Quasi-classical trajectory (QCT) calculations have been carried out for the reaction on an ab initio global potential energy surface for the ground electronic state, . Cross-sections, computed for collision energies between 0.001 and 1 eV, show no energy threshold and decrease with the increasing collision energy. Rate constants have been calculated in the 5–500 K temperature range. The thermal rate constant is in good agreement with previous theoretical results over a wide range of temperatures, and with the experimental data for temperatures above 300 K. Cross-sections and rate constants are found to be dependent on the initial rotation.