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 (Ag) and antisymmetric (Bu) 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.