Direct dynamics study on the hydrogen abstraction reactions N_2H_4+R- N_2H_3+RH (R=NH_2,CH_3)

摘要

We present a direct ab initio dynamics study on the hydrogen abstraction reactions N_2H_4+R ->N_H_3+RH (R=NH_2,CH_3),which are predicted to have six possible reaction channels for NH_2 abstraction and four for CH_3 abstraction caused by the different N_2H_4 isomers and various attacking orientations of foreign radical to N_2H_4.The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels are obtained at the UMP2(full)/6-31+G(d,p) level of theory.Energetic information of stationary points and the points along the MEPs is further refined by means of MC-QCISD method.The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method.The calculated results show that the favorable reaction channels are channels (n1) and (n4) as well as (c1) and (c3) (refer to Scheme 1) in the whole temperature range.The total ICVT/SCT rate constants of all channels for the two reactions at the MC-QCISD//UMP2(fuIl)/6-31 + G(d,p) level are both in good agreement with the available experimental data,and corresponding three-parameter expressions of k~(ICVT/SCT) in 220-3000 K are fitted as 6.46 x 10~(-15)(T/298)~(3.60) exp(-386/T) cm~3 mol~(-1) s~(-1) for NH_2 abstraction and 1.04 x 10~(-14)(T/298)~(4.00) exp(-2037/T) cm~3 mol~(-1) s~(-1) for CH_3 abstraction.Additionally,the long range interaction between the H atom of x-H bond in foreign radicals and the lone pair on the nonreactive N atom of the transition states is further discussed to explain the various transition-state numbers of the two similar hydrogen abstraction reactions.