Angular group induced bond alternation (AGIBA). A new face of substituent effect detected in molecular geometry

Angular Group Induced Bond Alternation is a new face of a substituent effect well documented by careful analysis of molecular geometry. The angular groups attached to monocyclic 6-pi-electron systems cause substantial changes to it-electron distribution in the ring. Different patterns are found for X-Y and X=Y types of groups. The overall effect is due to the rehybridization effect, the through space pi-electron interactions between a group and the ring, possibly also by other subtle effects, eg. the negative hyperconjugation. The effect is strongly conformational dependent. While the AGIBA is energetically weak, it is structurally strong enough that other interactions in monocyclic systems, eg. through - resonance type interactions do not reduce it. In polycyclic benzenoid hydrocarbons AGIBA works only locally while in non-cyclic pi-electron systems the effect does not work at all.