The structure of water dimer from molecular beam electric resonance spectroscopy

Molecular beams of hydrogen bonded water dimer, generated in a supersonic nozzle, have been studied using electric resonance spectroscopy. Radiofrequency and microwave transitions have been observed in (H2 16O)2, (D2 16O)2, and (H2 18O)2. Transitions arising from both pure rotation and rotation–tunneling occur. The pure rotational transitions have been fit to a rigid rotor model to obtain structural information. Information on the relative orientation of the two monomer units is also contained in the electric dipole moment component along the A inertial axis μa, which is obtained from Stark effect measurements. The resultant structure is that of a ’’trans-linear’’ complex with an oxygen–oxygen distance ROO of 2.98(1) Å, the proton accepting water axis is 58(6) ° with respect to ROO, and the proton donating water axis at −51(6) ° with respect to ROO. This structure is consistent with a linear hydrogen bond and the proton acceptor tetrahedrally oriented to the hydrogen bond. The limits of uncertainty are wholly model dependent and are believed to cover variations from the zero-point vibrational structure observed to the equilibrium structure. μa shows strong dependence on J and K and is about 2.6 D. Centrifugal distortion constants have been interpreted in terms of the monomer–monomer stretching frequency and give ω=150 cm−1.