Formation of nitriles in the interstellar medium via reactions of cyano radicals, CN(X 2Σ+), with unsaturated hydrocarbons

Crossed molecular beam experiments of cyano radicals, CN(X(2)Sigma (+), v = 0), in their electronic and vibrational ground state reacting with unsaturated hydrocarbons acetylene, ethylene, C2H2(X(1)Sigma (+)(g)), ethylene, C2H3((XAg)-Ag-1), methylacetylene, CH3CCH(X(1)A(1)), allene, H2CCCH2(X(1)A(1)), dimethylacetylene, CH3CCCH3(X(1)A(1)(')), and benzene, C6H6 (X(1)A(1g)), were performed at relative collision energies between 13.3 and 36.4 kJ mol(-1) to unravel the formation of unsaturated nitriles in the outflows of late-type AGB carbon stars and molecular clouds. In all reactions, the CN radical was found to attack the pi electron density of the hydrocarbon molecule with the radical center located at the carbon atom; the formation of an initial addition complex is a prevalent pathway on all the involved potential energy surfaces. A subsequent carbon-hydrogen bond rupture yields the nitriles cyanoacetylene, HCCCN (X (1)Sigma (+)), vinylcyanide, C2H3CN (X(1)A'), 1-methylcyanoacetylene, CH3CCCN (X(1)A(1)), cyanoallene, H2CCCH(CN) (X(1)A(')), 3-methylcyanoacetylene, HCCCH2CN(X(1)A'), 1,1-cyanomethylallene, H2CCC(CN)(CH3) (X(1)A'), and cyanobenzene, C6H5CN (X(1)A(1)). In case of acetylene and ethylene, a second reaction channel involves a [1, 2]-H atom shift in the initial HCCHCN and H2CCH2CN collision complexes prior to a hydrogen atom release to form cyanoacetylene, HCCCN (X(1)Sigma (+)), and vinylcyanide, C2H3CN (X(1)A'). Since all these radical-neutral reactions show no entrance barriers, have exit barriers well below the energy of the reactant molecules, and are exothermic, the explicit identification of this CN versus H atom exchange pathway under single collision conditions makes this reaction class a compelling candidate to synthesize unsaturated nitriles in interstellar environments holding temperatures as low as 10 K. This general concept makes it even feasible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in the interstellar medium. Here HCCCN, and C2H3CN, and CH3CCCN have been already observed; since is the common precursor to CH3CCCN is the common precursor to H2CCCH(CN)/CH3CCCN and the latter isomer has been assigned unambiguously toward TMC-1 and OMC-1, H2CCCH(CN) is strongly expected to be present in both clouds as well. The formation of iso-nitrile isomers was not observed in our experiments. Since all reactions to HCCNC, C2H3NC, CH3CCNC, H2CCCH(NC), H2CCC(NC)(CH3), and C6H5NC are either endothermic or the exit barrier is well above the energy of the reactants, neutral-neutral reactions of cyano radicals with closed shell unsaturated hydrocarbons cannot synthesize isonitriles in cold molecular clouds. However, in outflow of carbon stars, the enhanced translational energy of both reactants close to the photosphere of the central star can compensate this endothermicity, and isonitriles might be formed in these hotter environments as well.