Syn—gauche equilibrium of methyl vinyl sulphide studied by the combined use of microwave and raman spectroscopy, electron diffraction, and molecular force field and ab initio calculations

Abstract

Gaseous methyl vinyl sulphide has been reinvestigated using electron diffraction. Published results from ab initio calculations and microwave spectroscopy have been incorporated in the analysis. Raman spectra have been recorded at different temperatures, and a Urey—Bradley force field has been determined. A dynamic model has been used to account for the large amplitude motion due to the internal rotation. The relative merits of using such a model rather than one involving a small amplitude motion have been discussed. The energy difference, Δ E 0, between the syn and gauche (or a quasi-planar anti) conformation is determined to be 1.9(2) kcal mol −1 and 1.0(3) kcal mol −1, using a large and a small amplitude model, respectively. However, both models contain assumptions which affect the determination of Δ E 0, but not the magnitude of the standard deviations given in parentheses, which are derived from the least-squares analysis. Including these assumptions, Δ E 0 is most likely to be in the range 1.0–2.0 kcal mol −1. The most important structural parameters for the syn conformation are: r a(CC) = 1.343(1) Å, r a(C sp 2S) = 1.759(8) Å, r a(C sp 3S) = 1.795(8) Å, r a(C sp 2H) = 1.088(9) Å, r a(C sp 3H) = 1.106(8) Å, ∠ αCCS = 127.5(7)° and ∠ αCSC = 102.1(5)°. The torsion angles for the syn and gauche conformations are 0° and 135.8(6.5)°, respectively.