More than 100 reactions between stable molecules and free radicals have been shown to remain rapid at low temperatures. In contrast, reactions between two unstable radicals have received much less attention due to the added complexity of producing and measuring excess radical concentrations. We performed kinetic experiments on the barrierless N((4)S) + OH((2)Π) → H((2)S) + NO((2)Π) reaction in a supersonic flow (Laval nozzle) reactor. We used a microwave-discharge method to generate atomic nitrogen and a relative-rate method to follow the reaction kinetics. The measured rates agreed well with the results of exact and approximate quantum mechanical calculations. These results also provide insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar clouds.

Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N 2 , with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N 2 is difficult to detect spectroscopically through infrared or millimeterwavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N 2 H þ . Two main formation mechanisms, each involving two radical-radical reactions, are the source of N 2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N þ CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N 2 formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N 2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N 2 formation are inefficient, we argue that N 2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH 3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.astrochemistry | chemical kinetics E lemental nitrogen is present in the atmospheres of telluric planets predominantly in the form of N 2 . As a stable molecule with a high bond energy, it is hard to break N out of N 2 , so the amount of nitrogen available for the formation of complex prebiotic molecules in such environments depends on the presence of nitrogen in more labile forms such as NH 3 . Despite the high abundance of N 2 in the solar system, observations of N 2 beyond our own planetary system are scarce, as N 2 possesses no allowed rotational or vibrational transitions. Only one direct measurement (1) of N 2 in the far ultraviolet wavelength range (thereby accessing its electronic transitions) has been reported in a diffuse interstellar cloud, where low column densities allow light to pass and a star along the line of sight was used to probe the species within. In dense molecular clouds where temperatures as low as 10 K prevail, UV starlight is absorbed long before it can penetrate to the cloud core where N 2 is predicted to form efficiently (2, 3). Instead, gas-phase N 2 densities are inferred through observations of N 2 H þ , a product of the N 2 þ H 3 þ reaction (4-6). The major pathways for N 2 formation in dense clouds rely on reactions involving neutral radical species:Mechanism (II) Recent experimental and theoretical studies of reactions 1 (7) and 2 (8-10) indicate that these reactions are slower than previously thought, suggesting that current astrochemical models overestimate molecular nitrogen abundances produced by Mechanism (I) both at steady state and at specific times. The influence of Mechani...