Electronic Structural Analysis of the Infrared Intensity Generation and Enhancement Occurring upon Formation of the Bis(Pyridine)Iodonium Cation

Behavior of electrons is often essentially important for infrared intensities of strongly infrared-active vibrational modes, and elucidating its features is helpful in deepening our understanding on the electronic structural properties of the concerned molecular systems. Here, such an analysis based on electron density features is carried out for the bis(pyridine)iodonium cation (the pyridine‧‧‧I+‧‧‧pyridine complex), a representative system containing a three-center four-electron (3c4e) halogen bond. It is shown that, for the infrared intensity generation and enhancement of two vibrational modes that occur upon formation of this complex, vibration-induced partial transfer of electron density (called charge flux) plays an essential role. It is also shown that the effective electric charge of I+ in this complex is 0.6–0.7 e, which is significantly less than 1 e of an isolated I+ ion, suggesting that the 3c4e halogen bond of the N‧‧‧I+‧‧‧N part has a partially covalent character. The extent of anisotropy in the occupancies of electrons in the p orbitals of the outer-most shell of I+ is ~0.8 in the mixing ratio of px2py2pz0. Similarities and differences with the case of the protonated complex are also discussed.