On the correlation between bond-length change and vibrational frequency shift in hydrogen-bonded complexes:: A computational study of Y•••HCl dimers (Y = N2, CO, BF)

The H-Cl bond-length change and the harmonic vibrational frequency shift of the H-Cl stretch on formation of the linear isoelectronic Y center dot center dot center dot H-Cl complexes (Y = N-2, CO, EF) have been determined by ab initio computations at different levels of theory. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the H-Cl bond length from its equilibrium value in the isolated monomer. At the highest level of theory, blue shifts were obtained for BF center dot center dot center dot HCl and CO center dot center dot center dot HCl, while red shifts were obtained for FB center dot center dot center dot HCl, OC center dot center dot center dot HCl, and N-2 center dot center dot center dot HCl. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The widely believed correlation between the bond-length change and the sign of the frequency shift obtained on complexation is discussed and found to be unreliable.