20500 Turku (Finland) (Received 27 January 1992; in final form 26 February 1992)

Abstract The potential energy curves for the C-S torsion of methylvinyl sulfoxide and sulfone were calculated using ab initio methods (RHF/~2lG* and MP2/63lG*). In both cases two minima were found. In the case of methylvinyl sulfoxide the most stable conformation has the S = 0 group cis to the double bond. The other stable conformation has the lone-pair electrons of the sulfur atom cis to the double bond. The energy difference between the two minima is 6-16 kJmol-’ depending on the method used. In methylvinyl sulfone there are two equivalent minima with the S = 0 group cis to the double bond.

INTRODUCTION

The structural parameters and the conformations of the stable rotamers of methylvinyl sulfide have been widely studied, both experimentally [l-11] and theoretically [l&13]. The structure of methylvinyl sulfone has also been the subject of some experimental studies [l&16], whereas the conformations of methylvinyl sulfoxide have been sparsely studied [17]. For example, no structural parameters have been reported. Therefore it seemed of interest to study the effect of the oxygen atoms on the conformations of the stable rotamers of methylvinyl sulfoxide and sulfone as compared with those of methylvinyl sulfide. In this study the structural parameters and the potential-energy curve for the G-S torsion of methylvinyl sulfoxide and sulfone were calculated by using the semiempirical AM1 Hamiltonian [18]. We also used the ab initio program GAUSSIAN ss at the restricted HartreeFock (RHF) level with the 3-ZlG* [19] basis set and the second-order

Correspondence to: R. Kimmelma, Department of Chemistry and Biochemistry, University of Turku, SF-20506 Turku, Finland.

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@ 1992

ElsevierScience PublishersB.V. All rights reserved.

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Mraller-Plesset (MP2) model with the 6-31G* basis set in order to compare a few of the popular quantum-chemical methods. METHODS

Semiempirical methods, such as MNDO [20] or AM1 [18] have become standard tools for organic chemists in conformational analysis. One of the important advantages of the semiempirical methods is their applicability to a wide range of molecules, including very large ones for which more accurate calculations are not feasible. The results are quite reliable as long as the target molecules do not deviate appreciably from the set of model molecules used in the parameterization of the method. However, the semiempirical methods have a number of subtleties, which may affect the interpretation of the results. If, for example, the molecule contains heteroatoms the preferred geometries may be predicted erroneously [21-271. Practising theoretical chemists have to become familiar with the properties of the methods through testing. One of the purposes of this study is to gain an understanding of the behavior of the AM1 Hamiltonian. The standard ab initio methods also produce results to an accuracy which is characteristic of the method used. The behavior of the ab initio procedure is, however, better understood than that of the semiempirical methods. The performance of a model depends partly on the basis set and partly on the level of the theoretical model. In both cases it is generally well known how a certain improvement will affect the results and what type of model must be used to obtain the required accuracy. A split valence basis set like 3-21G [28,29] is a quite reliable method, but is also expensive. In many cases it is important to include polarization functions in the basic set, i.e. d functions on second-row atoms, resulting in a basis set like 3-21G* [19]. This will improve the flexibility of the electron distribution along the bonds and gives a better description of the charge polarization in the molecule. The properties of the 3-21G* basis set have been discussed extensively by Pietro et al. [19]. The RHF method was used in the present calculations. It is known to have many shortcomings because it is a one-particle model, in particular, when studying potential energy surfaces. Yet, the RHF method is normally adequate for studying conformational potential-energy surfaces. The semiempirical calculations were performed by using the AMPAC program [30] with the AM1 Hamiltonian [18]. The ab initio calculations were performed at the RHF/3-21G* and MP2/6-31G* levels using the GAUSSIAN 86 program [31]. The semiempirical calculations were performed on the VAX 8800 computer at the Computing Centre at Abe Akademi and the GAUSSIAN 86 calculations were done on the CRAY X-MP EA/416 computer at the Centre for Scientific Computing in Espoo.

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“\ /” /’ H 4z-p (‘EC

s3=04

/

H/c‘1H

,I

Fig. 1. Rotation of the MeSO group around the CS bond in methylvinyl sulfoxide. RESULTS AND DISCUSSION

A preliminary study of the potential-energy curve for the 8 torsion (the C=C-S-Me angle, Fig. 1).was performed using the semiempirical AMPAC method [30] with the AM1 Hamiltonian [18]. In these calculations the torsional angle 8 was varied in fixed steps and all other geometrical parameters were optimized at each point. A separate optimization of all parameters was performed at the principal minimum. In order to obtain comparable results, the energies are reported relative to the minimum conformational energy for each method. The relative energies are given in Table 1. The vibrational zero-point energy is not included. The geometry of the molecules at the principal minima are given in Table 2. A more accurate survey of the potential-energy curve was done using the TABLE 1 The relative energies (in kJ mol-‘) of methylvinyl sulfoxide and sulfone at different torsional angles (0) as calculated using the semiempirical AM1 Hamiltonian and the ab initio Hartree Fock method with the 3-21G* basis set and the MP2/6-31G* model

e(7 0

30 60 90 120 150 180 210 240 270 300 330

Methylvinyl sulfone

Methylvinyl sulfoxide AM1

3-21G*

10.9

31.6 27.9 23.3 16.1 14.7 21.6 27.7 13.8 0.3 8.1 28.8 35.5

8.3 5.0 4.8 6.9 9.5 9.1 4.2 0.2 1.8 7.7 11.4

631G*

10.0 21.7 (0)

AM1

3-21G*

6-31G*

13.6 10.1 5.9 1.6 0.3 4.2 7.5 4.2 0.3 1.6 5.9 10.1

24.7 24.7 18.5 4.4 0.6 13.1 26.5 13.1 0.6 4.4 18.5 24.7

15.2

19.2 (0)

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TABLE 2 The structures of methylvinyl sulfide, sulfoxide and sulfone molecules at the primary minimum as calculated using the semiempirical AM1 Hamiltonian and the ab initio HartreeFock method with the 3-21G* basis set and the MP2/631G* model for sulfoxide and sulfone and with the 44-31G basis set for the sulfide [12] Sulfide 44-31G

Bond length (pm) R cc 131.3 R cs 182.0 Rso R Shde 188.5 Bond angle (“) acc5 128.2 acsc acsO

102.4

Sulfoxide

Sulfone

AM1

3-21G*

631G*

AM1

3-21G*

6-31G*

Exp. [16]

132.7 172.3 159.9 175.7

131.3 116.7 149.3 179.3

133.3 178.6 151.0 180.9

132 175 163 167

131.4 174.0 143.9 175.5

133.4 117.0 146.8 178.2

134.7 177.4 142.9 177.6

122.5 99.1 103.9

119.3 96.4 106.6

119.0 95.0 107.4

123 113 112

121 102 108

120.0 102.7 107.8

118.4 103.2 110.0

program [31] and the 3-21G* basis set [19]. The optimization strategy was the same as for the semiempirical calculations. The relative energies are given in Table 1 and the geometrical parameters at the principal minima are given in Table 2. In both methylvinyl sulfoxide and sulfone the most stable conformation has the S=O group cis to the double bond (0 = 244.7’ for methylvinyl sulfoxide and 8 = 248.0’ for methylvinyl sulfone). The potential-energy curves are depicted in Figs. 2 and 3, respectively. In order to study the subtle details of the electronic structure of the species, we optimized their structure at the correlated MP2 level [32] of approximation by using the GAUSSIAN 86 [31] program and the 6-31G* basis set [33-351 at the critical points of the potential-energy surfaces. Slight improvements in the molecular structures and a significant lowering of the torsional barriers were found as compared to the RHF/3-21G* results. The sulfoxides turn out to be somewhat difficult to compute using ab initio methods with small basis sets. The 3-21G basis set predicts a zwitterionic electronic structure for the dimethyl sulfoxide (DMSO) molecule while the 3-21G* basis set predicts a more reasonable hypervalent electronic structure [19]. The calculated S=O bond distance in DMSO is reported to be 167.8 pm for the zwitterionic structure and 149.0 pm for the hypervalent structure, the latter being quite close to the experimental value of 147.7 pm. The present calculations give reasonable S=O bond distances in methylvinyl sulfoxide at the 3-21G* level of approximation. These calculations also give a plausible molecular and electronic structure, the calGAUSSIAN as

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0

Mol. Struct. (Theo&em)

120

160

Torsion

angle

240

276 (1992) 167-173

300

173

360

(deg)

Fig. 2. The potential-energy curve for the CS torsion of methylvinyl sulfoxide.

culated charges of the sulfur and oxygen atoms being + 1.5, and - 0.7, respectively. The 3-21G* basis set also reproduces quite well the experimental geometry, and the electronic structure is qualitatively correct according to these calculations, i.e. it agrees with the suggested simple model where the charged resonance structures O- S ++-O- and 0 = S’ -0 make

e-e M 0

200

100

Torsion

3-21G* AM1

I

300

angle

Fig. 3. The potential-energy curve for the GS torsion of methylvinyl sulfone.

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important contributions [36]. The quantitative details of the electronic structure are more difficult to obtain, as evidenced by the dipole moment value which is 1.633 D experimentally [36] and 2.3 D according to a calculation with the 3-21G* basis set. Even self-consistent-field (SCF) calculation using an extended basis set gives the value as 1.97D [37]. The structural parameters of methylvinyl sulfide, sulfoxide and sulfone are summarized in Table 2. When the structures obtained by different methods are compared, it can be seen that the largest differences are found for the S=O and S-CH, bond distances. Also, there is quite a large difference in the CSC bond angle. When these values for methylvinyl sulfone are compared with the experimental values it is found that the calculated 3-21G” values fit the experimental data best. This indicates that the bonding is correctly described. When the structural parameters of methylvinyl sulfide, sulfoxide and sulfone are compared, it can be seen that the CSC angle is surprisingly small in methylvinyl sulfoxide. It is of the same magnitude as in dimethyl sulfide [38]. This may result from different hybridization of the sulfur atom or the steric requirements of lone-pair electrons. Each of the molecules discussed here has two stable conformations. In methylvinyl sulfide the most stable one has the planar s-cis conformation [l-13]. According to electron-diffraction studies, this conformation is also stable in methylvinyl sulfone [16]. However, theoretical calculations give the most stable conformation of both methylvinyl sulfoxide and sulfone as the one with the S=O cis to the double bond (Figs. 2 and 3). In these conformations the S=O bond can interact with the double bond and thus stabilize these conformations. According to the present calculations the s-cis conformation (0 = 0’) is a transition state. A very small change in the Mulliken overlap populations was detected. Thus there is some discrepancy between the theoretical and experimental results. REFERENCES 1 R.E. Penn and R.F. Curl, J. Mol. Spectrosc., 24 (1967) 235. 2 J. Fabian, H. Krober and R. Mayer, Spectrochim. Acta, Part A, 24 (1968) 727. 3 C. Muller, W. Schafer, A. Schweig, N. Thon and H. Vermeer, J. Am. Chem. Sot., 98 (1976) 5440. 4 S. Samdal, H.M. Seip and T. Torgrimsen, J. Mol. Struct., 57 (1979) 105. 5 A.R. Katritzky, R.F. Pinxelli and R.D. Topsom, Tetrahedron, 28 (1972) 3441. 6 S. Samdal and H.M. Seip, Acta Chem. Stand., 25 (1971) 1903. 7 J.L. Derissen and J.M. Bijen, J. Mol. Struct., 16 (1973) 289. 8 S. Samdal and H.M. Seip, J. Mol. Struct., 28 (1975) 193. 9 E. Wyn-Jones, K.R. Crook and W.J. Orville-Thomas, Adv. Mol. Relaxation Proc., 4 (1972) 193. 10 B.A. Trofimov, V.M. Modonov, T.N. Baxhenova, B.I. Istomin, N.A. Nedolya, M.L. Al’pert, G.G. Efremova and S.P. Sitnikova, Bull. Akad. Sci. USSR, Div. Chem. Sci., (1979) 79.

M. Hotokka, R. KimmelmalJ. 11 12 I3 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

32 33 34 35 36 37 38

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D.G. Lister and P. Palmieri, J. Mol. Struct., 32 (1976) 355. J. Kao, J. Am. Chem. Sot., 100 (1978) 4685. R. Virtanen, Acta Chem. &and. B, 40 (1986) 313. E. Vajda, I. Hargittai and D. Hnyk, J. Mol. Struct., 162 (1987) 75. B. Nagel and A.B. Remizov, J. Gen. Chem., (1978) 1089. V.A. Naumov, R.N. Ziatdinova and E.A. Berdnikov, J. Struct. Chem., (1981) 382. SD. Kahn and W.J. Hehre, J. Am. Chem. Sot., 108 (1986) 7399. M.J.S. Dewar, E.G. Zoebisch, E.F. Healey and J.J.P. Stewart, J. Am. Chem. Sot., 107 (1985) 3902. W.J. Pietro, M.M. Francl, W.J. Hehre, D.J. DeFrees and J.A. Pople, J. Am. Chem. Sot., 104 (1982) 5039. M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899. S.P. McManus and M.R. Smith, Tetrahedron Lett., (1978) 1897. C.L. Combs and M.Rossie, Jr., J. Mol. Struct., 32 (1976) 1. H.J. Chiang and D. Worley, J. Electron Spectrosc. Relat. Phenom., 21 (1980) 121. K.Ya. Burstein and A.N. Isaev, Theor. Chim. Acta, 64 (1984) 397. T. Clark, A Handbook of Computational Chemistry, Wiley, New York, 1985. 0. Gropen and H.M. Seip, Chem. Phys. Lett., 11 (1971) 445. T.A. Halgren, D.A. Kleier, J.H. Hall Jr., L.D. Brown and W.N. Lipscomb, J. Am. Chem. Sot., 100 (1978) 6595. J.S. Binkley, J.A. Pople and W.J. Hehre, J. Am. Chem. Sot., 102 (1980) 939. M.S. Gordon, J.S. Binkley, J.A. Pople, W.J. Pietro and W.J. Hehre, J. Am. Chem. Sot., 104 (1982) 2797. Dewar Research Group and J.J.P. Stewart, QCPE 506, QCPE Bull., 6 (1986) 24. M.J. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachari, CF. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, L.R. Kahn, D.J. DeFrees, R. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fleuder and J.A. Pople, GAUSSIAN 86, Carnagie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. C. Moller and M.S. Plesset, Phys. Rev., 46 (1934) 618. W.J. Hehre, R. Ditchfield and J.A. Pople, J. Chem. Phys., 56 (1972) 2257. P.C. Hariharan and J.A. Pople, Theor. Chim. Acta, 28 (1973) 213. M.M. Francl, W. J. Pietro, W. J. Hehre, J.S. Binkley, M.S. Gordon, D. J. DeFrees and J.A. Pople, J. Chem. Phys., 77 (1982) 3654. D. Patel, D. Margolese and T.R. Dyke, J. Chem. Phys., 70 (1979) 2740. G.B. Bacskay, A.P.L. Rendell and N.S. Hush, J. Chem. Phys., 89 (1988) 5721. L. Pierce and M. Hagashi, J. Chem. Phys., 35 (1961) 479.