One-Dimensional Adiabatic Model Approach for Calculating Progressions in Vibrational Spectra of Ion–Water Complexes

Many water–anion complexes of the form X– ·(H2O), where X– is a polyatomic anion, display a peak progression in the OH stretch region of the vibrational spectra with spacings of 65–85 cm–1. These progressions result from strong anharmonic coupling between the OH stretch and a low-frequency intermolecular rock vibration. In this study, we calculate these progressions in HCO2 – ·(H2O), NO3 – ·(H2O), and CS2 – ·(H2O) by use of a one-dimensional adiabatic model with rock potentials generated from ab initio energies and frequencies. The importance of using a geometry-dependent reduced mass in calculating the peak spacings is demonstrated. We find that the one-dimensional adiabatic model is more successful in predicting peak spacings in the spectrum of HCO2 – ·(H2O) than for NO3 – ·(H2O), for which the rock vibration is highly anharmonic.