Examination of the product channels in the reactions of NH(a 1Δ) with H2 and D2

Abstract

A flash photolysis study (193 nm) of HNCO has been conducted and the mechanisms of the reactions NH(a 1Δ)+H2→NH2+H(1) and NH(a 1Δ)+D2→products (2) have been examined in detail at 295±3 K by monitoring NH(a 1Δ), H, D, NH2, and their D substituents via the laser induced fluorescence technique. From the pseudo-first-order analysis of the decay rate for NH(a 1Δ), rate constants have been determined as k1=(3.96±0.17)×10−12 and k2=(2.62±0.08)×10−12. (All the rate constants are expressed in units of cm3 molecule−1 s−1.) These rate constants are consistent with those determined from the time dependence of H and D atoms: they are k1=(3.76±0.70)×10−12 and k2=(2.78±0.17)×10−12. No pressure dependence has been observed for 10–100 Torr He. The branching fraction for H and D atoms as products for reaction (2) has been found to be [H]/[D] =0.24/0.76, where D production is more abundant than statistically predicted. This indicates that reaction (2) is dominated by insertion of NH(a 1Δ) into the D2 bond, but vibrational energy of the reaction intermediate NHD2 is still localized in newly formed N–D bonds before it passes through the exit barrier into NHD+D or ND2+H channels. NH2(X̃ 2B1) was observed in (0,0,0) and (0,1,0) vibrational states as a product of reaction (1), and the observed time dependence of both vibrational states could be satisfactorily simulated by solving the master equation for vibrational relaxation of NH2. This analysis has indicated that the vibrational energy partitioning in the product NH2 is nearly statistical.