Experimental, theoretical, and astrochemical modelling investigation of the gas-phase reaction between the amidogen radical (NH2) and acetaldehyde (CH3CHO) at low temperatures

Abstract

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH₂ with acetaldehyde (CH₃CHO) has been performed. Experiments were carried out using laser-flash photolysis and laser-induced fluorescence spectroscopy to create and monitor the temporal decay of NH₂ in the presence of CH₃CHO. Low temperatures relevant to the interstellar medium were achieved using a pulsed Laval nozzle expansion. Rate coefficients were measured over the temperature and pressure range of 29–107 K and 1.4–28.2 × 10¹⁶ molecules per cm³, with the reaction exhibiting a negative temperature dependence and a positive pressure dependence. The yield of CH₃CO from the reaction has also been determined at 67.1 and 35.0 K, by observing OH produced from the reaction of CH₃CO with added O₂. Ab initio calculations of the potential energy surface (PES) were combined with Rice–Rampsberger–Kessel–Marcus (RRKM) calculations to predict rate coefficients and branching ratios over a broad range of temperatures and pressures. The calculated rate coefficients were shown to be sensitive to the calculated density of states of the stationary points, which in turn are sensitive to the inclusion of hindered rotor potentials for several of the vibrational frequencies. The experimentally determined rate coefficients and yields have been used to fit the calculated PES, from which low-pressure limiting rate coefficients relevant to the ISM were determined. These have been included in a single-point dark cloud astrochemical model, in which the reaction is shown to be a potential source of gas-phase CH₃CO radicals under dark cloud conditions.