Reactions of hydrazine with the amidogen radical and atomic hydrogen

Abstract

The rate coefficient k1 for NH2 + N2H4 was measured to be (5.4 ± 0.4) × 10−14 cm3 molecule−1 s−1 at 296 K. NH2 was generated by pulsed laser photolysis of NH3 at 193 nm, and monitored as a function of time by pulsed laser-induced fluorescence excited at 570.3 nm under pseudo-first order conditions in the presence of excess N2H4 in an Ar bath gas. This reaction was also investigated computationally, with geometries and scaled frequencies obtained with M06-2X/6-311+G(2df,2p) theory, and single-point energies from CCSD(T)-F12b/cc-pVTZ-F12 theory, plus a term to correct approximately for electron correlation through CCSDT(Q). Three connected transition states are involved and rate constants were obtained via Multistructural Improved Canonical Variational Transition State Theory with Small Curvature Tunneling. Combination of experiment and theory leads to a recommended rate coefficient for hydrogen abstraction of k1 = 6.3 × 10−23 T3.44 exp(+289 K/T) cm3 molecule−1 s−1. The minor channel for H + N2H4 forming NH2 + NH3 was characterized computationally as well, to yield 5.0 × 10−19 T2.07 exp(-4032 K/T) cm3 molecule−1 s−1. These results are compared to several discordant prior estimates, and are employed in an overall mechanism to compare with measurements of half-lives of hydrazine in a shock tube.