Van der Waals interaction and hydrogen bond effects on molecular vibrational frequencies

Abstract

The frequency shift of an AZ two-atomic fragment upon molecule transfer from gas to solution or matrix is investigated. The nearest neighbour ligands are considered to form a cluster. The coupling of the ν s(AZ) mode with the cluster low-frequency oscillators (ν Q) is taken into account. One such oscillator can be the ν σ(AZ…B) intermolecular stretching frequency oscillator in the H-bonded or charge transfer complex. A general expression for the ν s(AZ) frequency shift (Δω) is obtained, which allows for both the usual van der Waals interaction and the (ν s, ν σ) and (ν s, ν Q) mode couplings. When the former prevails, the relative frequency shift Δω/ω f is invariant to the Z → Z* (e.g., H → D) isotopic substitution. When the mode coupling prevails, the quantity Δω/ω 2 f is invariant what is typical for H bonds. If the H-bonded or charge transfer complexes are absent, the frequency shift Δω is proportional to α(ϱ AZ + ϱ L) −6 where α is the polarizability of the ligand, ϱ AZ and ϱ L are the van der Waals radii of AZ and ligand, respectively. The additional ν s(AH) frequency shift upon transfer of the AH…B complex from the gas to solution seems to be caused by (ν s, ν Q) mode coupling.