The low carbonyl group frequency of thiocarboxylic S-esters has previously been attributed by Baker and Harris to delocalisation involving participation of sulphur 3d orbitals. However, a study of the displacement of the carbonyl frequencies of S-esters by solvation or by the introduction of electronegative substituents leads to the conclusion that such electronic participation is absent. The possibility that other factors might be responsible for the low carbonyl frequencies of S-esters was examined by use of simple vibrational models for methyl acetate and S-methyl thioacetate. The calculated values indicate that the lower carbonyl group frequency of the S-ester arises more from vibrational perturbations due to the mass of the sulphur atom and the properties of the C–S bond rather than from any substantial change in the force constant of the carbonyl bond.
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Journal of the Chemical Society B: Physical Organic
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