Chemical dynamics of d1-methyldiacetylene (CH3CCCCD; X (1)A(1)) and d1-ethynylallene (H2CCCH(C2D); X (1)A ') formation from reaction of C2D(X (2)Sigma(+)) with methylacetylene, CH3CCH(X (1)A(1))

The crossed beam reaction of the d1-ethynyl radical C2D(X (2)Sigma (+)), wi th methylacetylene, CH3CCH(X (1)A(1)), was investigated at an average colli sion energy of 39.8 kJ mol-1. Our experimental results were combined with e lectronic structure calculations. The chemical reaction dynamics are indire ct, involve three distinct channels, and are initiated via a barrierless ad dition of C2D to the acetylenic bond through long lived cis and trans CH3CC H(C2D), 1-ethynylpropen-2-yl, intermediates. The reduced cone of acceptance of the carbon atom holding the methyl group favors a carbon-carbon sigma b ond formation at the carbon atom adjacent to the acetylenic hydrogen atom. A crossed beam experiment of C2D with partially deuterated methylacetylene, CD3CCH, shows explicitly that the reactive intermediates decompose to form both methyldiacetylene, CD3CCCCD (channel 1, 70%-90%), and to a minor amou nt ethynylallene, D2CCCH(C2D) (channel 2; 10%-30%), isomers through exit tr ansition states located 7-15 kJ mol(-1) above the products. The computed re action energies to form both isomers are -135 and -107 kJ mol(-1), respecti vely, with respect to the separated reactants. A minor reaction pathway inv olves a H shift in CH3CCH(C2D) to an 1-ethynylpropen-1-yl radical which fra gments to methyldiacetylene via a barrier of 8.8 kJ mol(-1) (channel 3). Ne ither methyl group elimination nor the formation of the CC(CH3)(C2D) carben e was observed in our experiments. The experimentally observed "sideways sc attering" and ab initio investigation verify our conclusions of a predomina te formation of the methyldiacetylene isomer. These electronic structure ca lculations depict a hydrogen atom loss in the exit transition state to meth yldiacetylene almost parallel to the total angular momentum vector J as fou nd in our center-of-mass angular distribution. Since the title reaction and the corresponding reaction of the C2H radical with CH3CCH both have no ent rance barriers, are exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the ethynyl versus H atom exchange suggests the formation of both isomers under single collision conditions in extraterrestrial environments such as cold, molecular clouds as well as the atmosphere of Saturn's moon Titan. (C ) 2001 American Institute of Physics.