CHARGE FLUXES AND CHANGES IN ELECTRONIC-STRUCTURES AS THE ORIGIN OF INFRARED INTENSITIES IN THE GROUND AND EXCITED ELECTRONIC STATES

A theory for describing the mechanism giving rise to infrared (IR) int ensities in the ground and excited electronic states is presented, whi ch is applicable to modes representing the vibrational motions in the direction of the transition between two resonance structures having di fferent dipole moments, such as strongly IR active modes characteristi c of charged polyenes, protonated conjugated Schiff bases, and peptide s. A simple Hamiltonian based on a two-state model is used. The relati onship between charge fluxes giving rise to IR intensities and changes in electronic structures is examined in detail. The results derived f rom the simple model Hamiltonian are compared with those of ab initio molecular orbital calculations, which are regarded as solutions of mor e realistic Hamiltonians, for the pentadienyl cation, the heptatrienyl cation, the 2,4-pentadienylideneammonium cation, and N-methylacetamid e in the ground and excited electronic states.