Understanding the puzzling chemistry of bicyclo[2.1.0]pentaneElectronic supplementary information (ESI) available: geometries, harmonic vibration frequencies, energies, and natural orbital occupancies for all stationary points discussed in the paper. See http://www.rsc.org/suppdata/ob/b3/b310676d

Three thermal reactions of bicyclo[2.1.0]pentane have been studied by CASPT2–g3 and CASSCF electronic structure calculations. They are isomerization to cyclopentene, isomerization to 1,4-pentadiene, and cycloaddition to fumaronitrile. All three of these reactions exhibit unusual features that have prompted mechanistic debate. The present computational results provide a basis for understanding the experimental observations.