Density Functional Studies of the Formation of Nitrous Acid from the Reaction of Nitrogen Dioxide and Water Vapor

Reaction mechanisms for the production of nitrous acid (HONO) from the homogeneous gas-phase hydrolysis of nitrogen dioxide (NO 2 ) are examined by density functional theory calculations. The molecular structures and energies of the NO 2 −(H 2 O) n ( n = 1, 2, 3) and N 2 O 4 −(H 2 O) n ( n = 1, 2) systems corresponding to the stationary points on the potential energy surface along the reaction pathways are calculated using the B3LYP method with the 6-311+G(2d,p) basis set. These reaction pathways represent the homogeneous hydrolysis of NO 2 or N 2 O 4 with a varying number of water (H 2 O) molecules. The reactions of NO 2 with water produce HONO, along with the OH radical which was postulated to combine in the next step with a second NO 2 to form nitric acid (HNO 3 ). The simple NO 2 + H 2 O bimolecular reaction leads to the highly unstable OH radical which reacts reversibly with HONO without an energy barrier. The introduction of single solvating H 2 O molecule appears to stabilize the transition state as well as an intermediate that contains the OH radical. However, the energy barrier is found to be near 30 kcal mol -1 and is not affected by multiple additional H 2 O molecules. On the other hand, the reaction of N 2 O 4 with water leads directly to HONO and HNO 3 . The energy barrier for the N 2 O 4 reaction is above 30 kcal mol -1 and is also unaffected by additional H 2 O molecules. The study demonstrates that the gas-phase hydrolysis of NO 2 or N 2 O 4 is insignificant regardless of water vapor pressure. The physical origin responsible for the unusual hydrolysis reaction of NO 2 is explored with the contrasting examples of N 2 O 5 and SO 3 hydrolysis reactions.