GEOMETRICAL CONSEQUENCES OF INTERMOLECULAR HYDROGEN-BOND FORMATION IN THE FORMIC-ACID AND ACETIC-ACID DIMERS FROM AB-INITIO MO CALCULATIONS

Ab initio molecular orbital calculations on monomer/dimer formic and acetic acids have been performed with the 6-31G* and 6-31G** basis sets, including the electron correlation using second-order Moller-Plesset perturbation theory. Vibrational frequency analyses confirmed the stability of all the computed structures. The computed geometrical changes in dimeric formic and dimeric acetic acids, as compared to their monomers, are in good agreement with the notion of resonance-assisted intermolecular hydrogen bond formation. Trends in the structural changes obtained from electron diffraction were in general confirmed by the calculations, although they failed to predict the difference in the C-C bond length observed in the monomeric and dimeric forms of acetic acid, However, the experimental change in the O-H bond length upon hydrogen bond formation seems definitely exaggerated. Taking into account our previous results on 2-nitroresorcinol and 2-nitrophenol, this change is now expected to be not larger than 0.02 Angstrom.