ABINITIO STUDY OF SIGMA-EFFECTS AND PI-EFFECTS IN BENZENES FUSED TO 4-MEMBERED RINGS - REHYBRIDIZATION, DELOCALIZATION, AND ANTIAROMATICITY

The geometries of benzene incrementally fused to cyclobutenes, 3,4-dimethylenecyclobutenes, and cyclobutadienes are investigated with ab initio SCF-MO methods and by natural bond orbital (NBO)/natural resonance theory (NRT) analysis. These systems show various degrees of bond alternation in the six-membered ring with fused bonds elongated and bonds adjacent to the site of annelation contracted compared to those of benzene. NBO analysis reveals significant strain in the sigma-frame as evidenced by strong rehybridization at the annelated carbon centers. This effect accounts, in part, for the bond alternation in the cyclobutadieno-fused benzenes, but has negligible influence on that of the cyclobuteno- and 3,4-dimethylenecyclobuteno-fused analogs. The relative contributions of the two Kekule resonance forms are assessed by NRT analysis and are found to be in accord with the postulate originally presented by Mills and Nixon. Our results suggest that the principal source of bond alternation is either hyperconjugation (in cyclobuteno-fused benzenes) or conjugation (in benzenes annelated to 3,4-dimethylenecyclobutene or cyclobutadiene). In all cases, deletion of these delocalizing interactions from the wave function leads to a more symmetrical benzene geometry.