ORIGIN OF THE C=O STRETCH MODE SPLITTING IN THE FORMIC-ACID DIMER

The requirements of an electrostatic model of the intermonomer interac tions needed to reproduce the C=O stretch mode splitting, Delta v(C=O) , in the formic acid dimer have been reexamined in the light of their also being consistent with intermonomer Coulomb and van der Waals ener gies. The experimental value of Delta v(C=O) has been reevaluated and is determined to be 71 cm(-1). A complete cubic and quartic ab initio anharmonicity correction is found to be 9 cm(-1). The remaining harmon ic component of Delta v(C=O) can only be reconciled with the ab initio intermonomer Coulomb energy and the nonnegligible van der Waals energ y if atomic dipoles are added to the charges and the charge fluxes of the electrostatic model.