The crossed beam reaction of the d1-ethynyl radical C2D(X (2)Sigma (+)), with methylacetylene, CH3CCH(X (1)A(1)), was investigated at an average collision energy of 39.8 kJ mol-1. Our experimental results were combined with electronic structure calculations. The chemical reaction dynamics are indirect, involve three distinct channels, and are initiated via a barrierless addition of C2D to the acetylenic bond through long lived cis and trans CH3CCH(C2D), 1-ethynylpropen-2-yl, intermediates. The reduced cone of acceptance of the carbon atom holding the methyl group favors a carbon-carbon sigma bond 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 amount ethynylallene, D2CCCH(C2D) (channel 2; 10%-30%), isomers through exit transition states located 7-15 kJ mol(-1) above the products. The computed reaction energies to form both isomers are -135 and -107 kJ mol(-1), respectively, with respect to the separated reactants. A minor reaction pathway involves a H shift in CH3CCH(C2D) to an 1-ethynylpropen-1-yl radical which fragments to methyldiacetylene via a barrier of 8.8 kJ mol(-1) (channel 3). Neither methyl group elimination nor the formation of the CC(CH3)(C2D) carbene was observed in our experiments. The experimentally observed "sideways scattering" and ab initio investigation verify our conclusions of a predominate formation of the methyldiacetylene isomer. These electronic structure calculations depict a hydrogen atom loss in the exit transition state to methyldiacetylene almost parallel to the total angular momentum vector J as found in our center-of-mass angular distribution. Since the title reaction and the corresponding reaction of the C2H radical with CH3CCH both have no entrance 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.