STRUCTURE, BINDING-ENERGY, AND EQUILIBRIUM-CONSTANT OF THE NITRIC ACID-WATER COMPLEX

The nitric acid-water complex is studied by high level nb initio calcu lations. The equilibrium structure of the complex has an asymmetric do ubly hydrogen-bonded ring. A strong hydrogen bond is donated by the hy droxyl group of nitric acid to the oxygen atom of water, and a much we aker hydrogen bond is donated by an OH bond of water to a second oxyge n of nitric acid. The HNO3 unit in the complex is considerably distort ed with the OH stretching frequency red-shifted by over 300 cm(-1) fro m the isolated molecule and the infrared intensity enhanced by an orde r of magnitude. The binding energy of the complex is calculated as D-e = -9.5 kcal/mol. The equilibrium fraction of nitric acid complexed wi th water is predicted to be near 1% at the Earth's surface and falls w ith the increase of altitude.