An advanced quantum mechanical model for the treatment of the dynamics of a diatom-triatom molecule system is presented here. The model has been applied to the study of the reactive process H-2+C2H --> H+C2H2, using a four-dimensional (4-D) infinite-order sudden approximation scheme when seven of nine degrees of freedom are explicitly considered (the two others are being reduced by means of an averaging of the potential energy surface around the out-of-plane angles). Using a previously reported surface for this reaction [J. Chem. Phys. 101, 8646 (1994)] computations have been carried out to obtain total reactive probabilities, cross sections, and rate constants. A value of 2.68x10(-13) cm(3) molecule(-1) s(-1) was determined for the thermal rate constant at room temperature, in good agreement with the above referenced theoretical calculation as well as with available experimental data.