Rate Coefficients for the Reactions of Hydroxyl Radicals with Methane and Deuterated Methanes

Abstract

The rate coefficients for the reaction of OH with CH3D (k 1), CH2D2 (k 2), CHD3 (k 3), CD4 (k 4), and CH4 (k 5) as well as that of OD with CH4 (k 6) have been measured using the pulsed photolytic production of OH followed by its detection via pulsed laser induced fluorescence. k 1−k 4 and k 6 were measured between ∼220 and ∼415 K, while k 5 was measured down to 195 K. The measured rate coefficients do not strictly obey the Arrhenius expression. However, below 298 K, they can be represented by the expressions (in cm3 molecule-1 s-1): k 1 = (3.11 ± 0.44) × 10-12 exp[−(1910 ± 70)/T]; k 2 = (2.3 ± 1.2) × 10-12 exp[−(1930 ± 250)/T]; k 3 = (1.46 ± 0.22) × 10-12 exp[−(1970 ± 70)/T]; k 4 = (1.00 ± 0.22) × 10-12 exp[−(2100 ± 120)/T]; k 5 = (1.88 ± 0.11) × 10-12 exp[−(1695 ± 30)/T]; k 6 = (1.68 ± 0.12) × 10-12 exp[−(1640 ± 40)/T]. The obtained values of the rate coefficients and kinetic isotope effects are compared with values previously measured or calculated by other groups. The atmospheric implications of this data are briefly discussed.

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.…”

“…However, over the temperature range ∼250-420 K the rate constants measured in these studies are significantly lower than most of the earlier absolute measurements (Atkinson, 1994). Gierczak et al (1997) fit their data and the earlier data of Vaghjiani and Ravishankara (1991) from the same laboratory to a three-parameter expression, and obtained the rate expression k(methane)=1.85×10 −20 T 2.82 e −987/T cm 3 molecule −1 s −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%.…”

“…The kinetics of this reaction have been investigated and evaluated by Golden et al (1998), with the recommended rate constant being derived from the experimental data using an RRKM model. In this review and evaluation, a simpler (and somewhat more approximate) rate expression analogous to that used previously (Atkinson, 1989) has been derived from the recommended experimental rate constants tabulated by Golden et al (1998), of k(CO)=9.1×10 −19 T 1.77 e 580/T ×…”

On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

“…In the absence of a laboratory measurement for the ratio q, we resort to an estimate using analogous reactions. Recently Gierczak et al (1997) studied the abstraction reactions between OH and the deuterated isotopomers…”