The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol

TL;DR

This study measures and analyzes the reaction rate of C(3P) with methanol at low temperatures, revealing it as an efficient loss process for interstellar methanol, supported by experimental and theoretical agreement.

Contribution

It provides the first detailed kinetic and mechanistic analysis of the C(3P) + CH3OH reaction at interstellar temperatures, combining experimental data with ab-initio calculations.

Findings

Rate constant increases from 2.2 x 10^-11 to 20.0 x 10^-11 cm^3 molecule^-1 s^-1 from 296 K to 50 K

Major products are CH3 and HCO radicals under experimental conditions

Reaction significantly impacts interstellar methanol abundance in dense cloud models

Abstract

Rate constants for the C(3P) + CH3OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K to 296 K. C(3P) was created by the in-situ pulsed laser photolysis of CBr4, a multiphoton process which also produced some C(1D), allowing us to investigate simultaneously the low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H(2S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab-initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2 x 10-11 cm3 molecule-1 s-1 at 296 K to 20.0 x 10-11 cm3 molecule-1 s-1 at 50 K. CH3 and HCO are found to be the major products under our experimental conditions. As this reaction is not considered in current astrochemical networks, its influence on interstellar methanol abundances is tested using a model of dense interstellar clouds.