4π-Electron B-N Monocycles: Stability and (Anti)aromaticity.

This is a theoretical (DFT) study of the impact of electronic structural changes, induced by B-N/C-C isosterism, on two basic properties of 4π-electron antiaromatic system, that is, stability and antiaromaticity. The main driving force for the nonplanarity of B2N2 rings is electrostatic energy, and that for a ring with one B-N unit is the relief of Pauli repulsion. The charge-separation instability, inherent for a 1,3-B,N relationship, turns the ground state of the BCNC system to an aromatic triplet, which is less stable than the isomeric BNCC system, mostly because of larger Pauli interactions. The alternating BNBN connectivity is favoured primarily by orbital interaction energy and, secondarily, by better electrostatic attraction. The C--C → B-N substitution weakens the antiaromatic character, except that for a 1,3-B,N relationship, which results in increased antiaromaticity in the closed-shell state relative to that of cyclobutadiene.q [ABSTRACT FROM AUTHOR]