Photoionization and electronic structure of bridgehead alkenes

The vertical, valence ionization energies for several bicyclic alkenes containing geometrically distorted π C[dbnd]C bonds in bridgehead positions have been calculated (as free molecules) using high-level ab initio coupled-cluster method: IP-EOM-CCSD. We have selected bridgehead alkenes (BA) which have been either accessed as intermediates (e.g. in trapping reactions) or have been synthesized experimentally and classified as hyper-stable (based on reactivity deduced from theoretical calculations). We investigated changes in various parameters within isomeric pairs: one molecule being bridged the other non-bridged. The variations in vertical ionization energies were related to other electronic and structural parameters of BAs. The π-bond positioned at the junction of two rings (bridge) leads to its twisting and pyramidalization. This is reflected in the following changes in BA: decrease of the 1st ionization energy, reduction of HOMO-LUMO and singlet-triplet gaps, increased polarization of C[dbnd]C bond and increased standard enthalpy of formation of bridged vs. non-bridged isomer.