AB-INITIO INTERPRETATION OF CONFORMER STABILIZATION THROUGH S-CENTER-DOT-CENTER-DOT-CENTER-DOT-O AND C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-OBONDING IN THE ACETYL DERIVATIVES OF 2 REPRESENTATIVE HETEROCYCLIC METHINE BASES

Ab initio calculations (STO-3G and 3-21G basis sets) have been perfor med on 2 and 3 to determine the most favorable structure and to provid e an estimate of the barrier to internal rotation of the acyl group. T he Z configuration is preferred in 2, the E configuration in 3, with c alculated barriers to internal rotation of 79.9 and 68.7 kJ/mol, respe ctively, at the 3-21G level. The wave functions from the 3-21G* calcu lations are analyzed with the theory of atoms in molecules. The identi fication of bond critical points characteristic of a closed shelf inte raction establishes the existence of a weak bond between S ... O in 2 and between H ... O in 3, for the preferred configurations. The energy required to break this bond as well as the loss of extended conjugati on throughout the hetero ring and its side chain are responsible for t he asymmetry in the barrier. These findings provide a theoretical expl anation for experimental observations on this class of molecules in wh ich one conformer is preferred to any other and only one crystal struc ture is identified.