An ab initio conformational analysis of methyl vinyl sulfone (CH₂CHSO₂CH₃) has been carried out. Molecular geometry optimizations have been performed at the HF and MP2 levels of the theory. Relative energies of the stationary points have been determined by using different approaches, including electron correlation corrections up to the third order. The IR and Raman spectra of the liquid have been measured and a vibrational assignment is proposed. The height of the barrier to the methyl group internal rotation has been estimated. Theoretical calculations and vibrational spectra have shown that the predominant conformation of methyl vinyl sulfone has the C=C bond eclipsed with one of the S=O bonds. Similar eclipsed forms have been found in vinyl fluoro sulfone, vinyl chloro sulfone and divinyl sulfone by ab initio HF calculations.Facultad de Ciencias Exactas