Reactions of Methyl Radicals with Aniline Acting as Hydrogen Donors and as Methyl Radical Acceptors

Abstract

The present investigation theoretically reports the comprehensive kinetic mechanism of the reaction between aniline and the methyl radical over a wide range of temperatures (300-2000 K) and pressures (76-76,000 Torr). The potential energy surface of the C₆H₅NH₂ + CH₃ reaction has been established at the CCSD(T)//M06-2X/6-311++G(3df,2p) level of theory. The conventional transition-state theory (TST) was utilized to calculate rate constants for the elementary reaction channels, while the stochastic RRKM-based master equation framework was applied for the T- and P-dependent rate-coefficient calculation of multiwell reaction paths. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH₂ group of aniline (to form P1 (C₆H₅NH + CH₄)) has been found to compete with the CH₃-addition on the C atom at the ortho site of aniline (to form IS2) with the atmospheric rate expressions (in cm³ molecule^-1 s^-1) as k_a1 = 7.5 × 10^-23 T^3.04 exp[(-40.63 ± 0.29 kJ·mol^-1)/RT] and k_b2 = 2.29 × 10^-3 T^-3.19 exp[(-56.94 ± 1.17 kJ·mol^-1)/RT] for T = 300-2000 K and P = 760 Torr. Even though rate constants of several reaction channels decrease with increasing pressures, the total rate constant k_total = 7.71 × 10^-17 T^1.20 exp[(-40.96 ± 2.18 kJ·mol^-1)/RT] of the title reaction still increases as the pressure increases in the range of 76-76,000 Torr. The calculated enthalpy changes for some species are in good agreement with the available experimental data within their uncertainties (the maximum deviation between theory and experiment is ∼11 kJ·mol^-1). The T1 diagnostic and spin contamination analysis for all species involved have also been observed. This work provides sound quality rate coefficients for the title reaction, which will be valuable for the development of detailed combustion reaction mechanisms for hydrocarbon fuels.

Figure 1

Potential energy surface containing the favorable channels of the C₆H₅NH₂ + CH₃ reaction system calculated at the CCSD(T)//M06-2X/6-311++G(3df,2p) + ZPVE level of theory (energies are in kJ·mol^–1).

Figure 2

Plots of the predicted rate constants for the main reactions of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at a pressure of 76 Torr Ar.

Figure 3

Plots of the predicted rate constants for the main reactions of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at a pressure of 760 Torr Ar.

Figure 4

Plots of the predicted rate constants for the main reactions of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at a pressure of 7600 Torr Ar.

Figure 5

Plots of the predicted rate constants for the main reactions of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at a pressure of 76,000 Torr Ar.

Figure 6

Branching ratios for the main channels of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at 760 Torr (Ar) pressure. It should be noted that the branching ratios of some products (IS1, P2-P5, P8, P9) in this figure may not be visible due to their insignificant data. For more information, please refer to Table S14.

Figure 7

Plots of the predicted total rate constants of the C₆H₅NH₂ + CH₃ system in the temperature range of 300–2000 K and at different pressures of 76–76,000 Torr Ar. It should be noted that the invisibility of the kinetic lines at 76, 760, and 7600 Torr should be acknowledged because the calculated values at these pressures exhibit only minor differences as can be seen in Table S15.