The electronic structure of inorganic benzenes : Valence bond and ring-current descriptions

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X6H6 (X = C (1), Si (2)), X6 (X = N (3), P (4)), X3Y3H6 (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B3Y3H3 (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekule-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekule-like structures do not contribute. Of the heteronuclear compounds, only B3P3H6 (6) has some benzene-like features with a significant contribution of two Kekule-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planar geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current.