Knowledge Bank

The implications of H-bonding in halide-water complexes and its effects on infrared spectra have been of long standing interest, with studies focusing mainly on the OH stretching region. Until recently only the low-energy portion of the X$-$(H$2$O) spectra had been probed experimentally (X = F, Cl, Br).}}, 4952 (2003).}$,$}}, 571 (2005).} An interesting feature in this low-energy region, especially noticeable in the Cl$-$(H$2$O) spectrum, is the large intensity of the overtone for the out-of-plane bending mode. The potential and dipole moment surfaces along this internal coordinate are calculated at the MP2 level of theory (aug-cc-pVTZ basis), and the X$-$ vibrational wave functions and energies are the result of one-dimensional variational calculations.}}, 1982 (1992).} Our results reproduce the experimental findings very well, and we determine that the overtone intensities reflect the large shifts in charge distribution along the heavy-atom axis when the hydrogen moves out of the plane. This method, although only one-dimensional, shows excellent agreement with experiment and accurately explains the essential physics that give rise to these spectral features.