CONFORMATIONAL-ANALYSIS OF CARBONYL AND THIOCARBONYL ETHYL-ESTERS - THE HC(=X)Y-CH2CH3 (X, Y = O OR S) INTERNAL-ROTATION

The Y-CH2CH3 internal rotation in the HC(=X)YCH2CH3 (X, Y = 0 or S) series of molecules was studied by the ab initio SCF-MO method using 3-21G and 3-21G + d(zeta = 0.65S) basis sets. Energies and structures of several conformations of these molecules, determined by gradient geometry refinement, are reported and used to assess the effects of oxygen-by-sulphur substitution on molecular properties. The nature and relative importance of intramolecular interactions involving both the - CH2CH3 and the HC(=X) Y (X, Y = O or S) fragments are also discussed.