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 NH₂ 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

k_5a = (1.2 ± 0.5)10¹⁵ · exp[-(318±10) kJ mol⁻¹/RT] cm³/mol s

and for reaction (6) NH + M → N + H + M in the temperature range from 2500 to 3400 K

k₆ = (1.8 ± 0.8)10¹⁴ · exp[-(313 ± 15)kJ mol⁻¹/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) NH₂ + 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 ³CH₂.