A kinetic study of hydroxyl radical reactions with methane and perdeuterated methane

Abstract

We measured absolute rate coefficients for the reactions of the hydroxyl radical with methane (kl) and methaned4 (k2) using the laser photolysis/laser-induced fluorescence technique. We characterized kl and k2 over the temperature range 293-800 K at pressures between 400 and 750 Torr of helium. We find excellent agreement between our results and the recent determinations of kl at lower temperatures by Vaghjiani and Ravishankara.The measured rate coefficients, in the units cm3 molecule-' s-l, fit well to the three-parameter expressions k l ( T ) = 9.65 X lO-2OP1.58 exp(-1082/T) and k2(79 = 8.70 X 10-22T3.23 exp(-1334/T). The kinetic isotope effect for abstraction of the H and D atoms varies from 6.75 at 293 K to 1.96 at 800 K. We compare our results to recently reported calculations by Melissas and Truhlar.

Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

“…The recent studies of Bott and Cohen (1989), Vaghjiani and Ravishankara (1991), Finlayson-Pitts et al (1992), Lancar et al (1992), Dunlop and Tully (1993), Mellouki et al (1994), Gierczak et al (1997) and Bonard et al (2002) are in good agreement, as shown by the Arrhenius plot in Fig. 1.…”

“…This rate expression is plotted as the solid line in the Arrhenius plot ( Fig. 1), and provides an excellent fit to the data of Bott and Cohen (1989), Vaghjiani and Ravishankara (1991), Finlayson-Pitts et al (1992), Lancar et al (1992), Dunlop and Tully (1993), Mellouki et al (1994), Gierczak et al (1997) and Bonard et al (2002), agreeing with the 1234 K rate constant of Bott and Cohen (1989) to within 1% and with the 800 K rate constant of Dunlop and Tully (1993) and the 295-668 K rate constants of Bonard et al (2002) to within 10%. Accordingly, the rate expression of Gierczak et al (1997) The overall uncertainty in the rate constant at 298 K is estimated to be ±20%.…”

“…Figure 3 shows an Arrhenius plot of the absolute rate constants measured by Gordon and Mulac (1975), Dunlop and Tully (1993) and Gierczak et al (1997). The rate constant of Gordon and Mulac (1975) at 416 K is a factor of 2 lower than those of Dunlop and Tully (1993) and Gierczak et al (1997), which are in Dunlop and Tully (1993) and Vaghjiani and Ravishankara (1991) over the temperature range 223-800 K. c Fit of combined data sets of Gierczak et al (1997) and Vaghjiani and Ravishankara (1991) over the temperature range 195-420 K. excellent agreement over the temperature range common to both studies (293-413 K). The rate constants of Dunlop and Tully (1993) and Gierczak et al (1997) have been fitted to the three parameter expression k=AT 2 e −B/T , leading to the recommendation of…”

Tracking radical migration in large hydrogen deficient peptides with covalent labels: Facile movement does not equal indiscriminate fragmentation

“…Substitution of hydrogen for deuterium in direct abstraction reactions typically results in significant kinetic isotope effects. Measured kinetic isotope effects for hydrogen abstraction by ·OH range from 2.0 to 6.8 and decrease linearly with temperature [34]. Thus, monitoring deu- terium transfers provides a lower limit to the extent of hydrogen transfers that would occur in an unlabeled peptide.…”

Photocoupling of Methane in Water Vapor to Saturated Hydrocarbons

“…The resultant OH radicals are highly reactive such that they are able to abstract hydrogen from methane [19]. The methyl radicals thus formed are then able to couple to form ethane.…”