Structure, binding energy, and equilibrium constant of the nitric acid‐Water complex

The nitric acid‐water complex is studied by high level ab initio calculations. The equilibrium structure of the complex has an asymmetric doubly hydrogen‐bonded ring. A strong hydrogen bond is donated by the hydroxyl group of nitric acid to the oxygen atom of water, and a much weaker hydrogen bond is donated by an OH bond of water to a second oxygen of nitric acid. The HNO 3 unit in the complex is considerably distorted with the OH stretching frequency red‐shifted by over 300 cm −1 from the isolated molecule and the infrared intensity enhanced by an order of magnitude. The binding energy of the complex is calculated as D e = −9.5 kcal/mol. The equilibrium fraction of nitric acid complexed with water is predicted to be near 1% at the Earth's surface and falls with the increase of altitude.