An ab initio CASPT2//CASSCF study of the ground (1)A" potential energy surface (PES) for the title reactions has been carried out, The lowest energy barrier heights correspond to cis-arrangements in both reactive channels. Crossings with the first excited (1)A" PES are located, being complicated by spurious symmetry breaking of the complete active space self-consistent field (CASSCF) wave function, The rate constant values have been calculated employing transition state theory (TST) in the temperature range 500-5000 K, The main contribution to the total rate constant stems from the barrierless ground (1)A' PES, while the (1)A" PES is only relevant at the highest temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.