EXTENDED MOLECULAR MECHANICS CALCULATIONS OF THERMODYNAMIC QUANTITIES, STRUCTURES, VIBRATIONAL FREQUENCIES, AND INFRARED-ABSORPTION INTENSITIES OF FORMIC-ACID MONOMER AND DIMER

The molecular mechanics simulation of infrared absorption spectra utilizing the effective atomic charges and charge fluxes as both the potential and the intensity parameters has been applied to formic acid monomer and dimer. The thermodynamic properties, optimized geometries, vibrational frequencies, and infrared absorption intensities are consistently derived from the potential models in which the difference between the monomer and the dimer is empirically taken into account. The large splitting between the A(g) and the B(u) carbonyl stretching frequencies of the dimer and the extension of the carbonyl bond on the dimerization are simultaneously reproduced by introducing two charge fluxes partial-q(o = c)/partial-r(c = o) and partial-q(o...H)/partial-r(c = o). The charge flux partial-q(OH)/partial-r(OH) is responsible for the large frequency shift and intensification of the OH stretching band on the dimerization.