Experimental Gas-Phase Removal of OH Radicals in the Presence of NH₂C(O)H over the 11.7–353 K Range: Implications in the Chemistry of the Interstellar Medium and the Earth’s Atmosphere

Abstract

Formamide (NH 2 C(O)H) has been observed both in the interstellar medium (ISM), being identified as a potential precursor of prebiotic molecules in space, and in the Earth's atmosphere. In these environments where temperature is very distinct, hydroxyl (OH) radicals may play an important role in the degradation of NH 2 C(O)H. Thus, in this work, we report for the first time the experimental study of the temperature dependence of the gas-phase removal of OH in the presence of NH 2 C(O)H over the 11.7−353 K range. In the lowest temperature range (11.7− 177.5 K), of interest for the ISM chemistry, the kinetic study was performed using a pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus, while a thermostatized slow-flow reactor was employed in the kinetic study of the OH + NH 2 C(O)H reaction over the 273−353 K range, of interest in the Earth's troposphere below room temperature. The pulsed laser photolysis at 248 nm of a suitable OH-precursor (hydrogen peroxide, tert-butyl hydroperoxide, or acetylacetone) was used to generate OH radicals in the reactor. The temporal evolution of OH was monitored by laser-induced fluorescence at 310 nm. An almost independent k(T) between 273 and 353 K (temperatures of the Earth's troposphere extended to T > 298 K) is reported, being the OH + NH 2 C(O)H reaction the major degradation route with an atmospheric lifetime of around 1 day. At lower temperatures of interest in the ISM (11.7−177.5 K), the potential formation of NH 2 C(O)H dimers was evaluated. Thermodynamically, under equilibrium conditions, formamide would be fully converted into the dimer in that T range. However, the qualitative agreement of the observed increase of k(T) with computational studies on the OH + NH 2 C(O)H reaction down to 200 K let us to report, between 177.5 and 106.0 K, the following parameters commonly used in astrochemical modeling: α = (3.76 ± 0.62) × 10 −12 cm 3 s −1 , β = (3.07 ± 0.11), and γ = 0. At 11.7 K, a kinetic model reproducing the experimental data indicates that formamide dimerization could be important, but the OH-reaction with the monomer would be fast, 4 × 10 −10 cm 3 s −1 , and the OH-reaction with the dimer, relatively slow [(0.1−1.0) × 10 −11 cm 3 s −1 ]. Despite that, the impact of the gas-phase OH + NH 2 C(O)H in the relative abundances of NH 2 C(O)H in a dense molecular cloud (T ∼ 10 K) and after the warm-up phase in the surroundings of hot cores/corinos (here, 10−400 K) appears to be negligible.