The contributions of each element to every occupied π orbital of benzene, carborazine, and borazine calculated by Mulliken method are listed in Table . The contribution of the two low-lying orbitals (HOMO-1 and HOMO-2) of carborazine is dominated by N followed by C atoms, while the contribution of B atoms is very small. This is mainly due to the significantly lower energy of the 2pz orbitals of N and C relative to that of B (where z denotes the axis perpendicular to the molecular planes), so they naturally contribute more to the low-lying π orbitals. However, the 2pz orbital of B contributes obviously (more than one third) to the HOMO orbital. Therefore, on the whole, the 2pz orbitals of B, C, and N atoms are all heavily involved in the occupied π orbitals, which inevitably leads to significant six-center conjugation in the carborazine. By comparison, B only participates slightly in the lowest HOMO-1 orbital in borazine, and its participation in the two degenerate HOMO orbitals is even smaller. This observation implies that the global delocalization of π electrons in borazine should be conspicuously more difficult than that in carborazine and benzene.