Exploring the Origins of the Intensity of the OH Stretch-HOH Bend Combination Band in Water

While the intensity of the OH stretching fundamentaltransitionis strongly correlated to hydrogen-bond strength, the intensity ofthe corresponding transition to the state with one quantum of excitationin both the OH stretching and HOH bending vibrations in the same watermolecule shows a much weaker sensitivity to the hydrogen-bonding environment.The origins of this difference are explored through analyses of thecontributions of terms in the expansion of the dipole moment to thecalculated intensity. It is found that the leading contribution tothe stretch-bend intensity involves the second derivative ofthe dipole moment with respect to the OH bond length and HOH angle.While this is not surprising, the insensitivity of this derivativeto the hydrogen-bonding environment is unexpected. Possible contributionsof mode mixing are also explored. While mode mixing leads to splittingsof the energies of nearly degenerate excited states, it does not resultin significant changes in the sum of the intensities of these transitions.Analysis of changes in the partial charges on the hydrogen atoms upondisplacement of the HOH angles shows that these charges generallyincrease with increasing HOH angle. This effect is partially canceledby a decrease in the charge of the hydrogen atom when a hydrogen bondis broken. The extent of this cancellation increases with the hydrogenbond strength, which is reflected in the observed insensitivity ofthe intensity of the stretch-bend transition to hydrogen-bondstrength.