Semiempirical quantum-chemical calculations on the conformations of methyl formate and methyl thiolformate

Abstract

The cis, trans, and gauche conformations of the methyl esters of formic and thiolformic acids have been investigated by different semiempirical methods. Total geometry optimization (CNDO/2, MINDO/3) and bond angle optimization (PCILO, NDDO) have been performed for the O-alkyl ester. The S-alkyl ester has been studied by the MINDO/3 method at the total geometry optimization level and by CNDO/2 and PCILO methods at the bond angle optimization level. The influence of sulphur d orbitals on the optimized molecular geometry as well as on the magnitude and direction of the dipole moment vector has been investigated in the CNDO/2 framework. The total energy differences of the conformers are compared to the experimental and ab initio results. CNDO/2 and NDDO energy partitioning have been performed to obtain information on the origin of the cis— trans energy difference and of the rotation barrier. The extent of the lone-pair delocalization has been studied in the different conformations using localized molecular orbitals. Calculations have been performed on the staggered and eclipsed positions of the methyl group in the planar conformations of both esters.