VIBRATIONAL DISPERSION INTERACTIONS IN VANDERWAALS COMPLEXES - EFFECT UPON STABILITY AND INFRARED-SPECTRA

The vibrational analogue of London dispersion forces has been studied for a generalized bimolecular complex, and calculations have been performed for a series of van der Waals complexes comprised of subunits CO2, C2H4, and HCN. We present a generalized nonempirical perturbational methodology for evaluating the second-order stabilization resulting from the correlation between instantaneous monomeric vibrational motions using the harmonic approximation. We find that these second-order energies are typically on the order of 0.01 cm-1, with the relative ordering (CO2)2 > (CO2-HCN) > (HCN-C2H4) > (C2H4)2. In conjunction with first-order resonance energies, the shift in monomer vibrational energy levels upon complexation is evaluated. We conclude that although the effect of vibrational dispersion upon the stability of a complex is quite small, its effect upon infrared spectra can be potentially significant. The formalisms for electronic and vibrational dispersion are compared and contrasted. The limitations of this nonempirical scheme for the calculation of vibrational dispersion energies are discussed in light of the approximations that are needed to make the calculations tractable and applicable to large systems.