Azabenzenes (azines) — The nitrogen derivatives of benzene with one to six N atoms: Stability, homodesmotic stabilization energy, electron distribution, and magnetic ring current; a computational study

Abstract

The twelve azabenzenes — the species formed by replacing CH groups of benzene by N — were examined computationally (with emphasis on the six contiguous-nitrogen species). MP2(fc)/6-31G*, B3LYP/6-31G*, CCSD(T)/6-31G*//MP2(fc)/6-31G*, and G3(MP2) calculations were used to probe kinetic and thermodynamic stabilities, homodesmotic ring-opening energies, electron distribution (bond orders and lengths, Bird index), and NICS values, primarily to discern where the transition occurs from reasonably stable to highly fragile species. Some azabenzenes with three and those with four or more nitrogens are thermodynamically unstable, but kinetic stabilities indicated that, with the possible exception of hexaazabenzene, all the azabenzenes should be isolable, although some with four nitrogens might decompose at room temperature, and pentaazabenzene would require matrix isolation techniques. Although homodesmotic ring-opening reactions showed an almost monotonic decrease in the stability of the cyclic relative to the open-chain species (in appropriate ring-opening reactions) on going from benzene to hexaazabenzene, bond-order variation and NICS suggested this may be because of increasing destabilization by nitrogen lone pairs rather than because of a decrease in electron delocalization (in aromaticity). Key words: azines, azabenzenes, pentazine, hexazine, ab initio, DFT, aromaticity.