Kinetic isotope effects and rate constants for the gas-phase reactions of three deuterated toluenes with OH from 298 to 353 K

The rate constants for the gas-phase reactions of three deuterated toluenes with hydroxyl radicals were measured using the relative rate technique over the temperature range 298-353 K at about 1 atm total pressure. The OH radicals were generated by photolysis of H2O2, and helium was used as the diluent gas. The disappearance of reactants was followed by online mass spectrometry, which resulted in high time resolution, allowing for a large amount of data to be collected and used in the determination of the Arrhenius parameters. The following Arrhenius expressions have been determined for these reactions (in units of cm3 molecule-1 s-1): k=(6.42-0.99+1.17)x10-13exp [(661 +/- 54)/T] for toluene-d3, k=(2.11-0.69+1.03)x10-12exp [(287 +/- 128)/T]for toluene-d5, and k=(1.40+0.44-0.33)x10-12exp [(404 +/- 88)/T]for toluene-d8. The kinetic isotope effects (KIEs, kH/kD) of these reactions were 1.003 +/- 0.042 for all three compounds at 298 K. The KIE for toluene-d3 was temperature dependent; at 350 K, its KIE was 1.122+0.048-0.046. The KIE of toluene-d5 and toluene-d8 did not vary significantly with temperature. These KIE results suggest that methyl H-atom abstraction is more important than aromatic OH addition at higher temperatures. (C) 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 821827, 2012