Low Temperature NH(X 3Σ-) Radical Reactions with NO, Saturated, and Unsaturated Hydrocarbons Studied in a Pulsed Supersonic Laval Nozzle Flow Reactor between 53 and 188 K

Abstract

The reactions of ground-state imidogen radicals (NH(X ³Σ^-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53−188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k₁(NH + NO) = (4.11 ± 0.31) × 10^-11 × (T/300)^{(-0.30±0.17)} × exp^(77±21/T) cm³/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53−135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)^-n, and are as follows: k₄ = (2.3 ± 1.2) × 10^-12 × (T/300)^{(-1.09±0.33)} cm³/s, k₅ = (4.5 ± 0.3) × 10^-12 × (T/300)^{(-1.07±0.04)} cm³/s, k₆ = (5.6 ± 1.9) × 10^-12 × (T/300)^{(-1.23±0.21)} cm³/s, and k₇ = (7.4 ± 1.8) × 10^-12 × (T/300)^{(-1.23±0.15)} cm³/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k₃ = (6.8 ± 1.7) × 10^-12 cm³/s, while that for methane at the same temperature represents an upper bound of k₂ ≤ (1.1 ± 4.3) × 10^-12 cm³/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.