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

Abstract

To establish whether through-bond (TB) orbital interactions occur between the functional groups and the hydrocarbon skeleton in a series of end-functionalized oligo(cyclohexylidenes) 1-12, their He(I) photoelectron (PE) spectra were measured and analyzed. Vertical ionization energies, I(vj), of the highest occupied molecular orbitals (MOs) of 1-12 were assigned using ab initio RHF/6-31G* MO energies (-ε(j)) in combination with Koopmans' theorem. Excellent to good agreement was found between the PES and RHF/6- 31G* results. In addition, the I(vj), assignments were further corroborated by a comparison of the PES data of 1-12 with those previously reported for appropriate reference compounds 13-20. To assess contributions from through- bond (TB) and/or through-space (TS) interactions, RHF/6-31G*/NBO analyses were performed. 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) precanonical σ-MOs (σ-PCMOs) of the cyclohexyl-like moieties