Nitric acid-water complex: microwave spectrum, structure, and tunneling

Eight isotopic derivatives of the complex HNO3-H2O have been observed by microwave spectroscopy. The spectra are consistent with a structure in which the nitric acid forms a near-linear, 1.78 angstrom hydrogen bond to the oxygen of the water. A second, presumably weaker hydrogen bond is formed between a water hydrogen and one of the HNO3 oxygens. The resulting cyclic structure adopts a planar configuration except for the non-hydrogen bonded proton of the H2O. In complexes containing DOH, only the isomer with the deuterium in the plane is observed, confirming the contribution of the secondary O&middot&middot&middotH interaction to the overall stabilization of the system. Strong evidence for complex internal dynamics involving the water subunit is also presented. The a- and b-type transitions of the H2O and D2O containing species exhibit a doubling which disappears in the DOH complex, providing direct evidence for the existence of a proton interchange motion in the system. Moreover, c-type rotational transitions do not appear at their predicted rigid rotor positions, even for the DOH species, providing indirect evidence for a second motion which is interpreted as a large amplitude wagging of the non-hydrogen bonded proton of the water.