Theoretical Study of the Kinetics of the Gas-Phase Reaction between Phenyl and Amino Radicals

The potential energy surface (PES) of the C₆H₅ + NH₂ reaction has been investigated by using ab initio CCSD­(T)//B3LYP/6-311++G­(3df,2p) calculations. The conventional transition-state theory (TST) and the variable reaction coordinate-TST (VRC-TST) have been used to predict the rate constants for the channels possessing tight and barrierless transition states, respectively. The Rice–Ramsperger–Kassel–Marcus/Master equation (RRKM/ME) theory has been utilized to determine the pressure-dependent rate constants for these reactions. The PES shows that the reaction begins with an exothermic barrierless addition of NH₂ to C₆H₅ producing the vital intermediate state, namely, aniline (C₆H₅NH₂, IS1). Once IS1 is generated, it can further isomerize to various intermediate states, which can give rise to different products, including PR4 (4,5,6-trihydro-1-amino phenyl + H₂), PR5 (3,4,5,6-tetrahydro phenyl + NH₃), PR6 (2,3,5,6-tetrahydro-1-imidogen phenyl + H₂), PR9 (3,4,5,6-tetrahydro-1-imidogen phenyl + H₂), and PR10 (2,5,6-trihydro-1-amino phenyl + H₂), of which the most stable product, PR5, was formed by the most favorable channel going through the two advantageous transition states T1/11 (−28.9 kcal/mol) and T11P5 (−21.5 kcal/mol). The calculated rate constants for the low-energy channel, 1.37 × 10^–9 and 2.16 × 10^–11 cm³ molecule^–1 s^–1 at T = 300, P = 1 Torr and T = 2000 K, P = 760 Torr, respectively, show that the title reaction is almost pressure- and temperature-dependent. The negative temperature-dependent rate coefficients can be expressed in the modified Arrhenius form of k₁ = 8.54 × 10¹³ T^–7.20 exp (−7.07 kcal·mol^–1/RT) and k₂ = 2.42 × 10¹⁵ T^–7.61 exp (−7.75 kcal·mol^–1/RT) at 1 and 10 Torr, respectively, and in the temperature range of 300–2000 K. The forward and reverse rate coefficients as well as the high-pressure equilibrium constants of the C₆H₅ + NH₂ ↔ IS1 process were also predicted; their values revealed that its kinetics do not depend on pressure at low temperature but strongly depend on pressure at high temperature. Moreover, the predicted formation enthalpies of reactants and the enthalpy changes of some channels are in good agreement with the experimental results.