CO stretch mode splitting in the formic acid dimer: Electrostatic models of the intermonomer interaction

Abstract

The physical origin of the large (74 cm −1) splitting between the symmetric ( A g) and antisymmetric ( B u) components of the CO stretch mode in the formic acid dimer has previously been attributed to tautomerism effects, transition dipole—dipole coupling, or dynamical charge transfer through the hydrogen bonds. We show that an electrostatic model involving atomic charge—charge interactions can account for a splitting of 56 cm −1, provided the atomic partial charges are allowed to vary in magnitude during vibrational motion. The charges and charge derivatives have been obtained from ab initio Hartree—Fock calculations up to the 6-31G** level. An additional 13 cm −1 of the remaining discrepancy in the splitting of 69 cm −1, compared to the observed value of 74 cm −1.