Probing radical pathways in electrophilic addition of halogens: Classical vs. bridged intermediates

► Experimental observation of bridged iodoethyl radical. ► Trapping of key intermediate in prototypical electrophilic addition reaction of iodine and ethylene. ► Theory predicts bridged radical is global minimum on the C2H4I potential energy surface. ► MP2 provides a poor description of the potential energy surface due to spin-contamination effects.