Intramolecular Interactions in the Water Molecule: The Stretch–Stretch Interaction Force Constant of Water Molecules in Hydrogen-Bonded Systems

Abstract

A calculation is described for determining water molecule $F_{\mathrm{rr}}$ force constants from observed HOD, H₂O, and D₂O vibrational frequencies. The calculations for water in the vapor and in the hydrates MnCl₂·2H₂O, FeCl₂·2H₂O, CoCl₂·2H₂O, and CuCl₂·2H₂O indicate a large increase in $F_{\mathrm{rr}}$ with hydrogen bonding. Various contributions to $F_{\mathrm{rr}}$ are considered, and its increase is attributed to an enhancement of the effective charge on water hydrogens with increased hydrogen bonding.