Conformations of ethyl esters versus thiolesters

Ethyl formate and other substituted ethyl formates exist in stable anti and gauche conformations about the C-O-C-C dihedral angle, according to microwave spectroscopic studies. Similar studies of ethyl thiolformates characterize stable gauche conformations about the corresponding C-S-C-C dihedral angle in every compound studied, but the anti conformation is found only in ethyl fluorothiolformate and chlorothiolformate. Ab initio calculations that include electron correlation via MP2 or the B3LYP density functional model have been carried out for ethyl and methyl formate and thiolformate and their fluoroformate analogs. These calculations reveal that the potential energy minima at gauche and anti C-O-C-C configurations are well developed in every case. However, although the gauche minimum for the C-S-C-C torsional angle is clearly defined, the potential function near the anti C-S-C-C configuration corresponds to a potential energy plateau rather than a minimum. III the case of ethyl fluorothiolformate, a modest well is predicted at the anti C-S-C-C configuration, in agreement with experimental results.