Ab initio chemical kinetics for the NH 2 + HNO x reactions, part III: Kinetics and mechanism for NH 2 + HONO 2

The kinetics and mechanism for the reaction of NH 2 with HONO 2 have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single‐point calculations at the CCSD(T)/6‐311+G( 3df , 2p ) level based on geometries optimized at the B3LYP/6‐311+G( 3df , 2p ) level. The reaction producing the primary products, NH 3 + NO 3 , takes place via a precursor complex, H 2 N…HONO 2 with an 8.4‐kcal/mol binding energy. The rate constants for major product channels in the temperature range 200–3000 K are predicted by variational transition state or variational Rice–Ramsperger–Kassel–Marcus theory. The results show that the reaction has a noticeable pressure dependence at T < 900 K. The total rate constants at 760 Torr Ar‐pressure can be represented by k total = 1.71 × 10 −3 × T −3.85 exp(−96/T) cm 3 molecule −1 s −1 at T = 200–550 K, 5.11 × 10 −23 × T +3.22 exp(70/T) cm 3 molecule −1 s −1 at T = 550–3000 K. The branching ratios of primary channels at 760 Torr Ar‐pressure are predicted: k 1 producing NH 3 + NO 3 accounts for 1.00–0.99 in the temperature range of 200–3000 K and k 2 + k 3 producing H 2 NO + HONO accounts for less than 0.01 when temperature is more than 2600 K. The reverse reaction, NH 3 + NO 3 → NH 2 + HONO 2 shows relatively weak pressure dependence at P < 100 Torr and T < 600 K due to its precursor complex, NH 3 …O 3 N with a lower binding energy of 1.8 kcal/mol. The predicted rate constants can be represented by k −1 = 6.70 × 10 −24 × T +3.58 exp(−850/T) cm 3 molecule −1 s −1 at T = 200–3000 K and 760 Torr N 2 pressure, where the predicted rate at T = 298 K, 2.8 × 10 −16 cm 3 molecule −1 s −1 is in good agreement with the experimental data. The NH 3 + NO 3 formation rate constant was found to be a factor of 4 smaller than that of the reaction OH + HONO 2 producing the H 2 O + NO 3 because of the lower barrier for the transition state for the OH + HONO 2 . © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 69–78, 2010