We calculated Mayer bond order (MBO), which essentially represents effective number of shared electron pairs between two atoms, to characterize the bonding characters in benzene, carborazine, and borazine. The total MBO and the contributions of  and  electrons to it are shown in Table . From  contribution to MBO, it can be seen that C-C bonds in benzene exhibit more prominent  interactions than those in other molecules, which is in line with the well-known fact that benzene has strong  conjugation. The B-C, C-N, and N-B bonds in carborazine all shows more intensive  interaction than the B-N bond in borazine, indicating that overall  conjugation of carborazine may be stronger than that of borazine. It is worth to note that although the electronegativity of B and N atoms are very different, as shown by the no small  bond order, the sharing of  electrons between the two atoms cannot be underestimated. The LOL based only on π MOs, referred to as LOL-π, is a popular real-space function used to reveal the delocalization of π electrons in conjugated molecular units. The LOL-π isosurfaces of benzene, carborazine, and borazine in Figure The bond critical point (BCP) defined in atoms-in-molecules (AIM) theory is the most representative position of a chemical bond, and its properties are very useful for understanding bonding characteristics. Negative Laplacian of electron density (  ) and energy density (H), as well as eta index () larger than 1, usually imply a covalent bond; in addition, the more negative the    and H, as well as the larger the , the stronger the covalency. According to the data in Table , we find both C-C bond in benzene and C-N bond in carborazine are very typical covalent bond, B-C bond in carborazine has modest covalency, while N-B (also B-N) bond in carborazine and borazine shows most evident ionicity, which obviously is the consequence of the very large difference of electronegativities of the two elements. The data also suggest that the B-N (also N-B) bond in borazine shows stronger  interaction compared to that in carborazine.