Figure 2. Tripole models (left) which reproduces the EP maps (right) of isoelectronic molecules N2, CO and BF at the CCSD(T)/aV5Z level along with a schematic representation (left-bottom) of the EP surface of CO (distances in pm, 2n-pole units in e·Ån).
N2 has clearly three divided regions of negative-positive-negative EP in the whole space. CO having a weak dipole is practically three divided regions in the normal range of molecular interaction ranges (within 104 pm), while BF having moderate dipole moment has a small region (within 103 pm) of the negative EP around the F atom-end which is eventually surrounded by positive EP. Thus, BF behaves like a quadrupole in the near F atom-end, but like a dipole in the region far from the F atom-end as if F/B were positively/negatively charged. Here, one can note that the B in BF behaves like a singlet carbene with both positive and negative charges on B respectively along the bond-end and the radial directions of the B atom, while the C in CO partially shows such a behavior. For a comparative study, the tripole models for dihalogen atoms (F2, Cl2 and Br2) are also studied (Supplementary Information: Figure S1). It should be noted that these halogen cases show the opposite EP behaviors to the N2, CO and BF cases which give positive EP along the bond-ends but negative EP over the bond-cylindrical surface.