Inter- and Intramolecular Potentials and the Spectrum of Ice

Whereas the spectra of H2O and D2O ice cannot yet be explained unambiguously, the spectra of HDO in dilute solution in either H2O or D2O may be interpreted readily. In particular, since vOH occurs at 3275 cm—1 and 2 vOH at 6300 cm—1, nearly the harmonic value, the barrier to proton transfer lies well above the latter level and must exceed 23 kcal/mole. The width of vOD at 2416 cm—1 is only 20 cm—1, whereas that of vOH is about 80 cm—1 and 2 vOH about 600 cm—1. These widths can be explained by proton tunneling if the barrier height is near 32 kcal, in which case the second minimum must lie below the level vOH. It must therefore be less than 14 kcal/mole above the primary minimum. A doubling of vOD from OD···OD pairs was also observed and the magnitude of the splitting is consistent with an effective charge of 0.6e on the protons. It is clear from these results that the usual width of hydrogen bonded OH lines is not an intrinsic characteristic of the O–H···O bond but results largely from intramolecular coupling of the O–H motions.