A shock tube study of benzylamine decomposition: Overall rate coefficient and heat of formation of the benzyl radical

The decomposition rate of benzylamine (C₆H₅CH₂NH₂) and the heat of formation of the benzyl radical (C₆H₅-CH₂) were determined in shock tube experiments combined with RRKM calculations. To obtain the decomposition rate of benzylamine, the NH₂ mole fraction was measured using frequency-modulation absorption spectroscopy behind reflected shock waves. The initial slope of the NH₂ concentration is directly proportional to the decomposition rate and the initial concentration of benzylamine. The rate expression for the decomposition reaction for the temperature range 1225-1599 K and the pressure range 1.19-1.47 bar is k₁ = (5.49 × 10¹⁴)e^{-33110/[T(K)]} s^-1 with an uncertainty of ±15%. To obtain the high-pressure-limit rate expression for benzylamine decomposition we performed RRKM calculations using the parameters obtained from the experimental data of this study and those of the VLPP study of Golden et al.⁴ The resulting high-pressure-limit rate for the temperature range 1000-1600 K is k_∞ = (1.07 × 10^16.0)e^{-36470/[T(K)]} s^-1. From the RRKM calculations, we determined the C-N bond dissociation energy of benzylamine at 0 K to be 305 ± 4 kJ mol^-1, and with this value and the thermochemical properties of benzylamine and NH₂, the heat of formation of the benzyl radical was calculated. The heat of formation of benzyl radical at 298 K is 210 ± 5 kJ mol^-1, which agrees with the result of Ellison et al.⁵ and the value recommended by Tsang⁶.