Photoelectron Spectra of syn- and anti-Sesquinorbornene. Evidence for Vertical ?-? Delocalization in Bicycloheptene

Article abstract of DOI:10.1021/jo00169a015

The photoelectron spectra of syn- and anti-sesquinorbornene show the ? IP's to be 8.12 and 7.90 eV, respectively.Comparison of these IP's with those of other tetraalkylethylenes having the same number of carbon atoms indicates that both isomers are easier to ionize than expected for a C12 tetraalkylethylene.This is attributed to ?-? closed shell-closed shell repulsions between the ethano and methano bridges and the ? bond.The difference of 0.2 eV in the ? IP between the two is surprisingly large for two geometric isomers and is related to the molecular symmetry.The ? IP of norbornene, when compared with those of a series of cis olefins, is 0.12 eV easier to ionize than is expected for a cis-dialkylethylene containing seven carbons.This small reduction in ? IP which is attributable to vertical electronic factors may account for an increased reactivity of norbornene in processes involving electrophilic addition to the ? bond.