Kinetics and mechanism of the C₆H₅ + CH ₃CHO reaction: Experimental measurement and theoretical prediction of the reactivity toward four molecular sites

The kinetics and mechanism of the reaction of C₆H₅ with CH₃CHO have been investigated experimentally and theoretically. The total rate constant for the reaction has been measured by means of the cavity ring-down spectrometry (CRDS) in the temperature range 299-501 K at pressures covering 20-75 Torr. The overall bimolecular rate constant can be represented by the expression k = (2.8 ± 0.2) × 10¹¹ exp[-(700 ± 30)/T] cm³ mol^-1 s^-1, which is slightly faster than for the analogous C₆H₅ + CH ₂O reaction determined with the same method in the same temperature range. The reaction mechanism for the C₆H₅ + CH ₃CHO reaction was also explored with quantum-chemical calculations at various hybrid density functional theories (DFTs) and using ab initio high-level composite methods. The theories predict that the reaction may occur by two hydrogen-abstraction and two addition channels with the aldehydic hydrogen-abstraction reaction being dominant. The rate constant calculated by the transition state theory for the aldehydic hydrogen-abstraction reaction is in good agreement with the experimental result after a very small adjustment of the predicted reaction barrier (+0.3 kcal/mol^-1). Contributions from other product channels are negligible under our experimental conditions. For combustion applications, we have calculated the rate constants for key product channels in the temperature range of 298-2500 K under atmospheric-pressure conditions; they can be represented by the following expressions in units of cm³ mol^-1 s^-1: k_1,cho = 8.8 × 10³T^2.6 exp(-90/T), k_2,ch3 = 6.0 × 10⁷ T^3.3 exp(-950/T) k_3a(C₆C ₅OCH₃ + H) = 4.2 X 10⁵ T^0.6 exp(-410/T) and k_3b(C₆H₅CHO+CH₃) = 6.6 × 10⁹ T^-0.5 exp(-310/T).