### 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 _{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 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 _{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 _{-i} = 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 _{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 _{2}O + NO _{3} because of the lower barrier for the transition state for the OH + HONO _{2}.

Original language | English |
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Pages (from-to) | 69-78 |

Number of pages | 10 |

Journal | International Journal of Chemical Kinetics |

Volume | 42 |

Issue number | 2 |

DOIs | |

State | Published - 1 Feb 2010 |