Microwave and tunable far-infrared laser spectroscopy of OC-H_20: investigation of the water tunneling potential

The K_p=O → 1 rotation—tunneling bands of O^(12)C-H_2O and O^(12)C-D_2O have been measured in the region between 400 and 600 GHz. Two bands for each isotopomer were observed corresponding to ΔK_p = 1, b-type rotational transitions in the A and B water tunneling states. Each band was fit independently using a Watson A-reduced Hamiltonian yielding all three rotational and several distortion constants per band. The effective A-rotational constants, A^*, contain a contribution due to water tunneling. Assuming the tunneling splittings are the same in K_p=0 and K_p=1, A^*(A state) − A^*(B state) = 2ν_t, where ν_t is the tunneling splitting. We obtain tunneling splitting of 16.684 GHz for O^(12)C-H_2O and 1.012 GHz for O^(12)C-D_2O. These measurements are in good agreement with the predictions of Yaron (J. Chem. Phys. 92 (1990) 7095). Effective one-dimensional potentials have been employed to place constraints on the hydrogen bond geometry, to model the measured tunneling splittings, and to predict higher frequency vibration—rotation-tunneling transitions