The fast reaction between C( 3 P) and C 2 H 2 is thought to be an important process in dense interstellar clouds as it provides a mechanism for the growth of carbon chains. This feature article describes a complementary series of recent experimental and theoretical investigations on this reaction. This includes kinetic measurements of rate constants at low temperatures and crossed molecular beam determinations of integral and differential cross sections. The theory employs first-principles electronic structure computations and wave packet dynamics to calculate cross sections and rate constants for forming the linear and cyclic isomers of C 3 H which can be formed in the reaction. The rate constant and cross section measurements show that there are no barriers in the potential surface for the reaction, whereas the differential cross section experiments provide new evidence for the formation of C 3 + H 2 products. The theoretical results of overall rate constants and cross sections agree quite well with the experiments, and it is predicted that the linear isomer of C 3 H should be formed preferentially at low temperatures.

The reaction between carbon atoms (C(3P)) and acetylene has been studied by a reduced dimensionality approach, restricted to the initial addition channels, in the energy range between 5 and 70 kJ mol-1. Coupled cluster calculations with single and double substitution and a non iterative estimate of the triple excitation (CCSD(T)) have been used to generate the lowest triplet potential energy surface. The flux into two different reaction channels, leading to linear and cyclic isomers of C3H, has been calculated by solving the time dependent Schrödinger equation. Results show that linear C3H is preferentially formed, while cyclic C3H is formed only at the highest energy of the calculations. Furthermore, the adiabatic capture-coupled states approximation (ACCSA) has been employed to generate rate constants at low temperatures, using the CCSD(T) potential, giving an improvement on the results obtained by using the long-range part of the potential.

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