Kinetics of the CH 3 + HCl/DCl → CH 4/CH 3D + Cl and CD 3 + HCl/DCl → CD 3H/CD 4 + Cl reactions: An experimental H atom tunneling investigation

Abstract

The kinetics of the radical reactions of CH 3 with HCl or DCl and CD 3 with HCl or DCl have been investigated in a temperature controlled tubular reactor coupled to a photoionization mass spectrometer. The CH 3 (or CD 3) radical, R, was produced homogeneously in the reactor by a pulsed 193 nm exciplex laser photolysis of CH 3COCH 3 (or CD 3COCD 3). The decay of CH 3/CD 3 was monitored as a function of HCl/DCl concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature, typically from 188 to 500 K. The rate constants of the CH 3 and CD 3 reactions with HCl had strong non-Arrhenius behavior at low temperatures. The rate constants were fitted to a modified Arrhenius expression k = QA exp (− E a/ RT) (error limits stated are 1 σ + Students t values, units in cm 3 molecule −1 s −1): k(CH 3 + HCl) = [1.004 + 85.64 exp (−0.02438 × T/K)] × (3.3 ± 1.3) × 10 −13 exp [−(4.8 ± 0.6) kJ mol −1/ RT] and k(CD 3 + HCl) = [1.002 + 73.31 exp (−0.02505 × T/K)] × (2.7 ± 1.2) × 10 −13 exp [−(3.5 ± 0.5) kJ mol −1/ RT]. The radical reactions with DCl were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to a conventional Arrhenius expression k = A exp (− E a/ RT) (error limits stated are 1 σ + Students t values, units in cm 3 molecule −1 s −1): k(CH 3 + DCl) = (2.4 ± 1.6) × 10 −13 exp [−(7.8 ± 1.4) kJ mol −1/ RT] and k(CD 3 + DCl) = (1.2 ± 0.4) × 10 −13 exp [−(5.2 ± 0.2) kJ mol −1/ RT] cm 3 molecule −1 s −1. Curvature in the Arrhenius plots of the H-atom abstraction reactions at low temperatures was analyzed by considering H-atom tunneling through the reaction barrier and primary kinetic isotope effect. Contribution of tunneling in it was concluded to be negligible. In addition, secondary isotope effect was measured.