Enhancement of IR and VCD intensities due to charge transfer

Donor-acceptor interactions such as the one between the Cl^- base and the N-H σ* acceptor orbitals encountered in the complexation of Cl^- counterions to the [Co(en)₃]³⁺ transition metal complex, have been shown to cause huge enhancement (between 1 and 2 orders of magnitude) of the VCD intensities of N-H stretching modes. This effect has been fully analyzed, and could be attributed to increased charge flow from the Cl^- donors when the N-H bonds become stretched. The transfer of charge counteracts the movement of negative electronic charge that happens along with the motion of the H nuclei, effectively reversing the electronic part of the electric dipole transition moment (EDTM) in the direction of the charge flow (z, say), and of the magnetic transition dipole moment (MDTM) in the perpendicular direction. The consequences for the IR and VCD intensity follow: IR intensity is strongly increased if the EDTM is polarized in the z direction, e.g. in A₂ modes, but not so much if it is polarized in the xy plane (E modes), the VCD is strongly enhanced if the EDTM and MTDM are polarized in the xy plane (in E modes), but less so when they are polarized in the z direction (in A₂ modes). The explanation holds generally for complexation phenomena of this sort, including the donor-acceptor part of hydrogen bonding interactions, e.g. with solvent molecules.