The Thermal Decomposition of NH₂ and NH Radicals

Abstract

The formation of NH2 and NH radicals as well as H and N atoms during the thermal decomposition of hydrazine was investigated over a wide temperature range. For the first time frequency-modulated spectroscopy was used for the detection of NHp radicals behind shock waves. In comparison with the dual beam laser absorption technique, the detection limit has been improved by one and a half orders of magnitude.For reaction (5a) NH2+M + NH+H+M the rate constants were obtained in the temperature range from 2200 to 4000 K ks,=(1.2+0.5)10's~exp [-(318* 10) kJ mol-'/RT] cm'lmol s and for reaction (6) N H c M --t N + H + M in the temperature range from 2500 to 3400 K k6=(l.8+0.8)10'4.exp [-(313? 15) kJ mol-'/RT] cm3/mol s.The experimental apparent activation energies were found to be in good agreement with theoretical calculations and recently recommended enthalpies of formation for NH2 and NH. The competing NH2 decomposition channel (5b) NHz+M -+ N+H2+M was shown to be of minor importance ( < 5%). The kinetic behavior of the unimolecular decomposition of NH2 is compared with that of H20 and k H 2 .

Direct Measurements of the Reaction NH2 + H2 → NH3 + H at Temperatures from 1360 to 2130 K

“…Hydrazine decomposes practically completely into two amino radicals at temperatures above 1200 K [21,22]. The NH 2 concentration time profiles were evaluated by applying nearly the same mechanism as that given in a previous publication [1] for the NH 2 calibration procedure of the FM spectrometer. Fig.…”

“…The NH 2 concentration time profiles were evaluated by applying nearly the same mechanism as that given in a previous publication [1] for the NH 2 calibration procedure of the FM spectrometer. Part of the reaction mechanism used for simulation of the experimental profiles; rate constants (experimental data) are given in terms of k i ϭ A · T n · exp(ϪE a /RT); units are kJ, cm 3 , mol, s and K. The whole mechanism used (26 reactions) is given in our previous publication [1]. For the experiment shown in Fig.…”

“…For further details see the original literature [2Ϫ7] and the review of Hack et al [27]. For NH 2 the reported enthalpy data were adjusted to ∆ f H o 298 K ϭ 190 kJ/mol (∆ f H o 0 K ϭ 193.0 kJ/mol), the value obtained in our previous study of the thermal decay of NH 2 and NH radicals [1]. 3 represents the result of a theoretical calculation (ICVT/SCSAG on HF/BAC-MP4 potential surface) of Garret et al [8].…”

“…Rate constants for the reverse reaction were calculated automatically on the basis of the thermodynamic data set included in the Chemkin-II package [20]. For NH 2 and NH the reported enthalpy data were adjusted to values obtained in our previous study [1], i.e. ∆ f H o 298 K (NH 2 ) ϭ 190 kJ/mol and ∆ f H o 298 K (NH) ϭ 353 kJ/mol, respectively.…”

“…Herein, FM spectroscopy has been shown to be very sensitive for NH 2 detection behind shock waves [1]. Herein, FM spectroscopy has been shown to be very sensitive for NH 2 detection behind shock waves [1].…”