The crossed molecular beam technique was utilized to investigate the reaction of ground state carbon atoms, C(P-3(j)), with d3-methylacetylene, CD3CCH(X(1)A(1)), at an average collision energy of 21.1 kJ mol-1. Product angular distributions and time-of-flight spectra were recorded. Only the deuterium loss was observed; no atomic hydrogen emission was detected experimentally. Forward-convolution fitting of the data combined with electronic structure calculations show that the reaction is indirect and initiated by a barrierless interaction of the carbon atom to the pi -system of the methylacetylene molecule. Reactions with large impact parameters yield a triplet trans-methylpropene-1-diylidene complex whereas-to a minor amount-the formation of a triplet methylcyclopropenylidene intermediate is governed by smaller impact parameters. Both collision complexes rearrange via hydrogen migration and ring opening, respectively, to two distinct triplet methylpropargylene intermediates. A deuterium atom loss via a tight transition state located about 30 kJ mol-1 above the n-C4H3 product is a likely reaction pathway. The formation of the thermodynamically less stable cyclic isomer remains to be investigated closer. The D atom loss pathway represents an entrance barrierless and exothermic route to synthesize an extremely reactive C4H3 hydrocarbon radical in combustion processes and extraterrestrial environments. <(C)> 2001 American Institute of Physics.