Is the gas-phase OH+H 2 CO reaction a source of HCO in interstellar cold dark clouds? A kinetic, dynamic and modelling study.

Chemical kinetics of neutral-neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T∼10-100K). Scarce kinetic information is currently available for this kind of reactions at T<200 K. In this work we use the CRESU ( Cinétique de Réaction en Ecoulement Supersonique Uniforme , which means Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients ( k ) of the gas-phase OH+H ₂ CO reaction between 22 and 107 K. k values greatly increase from 2.1×10 ^-11 cm ³ s ^-1 at 107 K to 1.2×10 ^-10 cm ³ s ^-1 at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and ab initio potential energy surface, recently developed which generates highly accurate potential and includes long range dipole-dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+H ₂ CO reaction. In order to revisit the chemistry of HCO in cold dense clouds, k is reasonably extrapolated from the experimental results at 10K (2.6×10 ^-10 cm ³ s ^-1 ). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 10 ⁵ -10 ⁶ yrs. The different sources of production of HCO are presented and the uncertainties in the chemical networks discussed. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. The present reaction is shown to account for a few percent of the total HCO production rate. Extensions to photodissociation regions and diffuse clouds environments are also commented.