Direct Measurement and Theoretical Calculation of the Rate Coefficient for Cl + CH3 in the Range from T = 202-298 K

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• 作者：James K. Parker ; Walter A. Payne ; Regina J. Cody ; Fred L. Nesbitt ; Louis J. Stief ; Stephen J. Klippenstein ; Lawrence B. Harding
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：February 15, 2007
• 年：2007
• 卷：111
• 期：6
• 页码：1015 - 1023
• 全文大小：159K
• 年卷期：v.111,no.6(February 15, 2007)
• ISSN：1520-5215

文摘

The rate coefficient has been measured under pseudo-first-order conditions for the Cl + CH₃ associationreaction at T = 202, 250, and 298 K and P = 0.3-2.0 Torr helium using the technique of discharge-flowmass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl andCH₃ were generated rapidly and simultaneously by reaction of F with HCl and CH₄, respectively. Fluorineatoms were produced by microwave discharge in an approximately 1% mixture of F₂ in He. The decay ofCH₃ was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess overthe CH₃ concentration ([Cl]₀/[CH₃]₀ = 9-67). Small corrections were made for both axial and radial diffusionand minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range ofpressures studied. For example, at T = 202 K, the rate coefficient increases from 8.4 × 10^-12 at P = 0.30Torr He to 1.8 × 10^-11 at P = 2.00 Torr He, both in units of cm³ molecule^-1 s^-1. A combination of ab initioquantum chemistry, variational transition-state theory, and master-equation simulations was employed indeveloping a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonableempirical 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 k₂ = 6.0 × 10^-11 cm³ molecule^-1s^-1, independent of temperature in the range from 200 to 300 K.