Dynamical charge transfer through hydrogen bonds: Hartree–Fock calculations of vibrational properties of formic acid monomer and cyclic dimer

In order to explain the observed changes in the vibrational spectra of formic acid from monomeric to cyclic dimeric form the SCF Hartree–Fock method has been applied to calculate minimum energy geometries, quadratic force constants, and net atomic charges. The set of ab initio quadratic force constants turns out to be acceptable and accounts for the observed vibrational changes. The ab initio interaction force constant between carbonyl stretching motions for the dimer accounts for what is considered to be an unusually large observed splitting of the C=O stretching frequencies in carboxylic acid dimers. Calculations suggest that such a large splitting originates from a dynamical charge transfer through the hydrogen bond. The possible existence of this phenomenon should in general be considered when dealing with the vibrational properties of hydrogen bonded systems.