NH2 Radical Sources in the Gas Phase

NH2 radicals play some role in the environment. They are generated by photochemical processes in the atmosphere and by combustion processes. Moreover, NH2(X Bi) can be produced by many chemical reactions and via energy disposal in appropriate precursor molecules. 

The reaction OH + NH3NH2 +H2O can be used as an NH2 source In kinetic studies it produces besides the desired NH2 radical only H2O. The reaction with a room-temperature rate constant of about 1 x 10 cm -mol s is fast enough to suppress the consecutive reaction NH2 +OH- products (k=1.1xio cm -mor s see below) by a sufficiently high excess of NH3 over OH. This reaction has the disadvantage that another chemical reaction, F + H20- OH + HF or H + N02- OH + NO, or a photolysis process, H202 2 0H, NHO3 OH + NO2, is needed to produce OH radicals. The reaction OH + NH3 plays an Important role in flames (see p. 231). 

NH2-consuming reactions and the F atom concentration are essential to maximize the available NH2 concentration. The F atoms are produced in a microwave discharge (F2/He) in a flow reactor [31, 32] or by pulse radiolysis of SFg [33]. The reaction F + NH3 provides a clean NH2 source, since besides the radical NH2 wanted only the inert HF molecule is formed. Another hydrogen abstraction reaction, CF3 + NH3 CF3H + NH2, also produces NH2 radicals [34], however without being regarded to be an NH2 source. 

The chemical reactions described up to now all use NH3 as the precursor molecule, but N2H4 [35, 36] can also be used in a two-step mechanism 

The reaction of H atoms with HN3, which is isoelectronic with HNCO, produces electronically excited, highly vibrational-excited NH2(A A, V2 15) radicals in a direct elementary process [38]. 

---