Formation of nitriles in the interstellar medium via reactions of cyano radicals, CN(X (2)Sigma(+)), 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 unsatur ated hydrocarbons acetylene, ethylene, C2H2(X(1)Sigma (+)(g)), ethylene, C2 H3((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 la te-type AGB carbon stars and molecular clouds. In all reactions, the CN rad ical was found to attack the pi electron density of the hydrocarbon molecul e with the radical center located at the carbon atom; the formation of an i nitial addition complex is a prevalent pathway on all the involved potentia l energy surfaces. A subsequent carbon-hydrogen bond rupture yields the nit riles 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, H2C CC(CN)(CH3) (X(1)A'), and cyanobenzene, C6H5CN (X(1)A(1)). In case of acety lene 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 vinylcyani de, C2H3CN (X(1)A'). Since all these radical-neutral reactions show no entr ance barriers, have exit barriers well below the energy of the reactant mol ecules, and are exothermic, the explicit identification of this CN versus H atom exchange pathway under single collision conditions makes this reactio n class a compelling candidate to synthesize unsaturated nitriles in inters tellar environments holding temperatures as low as 10 K. This general conce pt makes it even feasible to predict the formation of nitriles once the cor responding unsaturated hydrocarbons are identified in the interstellar medi um. Here HCCCN, and C2H3CN, and CH3CCCN have been already observed; since i s the common precursor to CH3CCCN is the common precursor to H2CCCH(CN)/CH3 CCCN 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 wel l. 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 e nergy of the reactants, neutral-neutral reactions of cyano radicals with cl osed shell unsaturated hydrocarbons cannot synthesize isonitriles in cold m olecular clouds. However, in outflow of carbon stars, the enhanced translat ional energy of both reactants close to the photosphere of the central star can compensate this endothermicity, and isonitriles might be formed in the se hotter environments as well.