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

Abstract

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)/6311+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 2N..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 Arpressure 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 x 10 -23 x 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 2NO + 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 3N with a lower binding energy of 1.8 kcal/mol. The predicted rate constants can be represented by k -i = 6.70 10 -24 T +3.58exp(-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 1 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 2O + NO 3 because of the lower barrier for the transition state for the OH + HONO 2. © 2009 Wiley Periodicals, Inc.