Rotation and vibration spectra for the H2O dimer: Theory and comparison with experimental data

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Abstract
The ground state binding energy (BE), rotational and vibrational energy levels, and line strengths for radiative transitions between some of these energy levels are calculated for the [H2O]2 molecule. These quantities are computed for three intermolecular potentials published for the [H2O]2 molecule, two of which were calculated by MO SCF techniques and one which was derived from an empirical point charge model for the water molecule. The value of BE calculated for two of the potentials is approximately 6 kcal/mole, and for the third is approximately 3 kcal/mole. The total concentration of [H2O]2 in equilibrium with H2O vapor is calculated for the sets of energy levels determined for these potentials. Results are compared with available experimental data. Using calculated values of line strengths and experimental data for the integrated absorptions for two rotational transitions, an independent value is deduced for the equilibrium concentration of [H2O]2 in fair agreement with values calculated from two of the intermolecular potentials. The theoretical calculations for BE and the equilibrium concentration of [H2O]2 are also compared with those obtained from analysis of experimental data for the second virial coefficient B2(T) of H2O vapor. The theoretical values for BE bracket the value of 3–4 kcal/mole obtained from analysis of the data for B2(T). A discussion of our results is presented, along with suggestions for experimental work to search for the spectrum of [H2O]2 in the absorption spectrum of H2O vapor.