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

Author Institution: Harvard University, Cambridge, MA 02138.; Aerodyne Research Inc., 45 Manning Rd., Billerica, MA 01821.The nitric acid-water complex is studied by high level ab initi calculations. The calculations are carried out at the level of the second-order Moller-Plesset approxiamtion (MP2) using three different basis sets of increasing quality, $6-31 + G^{\ast}, 6-311 ++ G^{\ast\ast}$, and 6-311++G(2d, p). The equilibrium structure of the complex is determined to have an asymmetric doubly hydrogen-bonded ring, with a strong hydrogen bond donated from the hydroxyl group of the nitric acid to the oxygen atom of the water, and a much weaker hydrogen bond donated from an OH bond of the water to a second oxygen atom of the nitric acid. The ring is co-planar with the symmetry plan of the nitric acid, and the free OH bond of the water protrudes from the ring by an angle of about $50^{\circ}$, with a barrier of $80 cm^{-1}$ at the fully planar geometry. The $HNO_{3}$ unit in the complex is considerably distorted and OH stretching frequency is red-shifted by over $300 cm^{-1}$ form the isolated molecule. The binding energy of th complex with respect to the nitric acid and water monormers is calculated to $D_{c} = -9.1 kcal/mol$ after conterpoise corrections, using the 6-311++G(2d, p) basis set extended with a set of bond functions, 3s3p2d. The corresponding dissociation energy is $D_{0} = 7.0 kcal/mol$ where the zero-point vibrational energies estimated from calculated harmonic frequencies. The binding energy and vibrational frequencies are used to predict the equilibrium constant and mixing ratio of the complex in the stratosphere as well as in the ground level atmosphere. The results show that the association of nitric acid with water may have significant effects on the atmospheric chemistry involving nitric acid and other nitrogen compounds