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 (α and β, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H3O+)⋅(H2O), 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 (H3O)+ and H2O 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 β‐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. A metastable phase that forms in stratospheric clouds, nitric acid dihydrate (NAD) exists in two phases (α and β), each of which contains two types of (H3O+)⋅(H2O) complex with different H‐bonding characteristics. The figures show experimental RAIR spectra of NAD (•–•–) and DFT spectra calculated with two different functionals (blue and violet). Shaded areas indicate proton‐transfer motion between two water molecules.