Abinitio interpretation of conformer stabilization through bonding in the acetyl derivatives of two representative heterocyclic methine bases
Abinitio 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 shell interaction establishes the existence of a weak bond between [Formula: see text] in 2 and between [Formula: see text] 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. Keywords: nonbonded sulfur–oxygen interaction, closed shell interaction, hydrogen bonding, 3-21G* basis set, theory of atoms in molecules.