The conformation of alkyl sulfoxides

Parameters for sulfoxides used in force field MM1 were modified to be incorporated into force field MM2. The conformations of ten alkyl sulfoxides were then calculated using MM2 with these new parameters. The alkyl groups used were methyl, ethyl, isopropyl, and t-butyl. It was found that of the many possible conformations for these compounds, only one or two stable conformers exist, and that the number of these conformers agrees with the number of reported S-0 stretching frequencies in almost every case. No apparent correlation between the vibration frequency and the molecular structure was found. Although the conformation of thiane oxides has been the suliject of much experimental study, the conformation of dialkyl sulfoxides has received little attention. oki, Oka, and Sakaguchi' measured the S-0 stretching absorption frequencies of several simple sulfoxides and found that some showed one absorption maximum, whereas others showed two. They attributed this phenomenon to the rotational isomerism about the S-C axis and attempted to relate the absorption frequency to the conformation. They also pointed out that Cairns, Eglinton, and Gibson2 had made similar measurements and concluded that the multiplicity of the S--0 stretching frequency could not be explained in terms of the presence of conformers but was believed to be due to Fermi resonance: an overtone of some other vibration which occurs near one-half of the 6;-0 vibration frequency. A careful comparison of the papers published by the two groups showed that there was actually no inconsistency of interpretation of the spectra by the two groups. The second absorption peak reported by Cairns' group was at ca. 1020 cm-l, some 20-40 cm-' lower than the first one. Oki's group did not report an absorption in this range. The doublets observed by them were 7-10 cm-' apart and were observed in compounds, most of which were not studied by the other group. It became clear to us then that the doublets discussed by the groups referred to different pairs of absorption and that the doublets observed by Oki's group could very likely be due to the existence of rotational isomers as they concluded. Since a force field (MM1) for sulfoxides had been developed by Allinger and K~o,~ we thought we could shed some light on this problem by a systematic calculation of all stable conformations of the dialkyl sulfoxides. Just when calculations for this study using force field MM1 were completed, a new, improved force field, MM2, became a~ailable.~ Different parameters for the carbon and hydrogen atoms are used and certain calculational procedures are slightly modified in this new field. The MM2 force field gives improved results on a few selected compounds which could not be treated satisfactorily with force field MM1 due to their unique geometry. For most molecules studied, the two force fields give essentially the same results. In order to do calculations on sulfoxides using this new force field, the parameters for the sulfur and the oxygen atoms would have to be redetermined. We decided to take on this redetermination so that future calculations on sulfoxides may be done using force field MM2. Five pieces of experimental information were used to help determine the parameters: the geometries (bond length and angles) of dimethyl sulfoxide5 and of trans-1,4-dithiane-l,4dioxide6; the energy difference between 1 -axial