On factors influencing the geometry of hypericin and its tautomers

Hypericin and its tautomers were subject of several recent molecular orbita l studies. It has been established that the dominant factor influencing the ir thermodynamic stability is the cyclic conjugation of the pi-electrons. W e now go a step further and show that pi-electron conjugation determines al so many geometric features of the respective molecules. In particular, the carbon-carbon bond lengths are well correlated with the pi-electron bond or ders of the benzenoid system pertaining to the conjugated domain of the res pective tautomer. Pauling bond orders are found to be more suitable than Co ulson bond orders.