Kinetics and Product Branching Ratios of the Reaction of ¹CH₂ with H₂ and D₂

Abstract

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.

Experimental Studies of Gas-Phase Reactivity in Relation to Complex Organic Molecules in Star-Forming Regions

“…26 The removal of 1 CH 2 was instead dominated by quenching to 3 CH 2 at 100 K. Similar to Choi et al 221 , the authors used the reaction with H 2 as a calibration reaction, as they had previously measured the temperature dependent branching ratios for its reaction with 1 CH 2 in their laboratory. 223,224 Douglas et al 25 have also used LIF from OH to determine the branching ratios for quenching of 1 CH 2 to 3 CH 2 by H 2 and CH 4 versus reaction to other products. In these experiments,…”

Low temperature studies of the rate coefficients and branching ratios of reactive loss vs quenching for the reactions of 1CH2 with C2H6, C2H4, C2H2

“…Methane photolysis is also the major source of hydrocarbons radicals in the atmospheres of the giant planets. However, in these environments, 1 CH2 chemistry is dominated by the reaction with molecular hydrogen (R3) (Gannon et al, 2008;Hancock and Heal, 1992;Wagener, 1990).…”

“…Measurements indicate that relaxation accounts for only ~ 15 % of the overall loss process for reactions of 1 CH2 with H2, CH4, C2H2, C2H4, and C2H6, at around 300 K (Gannon et al, 2010a;Gannon et al, 2010b;Gannon et al, 2008). In a previous publication, we reported the branching ratio (BR) between electronic relaxation and chemical reaction as a function of temperature down to 73 K for the reaction of 1 CH2 with H2 (R3) and CH4 (R4) (Douglas et al, 2018), indicating that by 73 K relaxation had risen to become the dominant removal process, accounting for ~ 70 % of 1 CH2 loss.…”

“…In a previous publication, we reported the branching ratio (BR) between electronic relaxation and chemical reaction as a function of temperature down to 73 K for the reaction of 1 CH2 with H2 (R3) and CH4 (R4) (Douglas et al, 2018), indicating that by 73 K relaxation had risen to become the dominant removal process, accounting for ~ 70 % of 1 CH2 loss. BRs for the reaction of 1 CH2 with C2H2 (R7) and C2H4 (R8) have also been measured in this laboratory as function of temperature between 195 and 498 K (Gannon et al, 2010a;Gannon et al, 2010b;Gannon et al, 2008), and indicate that the fraction of reactive loss decreases from ~ 85 % at 300 K, down to ~ 30 % at 195 K. These results suggest that at the temperatures of Titan's photochemically active thermosphere (120 -160 K), in reactions of 1 CH2 with acetylene and ethylene, relaxation to 3 CH2 should be the dominant process, however no direct measurements at these temperatures have been made. For the reaction of 1 CH2 with ethane, there are currently no temperature dependant BR measurements.…”

Gas phase reactive collisions, experimental approach

“…Nevertheless, reactions for which only one exit channel is open can be used as references to determine the product branching ratio of another process. This idea has been used in the groups of Seakins [114][115][116][117][118] and Loison [119][120][121][122][123][124][125] to obtain the branching ratios of channels producing hydrogen atoms. Calibration reactions such as CN + H 2 → HCN + H, CH + CH 4 → C 2 H 4 + H or C + C 2 H 4 → C 3 H 3 + H can be used where H atoms are detected by VUV LIF.…”