On the basicity and π-electron delocalization of ‘hexaazabenzene’ N6 – Quantum-chemical studies

The HF, MP2, and DFT structures of neutral (N6) and monoprotonated (N6H+) hexazine, nitrogen analogues of benzene, were discussed. For neutral N6, the extended geometry-based HOMED (harmonic oscillator model of electron delocalization) index, being a measure of π-electron delocalization, is close to that of benzene. Protonation of one N atom in planar N6 has no important effect on the HOMED value. Contrary to benzene, planar N6H+ does not lose its aromatic character. π-Electrons of the ring are not engaged in the formation of the NH bond. The proton affinity (PA) of N6 seems to be considerably lower than that of benzene (by ca. 16 kcal mol−1 at the G2 level), indicating great NN lone pairs repulsion and electron-withdrawing effects of the N-aza groups (ca. 12 kcal mol−1 for each N atom).