A kinetic study of the gas-phase C(3P) + CH3CN reaction at low temperature. Rate constants, H-atom product yields and astrochemical implications

TL;DR

This study measures the rate constants and H-atom yields for the C(3P) + CH3CN reaction at low temperatures, combining experimental kinetics with quantum chemical calculations to assess its astrochemical relevance.

Contribution

It provides the first low-temperature kinetic data and quantum chemical insights for the C(3P) + CH3CN reaction, highlighting its impact on interstellar methyl cyanide abundance models.

Findings

Reaction is very fast with rate constants (3-4) x 10^-10 cm^3 s^-1.

Reaction rate shows little temperature dependence.

Including this reaction in astrochemical models reduces predicted CH3CN abundances.

Abstract

Rate constants have been measured for the C(3P) + CH3CN reaction between 50 K and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in-situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultra-violet laser induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + 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 triplet 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) x 10-10 cm3 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.