THEORETICAL-STUDIES ON THE DIAZACYCLOPENTADIENYLIDENES - AN ANALYSIS OF AROMATIC VERSUS NONAROMATIC SYSTEMS AND SINGLET VERSUS TRIPLET REACTIVITY

Ab initio molecular orbital calculations have been carried out on the various electronic states of 2,3- (6), 2,4- (7), 2,5- (8), and 3,4-dia zacyclopentadienylidene (9) at the fully geometry optimized 6-31 G le vel, with single point calculations being carried out at the MP2/6-31 G level. The calculated geometries are interpreted in terms of the de gree of occupancy and the nature of the pi and sigma-nonbonded MO's. A t the 6-31 G level the five pi-electron, pi,sigma-triplet states were calculated to be considerably lower in energy. At the MP2/6-31 G lev el, however, with 7 the six pi-electron singlet state is calculated to lie only slightly above the five pi-electron triplet (0.4 kcal mole-1 ), whereas with 8 and 9 the aromatic six pi-electron singlet states ar e calculated to be lower in energy (9.0 and 8.1 kcal mole-1). With 3 a nd 9 the aromatic six pi-electron sigma-triplet states lie only 3.6 an d 5.5 kcal mole-1 above the lowest energy states. It is concluded that , in general, the energy gained by having an electron in a lower energ y sigma-type MO instead of a higher energy pi MO effectively offsets t he energy gained by having an aromatic pi system. The results are disc ussed in terms of the observed chemistry of 6-9 and their substituted systems.