Long-range σ-π interactions in tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes). Photo-electron spectroscopy, ab initio SCF MO calculations, and natural bond orbital analyses

Long-range σ-π interactions in tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G(*) calculations. The vertical ionization energies I(vj) of the highest occupied molecular orbitals (MO's) were assigned using Koopmans' theorem (I(vj) = -ε(j)) and by correlation with the ionizations of related reference compounds. The experimental (PES) and theoretical (RHF/6-31G(*)) results are in good agreement. For tercyclohexylidene derivatives which contain two nonconjugated π-bonds splittings ΔI(v,j) of the π-bands in the range from ≃0.5 to 0.7 eV (Δ-ε(j) ~0.6 to 0.9 eV). For the bi- and tercyclohexylidene compounds containing two sulfur atoms at their α- and ω-end positions the π-type sulfur lone pair bands [Lpπ(S)] split significantly by ΔI(v,j) ~0.3 to 0.4 eV (Δ-ε(j) ~0.3 to 0.4 eV), i.e. σ-π interactions over distances of ca. 8 and 12 Å, respectively, occur. The magnitude of the interactions and the observed splittings are independent of the anti and syn conformations of the oligo(cyclohexylidene) hydrocarbon skeletons. RHF/6-31G(*) Natural Bond Orbital analyses reveal that the H(ax)-C-C-H(ax) precanonical MO's (PCMO's) centered on the cyclohexyl-type rings are paramount for the relay of the through-bond σ-π interactions; no through-space σ-π interactions were identified.