Reversible addition of the OH radical to p-cymene in the gas phase: kinetic analysis assuming formation of a single adduct. Part 1

Abstract

A flash photolysis-resonance fluorescence (FP-RF) technique was employed to study the kinetics and mechanism of the reaction of OH radicals with p-cymene at temperatures between 297 and 413 K in helium buffer gas. FP-RF experiments involved time-resolved detection of OH radicals by RF following vacuum-UV flash photolysis of H₂O–p-cymene–He and H₂O–He mixtures. Biexponential functions were fitted to decays of OH radicals according to reversible addition of OH radicals to p-cymene to form a single adduct. A rate constant of (15.7 ± 1.1) × 10⁻¹² is obtained (in units of cm³ s⁻¹) at room temperature (298 K) for the sum of the addition and abstraction channels (k_1a + k_1b) according to this simplified model. The Arrhenius plot reveals the step function typical of other aromatics and can be described using the expressions: 2 × 10⁻¹³ exp(+1300 K/T) at temperatures between 297 K and 324 K and 10⁻¹¹ exp(−250 K/T) at temperatures between 345 K and 413 K. After consideration of the abstraction channel an equilibrium constant of k_1a/k_−1a = 6 × 10⁻²⁶ exp(+9700 K/T) cm³ is obtained at temperatures between 297 and 325 K and 2 × 10⁻³⁶ exp(+17 000 K/T) cm³ at temperatures between 325 and 380 K.