Rate Constants for the Reactions of Hydroxyl Radical with Several Alkanes, Cycloalkanes, and Dimethyl Ether

Abstract

Relative rate experiments were used to measure ratios of rate constants as a function of temperature for the reactions of OH with propane, n -butane, n -pentane, n -hexane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, and dimethyl ether. To assure internal consistency, ratios were measured for seventeen reactant pairs among these reactants. All of the derived rate constants are based on an absolute rate constant of the OH + C 2 H 6 reaction using k (ethane) = 1.0 × 10 -11 exp(−1094/ T ) cm 3 /molecule s. The rate constants obtained are as follows. propane: 1.29 × 10 -11 exp(−730/ T ), k (298 K) = 1.11 × 10 -12 . n -butane: 1.68 × 10 -11 exp(−584/ T ), k (298 K) = 2.37 × 10 -12 . n -pentane: 1.94 × 10 -11 exp(−494/ T ), k (298 K) = 3.70 × 10 -12 . n -hexane: 2.60 × 10 -11 exp(−480/ T ), k (298 K) = 5.19 ×10 -12 . cyclopropane: 5.15 × 10 -12 exp(−1255/ T ), k (298 K) = 7.64 × 10 -14 . cyclobutane: 1.62 × 10 -11 exp(−611/ T ), k (298 K) = 2.08 × 10 -12 . cyclopentane: 2.57 × 10 -11 exp(−498/ T ), k (298 K) = 4.83 × 10 -12 . cyclohexane: 3.58 × 10 -11 exp(−500/ T ), k (298 K) = 6.69 × 10 -12 . dimethyl ether: 1.51 × 10 -11 exp(−496/ T ), k (298 K) = 2.86 × 10 -12 . These results are compared with previous literature data and are discussed in terms of trends in preexponential factors and activation energies. Also, rate constants and Arrhenius parameters are derived for methylene groups in the alkanes and cycloalkanes. In the low temperature regime, the present data illustrate a persistent discrepancy between absolute and relative rate measurements. The relative data show less curvature at low temperatures, and can be adequately fit with two-parameter Arrhenius expressions.