Reactions of Nitrogen–Hydrogen Radicals. III. Formation and Disappearance of NH Radicals in the Photolysis of Ammonia

Abstract

The rates of formation and disappearance of the NH biradical in the ground and first-excited vibrational states were measured by kinetic spectroscopy following flash photolysis of mixtures of ammonia and inert gases. The delay in the appareance of NH following a 25-!,sec photolysis flash shows that it is formed by secondary reactions of the primary photolysis products, either NH2+ NH2 or NH2+ H. A large proportion of the observed NH is initially formed in the v" = 1 vibrational state, which disappears an order of magnitude faster than NH(v"=O). The disappearance of NH (v" = 0) is pressure dependent and in the falloff region at 1 torr, 300 o K. The rate of disappearance is rapid and is consistent with NH + NHa insertion. At the highpressure lin1it the rate constant for this reaction is k,=1.0±0.3XlO IO Iiter/mole·sec. The reaction has a small negative energy of activation. The high rate of NH disappearance accounts for the small range of conditions under which it can be observed. Implications of the results to the mechanism of biradical insertion reactions are discussed.

Formation and Fate of NH(X3Σ−) in the Pulsed Vacuum UV Photolysis of NH3

“…cm3 s -l [13]. The present value for the upper limit of k, is not in agreement with the previous result of Mantei and Bair.…”

Absolute rate constants for reactions of free radicals in the high‐temperature photolysis of formamide vapor. II. Amino radicals

“…It has been suggested that reaction (4) is the source of the K H radicals observed in the flash photolysis of ammonia [8] and is important in the pulse radiolysis of ammonia [5]. The only estimate of its rate is of questionable accuracy, but corresponds to only about 1% of the rate of disappearance of NHz in the present system [19].…”

“…There is good evidence that reaction (3) is a major process, as it is very difficult to account for the formation of hydrazine in the mercury-photosensitized decomposition [6] and the pulse radiolysis [5,7] of ammonia by any other reaction. It has been suggested that reaction (4) is the source of the K H radicals observed in the flash photolysis of ammonia [8] and is important in the pulse radiolysis of ammonia [5]. The only estimate of its rate is of questionable accuracy, but corresponds to only about 1% of the rate of disappearance of NHz in the present system [19].…”

Quantum yields in the 58.4-nm photolyses of ammonia and water

“…If the neutralization of H(NH3)»+ is assumed to produce only H and nNHs the result of the primary photolysis on a time scale of a few hundreds of microseconds, such that dissociative recombination is essentially finished but neutral radical reactions have hardly begun, is given by 1.3NH, + hv -> 1.6H + NH, + 0.3NH (12) With the same assumption for the charge carriers in the H20 case we should have 1.03H,O + hv -1.28H + 0.72OH + 0.310 + 0.03H2 (13) The radicals on the right-hand side of ( 12) can take part in a large number of homogeneous and heterogeneous processes, for example16-17 NH* + NH* -N,H4 (14) NH + Nil•; -Xl.H, (15) NH + NH* -> N*II? (16) + N*H. -> NH, + Nil* (17) 2N*N3 -X*H; -X* 4• H* (18) and so on. Reaction 15 is rapid enough (ku ~1.7 X 10-12 cm3 molecule-1 sec-1 16) to result in appreciable N2H4 production under the conditions of these experiments; the absence of hydrazine from the products of both photolysis and radiolysis presumably results from the ease with which it can be removed by reactions such as (17).…”