REACTIONS OF THE CARBENE IONS CNN+ WITH LABELED METHANE - MECHANISTIC INTERPRETATION

The rate constants and branching ratios for the reactions of CnN+ ions (n = 4-8) with methane were determined by using Fourier transform mass spectrometry (FTMS). The CnN+ reactant ions are formed by the reaction of Cn-1 + ions (produced by direct laser vaporization of graphite) with HCN. A variety of products are formed in the title reactions, many related to the cyanopolyynes (HC2x+1N) and polyynes (C2xH2), species of interest for interstellar chemistry. Carbene attack on the C-H bond in methane, followed by hydrogen migration and C-C bond cleavage, can explain the formation of many of the observed products. However, studies with labeled 13CH4 revealed that more complex mechanisms are involved in the formation of some of the observed products. The results of MNDO calculations of the structures and heats of formation of the reactants, various intermediates, and products were used to explore possible mechanisms. It is suggested that a rearrangement mechanism involving a cyclopropenyl intermediate and formation of a cyano-carbene ion is responsible for these products. Reaction at the nitrogen end of the ion does not appear to be important. Electronic state effects are manifested in the reactions of the odd n reactant ions by the formation of radical products. © 1990, American Chemical Society. All rights reserved.