Water dimer hydrogen bond stretch, donor torsion overtone, and “in-plane bend” vibrations

We report the measurement and analysis of 64 new Ka=0←0,1 and Ka=1←0,1 transitions of (H2O)2 and 16 new Ka=0←0 transitions of (D2O)2 by terahertz laser vibration–rotation–tunneling spectroscopy of a planar supersonic expansion between 140.5 and 145.5 cm−1. The transitions in both isotopomers correspond to A′ vibrations assigned to the hydrogen bond stretch (translational) and donor torsion overtone vibrations. The interchange splitting is 56.3 GHz in Ka=0 of the excited state of (H2O)2, nearly 3 times the value of the ground state, and the bifurcation tunneling splitting is 1.8 GHz, over 2 times the value of the ground state. We compare the existing experimental spectra with calculations on state-of-the-art intermolecular potential energy surfaces and critically review the vibrational assignments reported in the literature. We show that the discrepancy between theory and experiment regarding the assignment of the feature near 103 cm−1 can be resolved by considering E2→E1 transitions, which had not been co...