Experimental and Theoretical Investigation of the Reaction of C₂H with Formaldehyde (CH₂O) at Very Low Temperatures and Application to Astrochemical Models

Rate coefficients for the reaction of C₂H with CH₂O were measured for the first time over the temperature range of 37-603 K, with the C₂H radicals produced by pulsed laser photolysis and detected by CH radical chemiluminescence following their reaction with O₂. The low temperature measurements (≤93 K) relevant to the interstellar medium were made within a Laval nozzle gas expansion, while higher temperature measurements (≥308 K) were made within a temperature controlled reaction cell. The rate coefficients display a negative temperature dependence below 300 K, reaching (1.3 ± 0.2) × 10^-10 cm³ molecule^-1 s^-1 at 37 K, while only a slight positive temperature dependence is observed at higher temperatures above 300 K. Ab initio calculations of the potential energy surface (PES) were combined with rate theory calculations using the MESMER master-equation program in order to predict rate coefficients and branching ratios. The three lowest energy entrance channels on the PES all proceed via the initial formation of a weakly bound prereaction complex, bound by ∼5 kJ mol^-1, followed by either a submerged barrier on the route to the H-abstraction products (C₂H₂ + CHO), or emerged barriers on the routes to the C- or O-addition species. MESMER calculations indicated that over the temperature range investigated (10-600 K) the two addition channels were uncompetitive, accounting for less 0.3% of the total product yield even at 600 K. The PES containing only the H-abstraction product channel was fit to the experimentally determined rate coefficients, with only a minor adjustment to the height of the submerged barrier (from -2.6 to -5.9 kJ mol^-1) required. Using this new submerged barrier height, and including the subsequent dissociation of the CHO product into CO + H in the PES, rate coefficients and branching ratios were calculated over a wide range of temperatures and pressures and these used to recommend best-fit modified Arrhenius expressions for use in astrochemical modeling. Inclusion of the new rate coefficients and branching ratios in a UMIST chemical model of an outflow from an asymptotic giant branch (AGB) star yielded no significant changes in the abundances of the reactants or the products of the reaction, however, removal of the C-addition channel currently in the UMIST Rate22 database did result in a significant reduction in the abundance of propynal (HCCCHO).