The walk rearrangement in bicyclo[2.1.0]pent-2-ene has been examined by ab initio calculations at Hartree-Fock, Møller-Plesset, and multiconfigurational levels of theory. At the multiconfigurational level of theory, it is found that the Woodward-Hoffman allowed reaction, occurring with inversion at the migrating carbon, is a concerted process, while that occurring with retension proceeds via a biradical intermediate. The difference in activation energies for the two reaction pathways is predicted to be at least 8 kcal/mol, favoring the Woodward-Hoffmann allowed reaction. The activation energy calculated at the MP2/6-31G* level for the reaction occurring with inversion agrees well with the value that can be estimated from experimental data. Calculations using spin contaminated unrestricted Hartree-Fock wave functions are found to give deviating results.
