Experimental\nGas-Phase Removal of OH Radicals in the\nPresence of NH<sub>2</sub>C(O)H over the 11.7–353 K Range:\nImplications in the Chemistry of the Interstellar Medium and the Earth’s\nAtmosphere

Formamide (NH<sub>2</sub>C(O)H) has been observed both\nin the interstellar\nmedium (ISM), being identified as a potential precursor of prebiotic\nmolecules in space, and in the Earth’s atmosphere. In these\nenvironments where temperature is very distinct, hydroxyl (OH) radicals\nmay play an important role in the degradation of NH<sub>2</sub>C(O)H.\nThus, in this work, we report for the first time the experimental\nstudy of the temperature dependence of the gas-phase removal of OH\nin the presence of NH<sub>2</sub>C(O)H over the 11.7–353 K\nrange. In the lowest temperature range (11.7–177.5 K), of interest\nfor the ISM chemistry, the kinetic study was performed using a pulsed\nCRESU (French acronym for Reaction Kinetics in a Uniform Supersonic\nFlow) apparatus, while a thermostatized slow-flow reactor was employed\nin the kinetic study of the OH + NH<sub>2</sub>C(O)H reaction over\nthe 273–353 K range, of interest in the Earth’s troposphere\nbelow room temperature. The pulsed laser photolysis at 248 nm of a\nsuitable OH-precursor (hydrogen peroxide, <i>tert</i>-butyl\nhydroperoxide, or acetylacetone) was used to generate OH radicals\nin the reactor. The temporal evolution of OH was monitored by laser-induced\nfluorescence at 310 nm. An almost independent <i>k</i>(<i>T</i>) between 273 and 353 K (temperatures of the Earth’s\ntroposphere extended to <i>T</i> > 298 K) is reported,\nbeing\nthe OH + NH<sub>2</sub>C(O)H reaction the major degradation route\nwith an atmospheric lifetime of around 1 day. At lower temperatures\nof interest in the ISM (11.7–177.5 K), the potential formation\nof NH<sub>2</sub>C(O)H dimers was evaluated. Thermodynamically, under\nequilibrium conditions, formamide would be fully converted into the\ndimer in that <i>T</i> range. However, the qualitative agreement\nof the observed increase of <i>k</i>(<i>T</i>)\nwith computational studies on the OH + NH<sub>2</sub>C(O)H reaction\ndown to 200 K let us to report, between 177.5 and 106.0 K, the following\nparameters commonly used in astrochemical modeling: α = (3.76\n± 0.62) × 10<sup>–12</sup> cm<sup>3</sup> s<sup>–1</sup>, β = (3.07 ± 0.11), and γ = 0. At 11.7 K, a kinetic\nmodel reproducing the experimental data indicates that formamide dimerization\ncould be important, but the OH-reaction with the monomer would be\nfast, 4 × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup>, and the OH-reaction with the dimer, relatively slow [(0.1–1.0)\n× 10<sup>–11</sup> cm<sup>3</sup> s<sup>–1</sup>]. Despite that, the impact of the gas-phase OH + NH<sub>2</sub>C(O)H\nin the relative abundances of NH<sub>2</sub>C(O)H in a dense molecular\ncloud (<i>T</i> ∼ 10 K) and after the warm-up phase\nin the surroundings of hot cores/corinos (here, 10–400 K) appears\nto be negligible.