Direct Measurement and Theoretical Calculation of the Rate Coefficient for Cl CH3in the Range from T202−298 K.

The rate coefficient has been measured under pseudo-first-order conditions for the Cl CH3association reaction at T202, 250, and 298 K and P0.3−2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2in He. The decay of CH3was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3concentration (Cl0/CH309−67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T202 K, the rate coefficient increases from 8.4 × 10-12at P0.30 Torr He to 1.8 × 10-11at P2.00 Torr He, both in units of cm3molecule-1s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin−orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k26.0 × 10-11cm3molecule-1s-1, independent of temperature in the range from 200 to 300 K. [ABSTRACT FROM AUTHOR]