ELECTRIC-FIELD EFFECTS ON THE OH VIBRATIONAL FREQUENCY AND INFRARED-ABSORPTION INTENSITY FOR WATER

The variations of the anharmonic OH frequency and the infrared absorption intensity with field strength have been calculated for the uncoupled OH stretching vibration of a water molecule in a static, homogeneous electric field using ab initio methods at the MP4 level with a nearly saturated basis set. The OH frequency is found to be virtually independent of the field components perpendicular to the vibrating OH bond. For the parallel component, the frequency vs field curve is close to quadratic, with a maximum for a slightly negative (directed from H to 0) field strength. The external field perturbation, defined as V(ext)(E(parallel-to), r(OH)) = V(tot)(E(parallel-to), r(OH)) - V(free)(r(OH)), is found to be closely linear in r(OH), except when the field strength Ell is both large and negative. The linear external force constant is almost perfectly accounted for by the sum of two terms, -E(parallel-to).dmu(parallel-to)free/dr(OH) and -1/2.E(parallel-to).partial derivative mu(parallel-to)induced/partial derivative r(OH). These derivatives are quite insensitive to the choice of basis-set. The partial derivative mu(parallel-to)induced/partial derivative r(OH) derivative is approximately proportional to E(parallel-to), and gives rise to the arclike shape of the frequency vs field curve. The frequency maximum occurs where partial derivative mu(parallel-to)tot/partial derivative r(OH) almost-equal-to 0. It is the sign of dmu(parallel-to)free/dr(OH) which determines that the frequency maximum occurs at a negative field strength for water (but at a positive field strength for OH-, for example), i.e., that a frequency red-shift (blue-shift for OH-) occurs when the molecule is bound. The linear relationship between the infrared absorption intensity and frequency of the water OH vibration is derived.