THE INFLUENCE OF LONE-PAIR REPULSIONS ON C-C BOND LENGTHS - A CRITICAL-EVALUATION OF THE EXPERIMENTAL AND THEORETICAL EVIDENCE

X-Ray structural data, as well as semiempirical and ab initio molecular orbital calculations, reveal no systematic and substantial difference between the C-C bond lengths of cis and trans 1,2-diketones. Additional results on various conformations of 1,2-diimines and 1,2-dithiones follow the same pattern. Therefore, lone-pair repulsions cannot be implicated in the observed lengthening of C-C bonds in isatin and several related molecules. Conjugation in these systems occurs peripherally avoiding the participation of the central C-C bond. Negative hyperconjugative interaction between the oxygen lone pairs and the adjacent C-C-sigma* orbital is suggested to be the principal reason for the relatively long C-C bond in diketones. This effect is found in both the cis and trans conformations.