Calculated structures,relative energies and electrostatic potentials of some tetraaza cyclic systems

A computational analysis of two tetraazapentalenes, a tetraazacyclooctatetraene, and some of their dinitro and difurazan derivatives is reported. Optimized structures were obtained at the 3-21G level with the ab initio SCF GAUSSIAN-82 procedure, and these were then used to compute STO-5G molecular electrostatic potentials. The tetraazapentalenes were found to have a high degree of electron delocalization, and to be much more stable than the tetraazacyclooctatetraene, which has localized double bonds. However the difurazan derivatives show a complete reversal of these relative stabilities. In all of the molecules, there are strong negative electrostatic potentials associated with the doubly coordinated nitrogens, indicative of basic character. These become weaker as other portions of the system become more electron withdrawing. A buildup of positive potential is observed above the bond between the triply coordinated bridgehead nitrogens in the tetraazapentalenes; this should be an attractive site for the initial approach of a nucleophile. The positive buildup becomes progressively stronger in going to the dinitro and the difurazan derivatives.