Reactions of Nitrogen–Hydrogen Radicals. III. Formation and Disappearance of NH Radicals in the Photolysis of Ammonia

Abstract

The rates of formation and disappearance of the NH biradical in the ground and first‐excited vibrational states were measured by kinetic spectroscopy following flash photolysis of mixtures of ammonia and inert gases. The delay in the appareance of NH following a 25‐μsec photolysis flash shows that it is formed by secondary reactions of the primary photolysis products, either NH₂ + NH₂ or NH₂ + H. A large proportion of the observed NH is initially formed in the $\text{υ″\hspace{0.17em}=\hspace{0.17em}1}$ vibrational state, which disappears an order of magnitude faster than $\mathrm{NH}\text{(υ″\hspace{0.17em}=\hspace{0.17em}0)}$ . The disappearance of $\mathrm{NH}\text{(υ″\hspace{0.17em}=\hspace{0.17em}0)}$ is pressure dependent and in the falloff region at 1 torr, 300°K. The rate of disappearance is rapid and is consistent with NH + NH₃ insertion. At the highpressure limit the rate constant for this reaction is $k_4{\text{\hspace{0.17em}=\hspace{0.17em}1.0\hspace{0.17em}±\hspace{0.17em}0.3\hspace{0.17em}×\hspace{0.17em}10}}^{10}\mathrm{liter}/\mathrm{mole}·\sec$ . The reaction has a small negative energy of activation. The high rate of NH disappearance accounts for the small range of conditions under which it can be observed. Implications of the results to the mechanism of biradical insertion reactions are discussed.