Zhang Xin, Li Qian Shu
School of Chemistry and Environment, South China Normal University, Guangzhou 510631, People's Republic of China.
J Phys Chem A. 2006 Oct 19;110(41):11636-44. doi: 10.1021/jp063540c.
We present a systematic direct ab initio dynamics investigation of the reaction between N2H4 and F atom, which is predicted to have three possible reaction channels. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels were calculated at the UB3LYP/6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs was further refined by means of the CCSD(T)/aug-cc-pVTZ method. The calculated results revealed that the first two primary channels (N2H4+F-->N2H3+HF) are equivalent and occur synchronously via the formation of a pre-reaction complex with Cs symmetry rather than via the direct H abstraction. The pre-reaction complex then evolves into a hydrogen-bonding intermediate through a transition state with nearly no barrier and a high exothermicity, which finally makes the intermediate further decompose into N2H3 and HF. Another reaction channel of minor role (N2H4+F-->NH2F+NH2) was also found during the calculations, which has the same Cs pre-reaction complex but forms NH2F and NH2 via another transition state with high-energy barrier and low exothermicity. The rate constants of these channels were calculated using the improved canonical variational transition state theory with the small-curvature tunneling correction (ICVT/SCT) method. The three-parameter ICVT/SCT rate constant expressions of k(ICVT/SCT) at the CCSD(T)/aug-cc-pVTZ//UB3LYP/6-31+G(d,p) level of theory within 220-3000 K were fitted as (7.64x10(-9))T (-0.87) exp(1180/T) cm3 mole-1 s-1 for N2H4+F-->N2H3+HF and 1.45x10(-12)(T/298)(2.17) exp(-1710/T) cm3 mole-1 s-1 for N2H4+F-->NH2F+NH2.
我们对N₂H₄与F原子之间的反应进行了系统的直接从头算动力学研究，预计该反应有三个可能的反应通道。在UB3LYP/6 - 31 + G(d,p)理论水平下计算了所有反应通道的驻点以及沿最小能量路径（MEP）各点的结构和频率。通过CCSD(T)/aug - cc - pVTZ方法进一步细化了驻点和沿MEP各点的能量信息。计算结果表明，前两个主要通道（N₂H₄ + F→N₂H₃ + HF）是等效的，通过形成具有Cs对称性的预反应复合物同步发生，而不是通过直接氢提取。预反应复合物然后通过一个几乎无势垒且放热高的过渡态演变成氢键中间体，最终使中间体进一步分解为N₂H₃和HF。在计算过程中还发现了另一个次要反应通道（N₂H₄ + F→NH₂F + NH₂），它具有相同的Cs预反应复合物，但通过另一个具有高能垒和低放热的过渡态形成NH₂F和NH₂。使用改进的正则变分过渡态理论结合小曲率隧道效应校正（ICVT/SCT）方法计算了这些通道的速率常数。在220 - 3000 K范围内，CCSD(T)/aug - cc - pVTZ//UB3LYP/6 - 31 + G(d,p)理论水平下，N₂H₄ + F→N₂H₃ + HF的三参数ICVT/SCT速率常数表达式拟合为(7.64x10⁻⁹)T⁻⁰.⁸⁷ exp(1180/T) cm³ mole⁻¹ s⁻¹，N₂H₄ + F→NH₂F + NH₂的拟合为1.45x10⁻¹²(T/298)².¹⁷ exp(⁻1710/T) cm³ mole⁻¹ s⁻¹。
