The crystal structures of the low-temperature and high-pressure polymorphs of nitric acid

A new high-pressure phase of pure nitric acid (HNO(3)) has been characterised at 1.6 GPa at room temperature by high-pressure neutron powder and X-ray single-crystal diffraction techniques. This is the first crystalline phase obtained upon compression of liquid nitric acid at room temperature and appears to be the stable phase up to pressures of at least 4 GPa. The crystal structure of this new phase shows some similarities to that of the low-temperature phase of nitric acid at ambient pressure, which has been redetermined as part of this study. Both structures share a herringbone packing of hydrogen-bonded molecular catemers, although the presence of disorder within the hydrogen bonds within one of the catemers of the low-temperature phase makes its structure comparatively more complex.

Theoretical study on hydrogen-bond effects in IR spectra of high- and low-temperature phases of nitric acid dihydrate

The low- and high-temperature phases (alpha and beta, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H(3)O(+)) x (H(2)O), designated A and B, with different hydrogen-bonding (HB) characteristics. The theoretical study reveals that type A complexes are weakly bound and could be described as (H(3)O)(+) and H(2)O aggregates, with decoupled vibrational modes, whereas in type B structures the proton is situated close to the centre of the O...O bond and induces strong vibrational coupling. The proton-transfer mode is predicted at quite different wavenumbers in each complex, which provides an important differentiating spectral feature, together with splitting of some bands in beta-NAD. Theoretical spectra are estimated by using two GGA parameterizations, namely, PBE and BLYP. The potential-energy surface for each type of HB in NAD is also studied, as is the spectral influence of displacement of the shared H atom along the O-O bond. The results are compared to literature infrared spectra recorded by different techniques, namely, transmission and reflection-absorption, with both normal and tilted incident radiation. This work provides a thorough assignment of the observed spectra, and predictions for some spectra not yet available. The usefulness of high-level theoretical calculations as performed herein to discriminate between two phases of a solid crystal is thus evidenced.