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

Ab initio calculations (STO-3G and 3-21G* basis sets) have been performed on 2 and 3 to determine the most favorable structure and to provide an estimate of the barrier to internal rotation of the acyl group. The Z configuration is preferred in 2, the E configuration in 3, with calculated barriers to internal rotation of 79.9 and 68.7 kJ/mol, respectively, at the 3-21G* level. The wave functions from the 3-21G* calculations are analyzed with the theory of atoms in molecules. The identification of bond critical points characteristic of a closed shelf interaction 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 conjugation throughout the hetero ring and its side chain are responsible for the asymmetry in the barrier. These findings provide a theoretical explanation for experimental observations on this class of molecules in which one conformer is preferred to any other and only one crystal structure is identified.