Mixtures of fluoromethane, chlorine, and oxygen were subjected to continuous UV photolysis and monitored with Fourier Transform infrared (FT-IR) spectroscopy at a resolution of 0.5 cm-1. Reactant mixtures were prepared within Pyrex bulbs manometrically to a total pressure of ca. 1 atm made up with argon bath gas. The process of UV photolysis was conducted, using a mercury pen-light, directly within a white-type long path cell of 100 cm3 volume and a path length of 240 cm. The dominant products of these experiments are hydrogen chloride (HCl), chlorofluoromethane (CH2ClF), and formyl fluoride (HCOF). Determination of the absolute yield of HCOF permits the first measurement of its absolute IR band intensities (S). The carbonyl stretching mode for HCOF at 1780-1880 cm-1 has S = 4.2x10-17 cm molecule-1. These data were employed to study competition between the reactions of fluoromethyl (CH2F) radicals with chlorine, which leads to the production of CH2ClF, and oxygen, which leads via several steps to the production of HCOF. The chlorine mechanism is found to be 3.7 times slower than the oxygen pathway at equivalent concentrations. This mechanism constrains the ratio of the rate constants for the elementary reactions: CH2F + Cl2 → CH2ClF + Cl and CH2F + O2 (+ Ar) → CH2FOO (+ Ar), neither of which has been determined before. Relative kinetics yield the rate constant for the initial Cl + CH3F step, and also the kinetic isotope effect for Cl reaction with CD3F. A quantitative mechanism for atmospheric oxidation of fluoromethane, a simple example of halogenated hydrocarbons which are important in global warming and stratospheric ozone loss, has been developed through these experiments.

Spectroscopic and relative rate studies of the oxidation of fluoromethane initiated by atomic chlorine

2019 Spring National Meeting

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