COMPARISON OF THIOL AND SELENOL ESTERS WITH OXY ESTERS IN SPECTROSCOPIC PROPERTIES

Abstract

The infrared and ¹³C nmr spectra of the title compounds are reported in relation to the weights of the various resonance structures that describe the ester in question. The infrared spectra of thiolesters show a somewhat reduced response to change in substituent in Ph[sbnd]CO[sbnd]S(O)[sbnd]Ar than oxyesters, consistent with the view that sulfur does interact with carbonyl, but less so than oxygen. The ¹³C shieldings for CO of thiol- and selenoesters are similar to ketones, and quite unlike oxyesters. These chemical shifts (corrected for anisotropy effects) lie in the reverse order than expected on the basis of electronegative effects, i.e. 0 < S < Se. However, this order is consistent with the inhibition of charged resonance forms, (i.e. C⁺ - O⁻) for oxyesters, in agreement with infrared and chemical reactivity data. In thiol- and in oxyesters, reduced lone pair delocalization into the Ph[sbnd]CO[sbnd]S(O)[sbnd]Ar ring is evident by ¹³C shieldings, which suggests that both types of compounds have lone pairs delocalized into carbonyl. The response of the carbonyl ¹³C to hydrogen bonding solvents is in disagreement with the supposed importance of resonance forms such as R[sbnd]C(O⁺)[sbnd]S⁻R.