The Nitric Acid−Water Complex:  Microwave Spectrum, Structure, and Tunneling

Eight isotopic derivatives of the complex HNO 3 −H 2 O have been observed by microwave spectroscopy. The spectra are consistent with a structure in which the nitric acid forms a near-linear, 1.78 Å hydrogen bond to the oxygen of the water. A second, presumably weaker hydrogen bond is formed between a water hydrogen and one of the HNO 3 oxygens. The resulting cyclic structure adopts a planar configuration except for the non-hydrogen bonded proton of the H 2 O. In complexes containing DOH, only the isomer with the deuterium in the plane is observed, confirming the contribution of the secondary O···H 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 H 2 O and D 2 O 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.