Kinetic Study of the Gas-Phase C(³P) + CH₃CN Reaction at Low Temperatures: Rate Constants, H-Atom Product Yields, and Astrochemical Implications

Abstract

Rate constants have been measured for the C( 3 P) + CH3CN reaction between 50 K and 296 K using a continuous-flow supersonic reactor. C( 3 P) atoms were created by the in-situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C( 3 P) atom loss were followed by direct vacuum ultra-violet laser induced fluorescence at 115.8 nm. Secondary measurements of product H( 2 S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C( 3 P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states relevant to the title reaction over the 3 Aʹʹ potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range (3-4) ´ 10 -10 cm 3 s -1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.

Kinetic Study of the Gas-Phase O(1D) + CH3OH and O(1D) + CH3CN Reactions: Low-Temperature Rate Constants and Atomic Hydrogen Product Yields

“…The derived k 1st values were plotted as a function of the coreagent concentration as shown in Figure 2 for CH 3 OH and in Figure 3 for CH processes are not expected to modify the derived second-order rate constants for either of these reactions, which are determined from the slopes of individual fits. In common with an earlier study on the C( 3 P) + CH 3 CN reaction, 41 cluster formation occurred readily in the experiments employing CH 3 CN due to its large electric dipole moment. Although it was possible to derive rate constants for the C( 3 P) + CH 3 CN reaction to temperatures as low as 50 K, the large carrier gas quenching contribution of O( 1 D) atoms in the present work made measurements at this temperature unreliable.…”

“…In common with our earlier studies of the reactions of ground state atomic carbon with CH 3 OH and CH 3 CN, the concentration ranges that could be used during these experiments were limited by the onset of cluster formation in the supersonic flow. Consequently, only those datapoints yielding a pseudo-first-order rate constant that displayed a linear dependence on the coreagent concentration were used in the final analysis to obtain temperature dependent rate constants.…”

“…In previous work on the reactions of C­( 1 D) and C­( 3 P) atoms with hydrogen bearing molecules, , absolute H-atom product-branching ratios were obtained by comparing the H-atom formation curves produced by the target reaction to the ones produced by a reference reaction with a known H-atom yield. During these experiments, the excess coreagent concentration was varied so that it was possible to compare traces with similar H-atom production rates.…”

Kinetic Study of the Gas-Phase Reaction between Atomic Carbon and Acetone: Low-Temperature Rate Constants and Hydrogen Atom Product Yields

“…Indeed, earlier studies of the barrierless reactions of C( 3 P) with other reagents involving complex formation and a submerged barrier toward adduct formation 5,14,31 present much stronger temperature dependences than those C-atom reactions where these features are absent. 15 Despite this, the measured reaction rates at 75 and 50 K are slightly lower than those obtained at a higher temperature, although the differences are close to being covered by the combined measurement uncertainties. In this respect, for low-temperature interstellar modeling purposes, we recommend the use of a temperature-independent value for the rate constant, k C+CHd 3 COCHd 3 = 2.2 × 10 −10 cm 3/ s. Mechanistically, the observed rate constant decrease could be indicative of the opening of the reactive channel via TS2 as the temperature falls (Figure 1), with part of the reactive flux being trapped in MIN2 rather than passing by the other barrierless channels via MIN1 and MIN9.…”

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…In the reaction between C( 3 P) and CH 3 CN, both the rate of C( 3 P) decay and the formation of H( 2 S) products were (separately) detected. 95 This allowed temperature-dependent product branching ratios to be determined—paving the way for possible future measurements of the internal energy distribution in products. The combination of CRESU methods with a chirped-pulse Fourier-transform microwave spectrometer (CP-FTMW) and a continuous-wave cavity ring down spectrometer have further extended the applicability of CRESU to the study of CN radical reaction dynamics.…”