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

Abstract

The kinetics of the reactions of the first excited state of methylene, 1 CH2, with C2H2, C2H4, and C2H6, have been measured over the temperature range 43-298 K by pulsed laser photolysis, monitoring 1 CH2 removal by laser induced fluorescence. Low temperatures were obtained using a pulsed Laval expansion (43-134 K), while a slow flow reaction cell was used for temperatures of 160 K and above. The rate coefficients for the reactions with C2H2, C2H4, and C2H6, all showed a strong negative temperature dependence. In combination with other literature data, the coefficients can be parameterized as: kC 2 H 2 (43 ≤ T/K ≤ 298) = (3.22 ± 0.15) × 10-10 × (T/298) (-0.394 ±0.066) kC 2 H 4 (43 ≤ T/K ≤ 298) = (2.16 ± 0.14) × 10-10 × (T/298) (-0.612 ±0.089) kC 2 H 6 (43 ≤ T/K ≤ 298) = (1.78 ± 0.10) × 10-10 × (T/298) (-0.545 ±0.078) Branching ratios for reactive removal of 1 CH2 vs quenching to ground state were also determined for all three colliders and for H2 and CH4, at temperatures between 100 and 298 K. The values measured show that the dominant removal process of 1 CH2 by H2, C2H2, and C2H4, changes from reactive removal to quenching to ground state 3 CH2 as the temperature decreases from 298 K to 100 K, while for CH4 and C2H6, reactive removal drops from around 85 % to around 55 %. The impacts of the new measurements for Titan's atmosphere are examined using a 1D chemistry and transport model. A significant increase (~25%) in the mixing ratio of benzene between 500 and 1550 km is calculated, due to the increased production of C3H3 from the reaction of 1 CH2 with C2H2.

A major ice component in Pluto’s haze

“…Models The photochemical model is based on previous studies of Titan's photochemistry 46 , updated with high-resolution energy deposition calculations 6 and the latest chemical reaction rates 2,47,48 . The model solves the continuity equation taking into account atmospheric mixing and molecular diffusion, the chemical production and loss processes, and loss/gain due to condensation/evaporation, for each species considered.…”

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

“…Versions based on electromagnetic valves supplying gas into the reservoir upstream of the Laval nozzle require very small reservoir volumes (∼1 cm 3 ) owing to the limited flow rate of such valves. Instead of being essentially at rest, the gas is already moving fast and turbulently in the reservoir, impacting the quality of the uniform supersonic flow and limiting the lowest achievable temperature to 43 K. , The use of piezoelectric actuator valves, which allow higher gas flow rates and thus somewhat larger reservoirs, or rotating disk valves, which interrupt the flow close to the Laval throat, enable better quality flows and temperatures as low as 20 and ∼12 K to be achieved, respectively.…”

“…Using this method, the authors found room temperature branching ratios to H atom channels of (1.08 ± 0.13) and (1.04 ± 0.11) for the reactions of CN with C 2 H 2 and C 2 H 4 , respectively. The H atom calibration technique has been applied to a handful of reactions at T < 300 K: by the group of Seakins and co-workers at the University of Leeds, to study reactions of 1 CH 2 , 26,222 and by Hickson and co-workers at the Universited e Bordeaux for reactions involving C( 3 P), 139,142,144 C( 1 D), 149 and O( 1 D). 158,160 In the former set of experiments, the authors found that the H atom branching ratio for the reaction of 1 CH 2 with both acetylene and ethene increases with temperature between 195 and 298 K. The temperature range was later extended down to 100 K using a pulsed CRESU apparatus, and the H atom branching ratio was shown to continue to decrease at lower temperatures.…”

“…The H atom calibration technique has been applied to a handful of reactions at T < 300 K: by the group of Seakins and co-workers at the University of Leeds, to study reactions of 1 CH 2 , , and by Hickson and co-workers at the Université de Bordeaux for reactions involving C­( 3 P), ,, C­( 1 D), and O­( 1 D). , In the former set of experiments, the authors found that the H atom branching ratio for the reaction of 1 CH 2 with both acetylene and ethene increases with temperature between 195 and 298 K. The temperature range was later extended down to 100 K using a pulsed CRESU apparatus, and the H atom branching ratio was shown to continue to decrease at lower temperatures . The removal of 1 CH 2 was instead dominated by quenching to 3 CH 2 at 100 K. Similar to Choi et al, 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. , Douglas et al 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.…”

Impact of pyruvic acid photolysis on acetaldehyde and peroxy radical formation in the boreal forest: theoretical calculations and model results

“…The reaction with pyruvic acid itself is also briefly explored. The rovibrational characteristics and energetics of all critical points on the potential energy surface were characterized at the CCSD(T)/aug-cc-pVTZ//M06-2X-D3/aug-cc-pVTZ level of theory with a wavenumber scaling factor of 0.971 (Zhao and Truhlar, 2008;Dunning, 1989;Purvis and Bartlett, 1982;Grimme et al, 2011; Database of Frequency Scale Factors for Electronic Model Chemistries (Version 4); Alecu et al, 2010). This method compares favourably with the more rigorous focal point analysis of Schreiner et al (2011), with energy differences in the singlet state unimolecular chemistry of less than 0.7 kcal mol −1 , indicating that the method is reliable for kinetic predictions under atmospheric temperatures.…”