Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X (1)A(')) formation from reaction of C6H5(X (2)A(1)) with methylacetylene, CH3CCH(X (1)A(1))

The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X ( 1)A(')), via reactive collisions of the phenyl radical C6H5(X (2)A(1)) with methylacetylene, CH3CCH(X (1)A(1)), are unraveled under single collision c onditions in a crossed molecular beam experiment at a collision energy of 1 40 kJ mol(-1). The laboratory angular distribution and time-of-flight spect ra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hy drogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X (1)A(1)), was used to identify the site of the carbon-hydrogen bond cleavag e. Only the loss of the acetylenic hydrogen atom was observed; the methyl g roup is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol(-1). Forward- convolution fitting of our data shows that the chemical reaction dynamics a re on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the pi electron de nsity of the C1 carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C6H5)HCCCH3 intermediate. The latte r loses a hydrogen atom to form the phenylmethylacetylene molecule on the ( 2)A(') surface. The phenylallene isomer channel was not observed experiment ally. The dynamics of the title reaction and the identification of the phen yl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photosp heres of carbon stars and oxygen poor combustion systems. (C) 2000 American Institute of Physics. [S0021-9606(00)01409-4].