Rate Constants for Abstraction of H from the Fluoromethanes by H, O, F, and OH

Abstract

In this work, we compiled and critically evaluated rate constants from the literature for abstraction of H from the homologous series consisting of the fluoromethanes (CH3F, CH2F2, and CHF3) and methane (CH4) by the radicals H atom, O atom, OH, and F atom. These reactions have the form RH + X → R + HX. Rate expressions for these reactions are provided over a wide range of temperatures (300–1800 K). Expanded uncertainty factors f (2σ) are provided at both low and high temperatures. We attempted to provide rate constants that were self-consistent within the series—evaluating the system, not just individual reactions. For many of the reactions, the rate constants in the literature are available only over a limited temperature range (or there are no reliable measurements). In these cases, we predicted the rate constants in a self-consistent manner employing relative rates for other reactions in the homologous series using empirical structure–activity relationships, used empirical correlations between rate constants at room temperature and activation energies at high temperatures, and used relative rates derived from ab initio quantum chemical calculations to assist in rate constant predictions.

Theory-Based Mechanism for Fluoromethane Combustion I: Thermochemistry and Abstraction Reactions

“…Figure compares the current results with the prior literature for OH + fluoromethane, including both experimental − and theoretical results ,,,− (to improve the clarity, theoretical results prior to the year 2000 are not shown). As with abstraction via H radicals, the barrier heights for abstraction via OH were increased slightly: OH + CH 3 F, +2.0 kJ/mol; H + CH 2 F 2 , +1.5 kJ/mol; H + CHF 3 , +2.0 kJ/mol.…”

“…The results for H abstraction via H and O atoms from each specific fluoromethane are summarized in Figure . This figure includes both the preferred experimental data − (as recommended by Burgess and Manion) as well as prior theoretical predictions − (to improve the clarity, theoretical results prior to the year 2000 are not shown ,, ). In each subplot, the solid black line is the current work, and the dotted blue line is the recent recommendation from ref along with the recommended uncertainty value (shaded region).…”

“…This figure includes both the preferred experimental data − (as recommended by Burgess and Manion) as well as prior theoretical predictions − (to improve the clarity, theoretical results prior to the year 2000 are not shown ,, ). In each subplot, the solid black line is the current work, and the dotted blue line is the recent recommendation from ref along with the recommended uncertainty value (shaded region). Modest adjustments were made to the ANL0 barrier heights for the H radical abstraction to improve the agreement with the experimental data: H + CH 3 F, +1.0 kJ/mol; H + CH 2 F 2 , +0.5 kJ/mol; H + CHF 3 , +0.5 kJ/mol.…”

“…Fluoromethanes (CH 3 F, CH 2 F 2 , CHF 3 , and CF 4 ) have been used extensively as flame suppressants and refrigerant working fluids. The thermal stability and gas-phase decomposition kinetics of fluoromethanes has been the subject of numerous experimental and theoretical studies. − In recent years, much of this work has been performed by the Chemical Sciences Division at the National Institute for Standards and Technology (NIST) as part of their program on the kinetics of flame suppressants and refrigerants. − ,− …”

“…The heats of formation for small fluorinated hydrocarbons have been investigated using a variety of computational methods. ,− The elementary rate constants for H abstraction by H, O, F, and OH were recently reviewed by Burgess and Manion, and the reader is directed to the references and discussion therein for a thorough review. The present work is a complement to their review in two respects.…”