Microwave spectrum, structure, and internal dynamics of the nitric acid dihydrate complex

A-type rotational spectra of the complex HNO₃- (H ₂O)₂ have been observed by rotational spectroscopy in a supersonic jet. Extensive isotopic substitution and analysis of the resulting moments of inertia reveals that the complex adopts a cyclic geometry in which a second water inserts into the weak secondary hydrogen bond of the (also cyclic) HNO₃-H₂O dimer. The complex is planar, except for one free proton from each water unit that lies above or below the plane. The primary hydrogen bond, formed between the HNO₃ proton and the first water molecule in the trimer, is 1.643(76) Å in length. All intermolecular distances are smaller than those of the constituent dimers. Internal motion, inferred from spectral doubling and studied by isotopic substitution experiments, likely corresponds to proton interchange involving the second water unit, but no such motion is revealed by the a-type spectrum for the first water unit. The degree of proton transfer across the hydrogen bond is discussed in terms of the proton-transfer parameter, ρPT, which assesses the degree of ionization on the basis of interatomic distances. Measured in this way, the complex is best described as hydrogen bonded, in accord with numerous theoretical predictions. However, an increase in the degree of ionization relative to that in HNO₃-H₂O is discernible. Using ρPT as a metric, two water molecules do less to ionize nitric acid than one water does to ionize sulfuric acid.