For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented. Resolving energetics and properties of these intermediates is essential for the understanding of the reaction of C(P-3) with C2H2 to form interstellar cyclic and linear isomers of C3H. Computed reaction energies agree with results from molecular beam experiments. The combination of crossed molecular beam experiments and ab initio calculations allows us to identify two reaction channels for the carbon-hydrogen exchange and to explain astronomical observations of a higher c-C3H to 1-C3H ratio in dark clouds as compared to hotter envelopes of carbon stars.