Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion

The dynamics of some elementary reactions of N(D-2), C(P-3,D-1) and CN(X (2 )Sigma (+)) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection . The novel capability of producing intense, continuous beams of the radica l reagents by a radio-frequency discharge beam source was exploited. From a ngular and velocity distribution measurements obtained in the laboratory fr ame, primary reaction products have been identified and their angular and t ranslational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(D-2) + CH4, which is found to lead to H + C H2NH (methylnitrine) and (H + CH3N methylnitrene); no H-2 elimination is ob served. In the reaction N(D-2) + H2O the N/H exchange channel has been foun d to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H-2 is negligible. Formation of H + H2CCCH (propargyl) i s the dominant pathway, at low collision energy (E-c), of the C(P-3) + C2H4 reaction, while at high E-c formation of the less stable C3H3 isomers (cyc lopropenyl and/or propyn-1-yl) also occurs; the H-2 elimination channel is negligible. The H elimination channel has also been found to be the dominan t pathway in the C(P-3,D-1) + CH3CCH reaction leading to C4H3 isomers and, again, no H-2 elimination has been observed to occur. In contrast, both H a nd H-2 elimination, leading in comparable ratio to C3H + H and C-3(X (1)Sig ma (+)(g)) + H-2(X (1)Sigma (+)(g)), respectively, have been observed in th e reaction C(P-3) + C2H2(X (1)Sigma (+)(g)). The occurrence of the spin-for bidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between tri plet and singlet manifolds of the C3H2 potential energy surfaces. The react ion CN(C (2)Sigma (+)) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been di scussed in the light of recent theoretical calculations on the relevant pot ential energy surfaces. Previous, lower resolution studies on C and CN reac tions carried out using pulsed beams are noted. Finally, throughout the pap er the relevance of these results to combustion chemistry is considered.