The OH stretching frequency in LiClO 4·3H 2O(s) from ab initio and model potential calculations

Abstract

The “in-crystal” frequency of the anharmonic and uncoupled OH stretching vibration of HDO molecules in LiClO 4·3H 2O(s) has been calculated by quantum-mechanical ab initio and model potential methods and compared with the experimental infrared frequency from isotope-isolated HDO molecules. The effects of the nearest neighbours as well as of the crystalline environment have been investigated by the two computational techniques. In both cases, the one-dimensional potential for an anharmonic OH oscillator was constructed from point-wise energy calculations and the Schrödinger equation for the protonic motion in this potential well was solved by a variational procedure. In the ab initio calculations, vibrational potentials were constructed from RHF and MP2 type calculations of point-charge embedded ClO − 4·HDO and (Li +) 2·(ClO − 4) 2·HDO clusters using DZP and TZP basis sets. For the LiClO 4·3H 2O(s) crystal, the ab initio OH frequency is in close quantitative agreement with experiment when electron correlation by MP2 and the crystal field are included: 3537 cm −1 (MP2(TZP)) versus the experimental value of 3556 cm −1. Inclusion of the crystal field is essential and can in this crystal be satisfactorily represented by Ewald field-consistent point charges outside the hydrogen-bonded ClO − 4…HDO cluster. In the model potential calculations, analytical intermolecular pair potential functions from the literature were used in conjunction with an experimental intramolecular potential function for the OH stretching motion. The particular intermolecular model chosen here yields an absolute OH frequency 160 cm −1 below experiment. These calculations exemplify some of the difficulties encountered when employing analytical model potentials in vibrational studies.