The electronic structure of heteroaromatic molecules. Part 3. A comparison of benzene, borazine, and boroxine

Abstract

Spin-coupled theory has been applied to the π electrons of the heteroaromatic molecules borazine and boroxine. Benzene is also reconsidered with more elaborate calculations than were used in previous work. The spin-coupled description of benzene, with semi-localized orbitals, is the same as that found previously. The close relationship between the spin-coupled wavefunctions and those from multiconfiguration SCF calculations is explored using large basis sets.

Marked differences are found between the spin-coupled descriptions of organic benzenoid systems and of these inorganic rings. Whereas the benzenoid aromatic molecules are stabilized by the mode of coupling the spins of the π electrons, this is not an important effect in borazine and boroxine.

The spin-coupled π orbitals for the boron heterocycles take the form of two distorted 2p orbitals on each nitrogen or oxygen atom. One orbital is very localized while the other shows very significant delocalization onto neighbouring boron centres. Any special stabilization of these two molecules must arise almost entirely from distortion effects in the orbitals. However, we find that borazine and boroxine are very similar, and we suggest that neither molecule has significant aromatic character.