The nitric acid hydrates:: Ab initio molecular study, and RAIR spectra of the solids

The molecular complexes formed by nitric acid with one, two, and three water molecules have been investigated by high level ab initio calculations. The B3LYP/aug-cc-pVTZ method and basis set have been used to derive equilibrium geometries and binding energies and to predict the infrared spectra of these complexes. It has been found that the complexes become stable through the formation of several hydrogen bonds. The main one is established between the acid, which acts as a donor, and the first water molecule, but further bonds are created between successive water molecules, with a weaker bond making a closed structure between the last water unit and the acid. Films of nitric acid mono-, di-, and trihydrates (NAM, NAD, and NAT) have been prepared by deposition on a cold surface and annealing to induce crystallization. RAIR spectra have been taken of these films. The spectra are compared to previous investigations, and the assignments of the spectra and the structure of the crystals are discussed. A global good agreement is obtained with previous transmission and RAIR spectra of thin films and extinction spectra of aerosols.