NASA - National Aeronautics and Space Administration

Kinetic isotope effect (KIE) and reaction rate coefficients, k1−k4, for the gasphase reaction of Cl atoms with 12CH3D (k1), 12CH2D2 (k2), 12CHD3 (k3), and 12CD4 (k4) over the temperature range 223−343 K in 630 Torr of synthetic air are reported. Rate coefficients were measured using a relative rate technique with 12CH4 as the primary reference compound. Fourier transform infrared spectroscopy was used to monitor the methane isotopologue loss. The obtained KIE values were 12CH3D: KIE1(T) = (1.227 ± 0.004) exp((43 ± 5)/T); 12CH2D2: KIE2(T) = (1.14 ± 0.20) exp((191 ± 60)/T); 12CHD3: KIE3(T) = (1.73 ± 0.34) exp((229 ± 60)/T); and 12CD4: KIE4(T) = (1.01 ± 0.3) exp((724 ± 19)/T), where KIEx(T) = kCl+12CH4(T)/kx(T). The quoted uncertainties are at the 2σ (95% confidence) level and represent the precision of our data. The following Arrhenius expressions and 295 K rate coefficient values (in units of cm3 molecule−1 s−1) were derived from the above KIE using a rate coefficient of 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1 for the reaction of Cl with 12

CH4: k1(T) = (5.95 ± 0.70) × 10−12 exp(−(1323 ± 50)/T), k1(295 K) = (6.7 ± 0.8) × 10−14; k2(T) = (6.4 ± 1.3) × 10−12 exp(−(1471 ± 60)/T), k2(295 K) = (4.4 ± 0.9) × 10−14; k3(T) = (4.2 ± 1.0) × 10−12 exp(−(1509 ± 60)/T), k3(295 K) = (2.53 ± 0.6) × 10−14; and k4(T) = (7.13 ± 2.3) × 10−12 exp(−(2000 ± 120)/T), k4(295 K) = (0.81 ± 0.26) × 10−14. The reported uncertainties in the pre-exponential factors are 2σ and include estimated systematic errors in our measurements and the uncertainty in the reference reaction rate coefficient. The results from this study are compared with previously reported room-temperature rate coefficients for each of the deuterated methanes as well as the available temperature dependent data for the Cl atom reactions with CH3D and CD4. A two-dimensional atmospheric chemistry model was used to examine the implications of the present results to the atmospheric lifetime and vertical variation in the loss of the deuterated methane isotopologues. The relative contributions of the reactions of OH, Cl, and O(1D) to the loss of the isotopologues in the stratosphere were also examined. The results of the calculations are described and discussed.