Through-bond orbital coupling in end-functionalized bicyclohexylidenes - Photoelectron spectroscopy and ab initio SCF-MO calculations

To establish whether through-bond (TB) orbital interactions occur between t he functional groups and the hydrocarbon skeleton in a series of End-functi onalized oligo(cyclohexylidenes) 1-12, their He-1 photoelectron (PE) spectr a were measured and analyzed. Vertical ionization energies, I-vj, of the hi ghest occupied molecular orbitals (MOs) of 1-12 were assigned using ab init io RHF/6-31G* MO energies (-epsilon(j)) in combination with Koopmans' theor em. Excellent to good agreement was found between the PES and RHF/6-31G* re sults. In addition, the I-vj, assignments were further corroborated by a co mparison of the PES data of 1-12 with those previously reported for appropr iate reference compounds 13-20. To assess contributions from through-bond ( TB) and/or through-space (TS) interactions, RHF/G-31G*/NBO analyses were pe rformed. The results show that in the cases of 1-12, TS interactions do not occur. TB interactions were unequivocally identified for 1-4, 8, 10 and 11 -12. These TB interactions were found to be relayed via the H-ax-C-C-H-ax p recanonical sigma-MOs sigma-PCMOs) of the cyclohexyl-like moieties.