To gain a deeper insight into the electronic structures of 3a and 3b, a topological analysis of the electron density,  (e × rBohr -3 ), was performed using the quantum theory of atoms in molecules (AIM) method based on the results of density-functional theory (DFT) calculations conducted at the PBE0/6-311G(d,p)//B97X-D/6-31G(d) level of theory. The following discussions are focused on the results for 3a, shown in Figure , as almost identical conclusions were confirmed with 3b (Figure ). Figure depicts the bond paths (solid lines connecting atoms), bond critical points (BCPs; green dots), and the contour plots of the Laplacian of , ∇ 2  (e × rBohr -5 ), through a plane defined by the C1, B, and N3 atoms. For BCP1 to BCP8, values of , ∇ 2 , the delocalization indices (BCP), and energy densities H (Eh × rBohr -3 ) are also shown.