C-O And C-S Bonds: Stability, Bond Dissociation Energies, And Resonance Stabilization

The structures and energies of the compounds X-C(=O)OH, X-C(=S)OH, X-C(=O)SH, and X-C(=S)SH, where X = CH3, NH2, OH, and F, were obtained at the MP2/6-31+G(2d) theoretical level, and energies were calculated at the MP3/6-311++G(2d,p), CBS-4, and G2 theoretical levels. These data show that X-C(=O)SH is preferred over X-C(=S)OH by 5 kcal/mol with X = CH3, and the preference increases to 12 kcal/mol with X = F. The C-O(H) bond dissociation energies were greater than the C-S(H) energies by approximately 30 kcal/mol and were only weakly affected by the nature of X attached to the carbonyl carbon. Calculated bond dissociation energies reveal that a C=O bond is significantly stronger than a C=S bond (by about 40 kcal/mol). The calculated atomic charges and bond orders demonstrated that a double bond to oxygen had a much larger effect on the attached carbon than did a single bond to oxygen. The C=O covalent bond orders were about 1.2, in accord with their considerable ionic character, while the C=S covalent bond orders were close to 2.0. Differences between the MP2/6-31+G(d) charge density for the ground state and the bond rotation transition state were used to investigate the nature of the electronic reorganization that occurs during bond rotation. This approach revealed that intramolecular charge transfer was greater for C-N bond rotation than for C-O bond rotation and greater for C-O rotation than for C-S rotation. Furthermore, polarization was greater for C=S than for C=O with all rotating groups by almost a factor of 2, in accord with the greater amount of charge transfer known to accompany bond rotation in thioamides as compared to amides. In all cases, the ground state exhibited charge transfer in the pi system to the terminal carbonyl oxygen or thiocarbonyl sulfur atom that was partially counteracted by charge transfer in the sigma system in the opposite direction