A theoretical study on mechanism and kinetics of the reaction between para-aminotoluene and OH radical

Abstract We conducted an ab initio exploration of the potential energy landscape for the para-aminotoluene reaction with OH, employing the CCSD(T)//M06-2X/6-311 + + G(3df,2p) level of theory. Additionally, thermal rate constants and branching ratios spanning the 300–2000 K temperature range were calculated. These calculations relied on the CCSD(T) energies and the M06-2X structures of the involved species, utilizing statistical theoretical TST and RRKM master equation computations. The calculated results reveal that the reaction channel para-aminotoluene + OH → NH-C₆H₄-CH₃ + H₂O prevails under the 300–2000 K temperature range, with the branching ratio in the range of 41%- 72% at P = 760 Torr. The overall second-order rate constants of the titled reaction obtained at the pressure 760 Torr (N₂) can be illustrated by the modified Arrhenius expression of k_total(T) = 2.04×10⁻ ¹⁸ T2.07 exp[(11.2 kJ.mol⁻ ¹)/RT]. The total rate constant at the ambient conditions in this work, 2.16×10⁻ ¹¹ cm³ molecule⁻ ¹ s⁻ ¹, is about one order of magnitude larger than those for the toluene reaction with OH at the same conditions measured by Davis et al. (6.11 ± 0.4×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹), Doyle et al. (4.15 ± 1.49×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹), Hansen et al. (5.78 ± 0.58×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹), Tully et al. (6.36 ± 0.69×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹), Knispel et al. (7.0 ± 0.41×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹) or calculated by Zhang et al. (5.69×10⁻ ¹² cm³ molecule⁻ ¹ s⁻ ¹). However, the value 2.16×10⁻ ¹¹ cm³ molecule⁻ ¹ s⁻ ¹ is smaller than those of the aniline + OH reaction measured by Atkinson and co-workers (1.18 ± 0.11×10⁻ ¹⁰ cm³ molecule⁻ ¹ s⁻ ¹), Rinke et al. (1.20 ± 0.24×10⁻ ¹⁰ cm³ molecule⁻ ¹ s⁻ ¹), and Witte et al. (1.23×10⁻ ¹⁰ cm³ molecule⁻ ¹ s⁻ ¹) or calculated by Abdel-Rahman et al. (4.4×10⁻ ¹⁰ cm³ molecule⁻ ¹ s⁻ ¹) and Mai et al. (1.3×10⁻ ¹⁰ cm³ molecule⁻ ¹ s⁻ ¹). This study provides a thorough comprehension of the mechanisms and kinetics associated with the interaction between para-aminotoluene and OH radical.