New kinetic data and product distributions have been obtained using the experimental CRESU technique combined with a theoretical analysis of the reaction mechanism. The astrophysical implications of fast CH3O and CH2OH formation are discussed.

In the last years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than previously thought. One example is the reaction between OH and CHOH, which has been recently found to be accelerated at low temperatures yielding CHO as main product. This finding opened the question of whether the CHO observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CHOH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CHO with little success. Here we report for the first time rate coefficients for the gas-phase reaction of OH and CHOH down to a temperature of 22 K, very close to those in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis-laser induced fluorescence technique were used to determine rate coefficients in the temperature range 22-64 K. The temperature dependence obtained in this work can be expressed as (22-64 K) = (3.6 ± 0.1) × 10(300 K) cm molecule s. Implementing this expression in a chemical model of a cold dense cloud results in CHO/CHOH abundance ratios similar or slightly lower than the value of ∼ 3 × 10 observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CHOH is an important contributor to the formation of interstellar CHO. The role of grain-surface processes in the formation of CHO, although it cannot be fully neglected, remains controversial.

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 (, which means Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients () of the gas-phase OH+HCO reaction between 22 and 107 K. values greatly increase from 2.1×10 cm s at 107 K to 1.2×10 cm s 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 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+HCO reaction. In order to revisit the chemistry of HCO in cold dense clouds, is reasonably extrapolated from the experimental results at 10K (2.6×10 cm s). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 10-10 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.

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