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

Long-range sigma-pi interactions in tetrahydro-4H-thiopyran end-capped olig a(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PE S) and ab initio RHF/6-31G* calculations. The vertical ionization energies I-v,I-j of the highest occupied molecular orbitals (MO's) were assigned usi ng Koopmans' theorem (I-v,I-j = -epsilon(j)) and by correlation with the io nizations of related reference compounds. The experimental (PES) and theore tical (RHF/6-31G*) results are in good agreement. For tercyclohexylidene de rivatives which contain two nonconjugated pi-bonds splittings Delta I-v,I-j of the pi-bands in the range from similar to 0.5 to 0.7 eV (Delta-epsilon( j) similar to 0.6 to 0.9 eV). For the bi- and tercyclohexylidene compounds containing two sulfur atoms at their alpha- and omega-end positions the pi- type sulfur lone pair bands [Lp(pi)(S)] split significantly by Delta I-v,I- j similar to 0.3 to 0.4 eV (Delta-epsilon(j) similar to 0.3 to 0.4 eV), i.e . sigma-pi interactions over distances of ca. 8 and 12 Angstrom, respective ly, occur. The magnitude of the interactions and the observed splittings ar e independent of the anti and syn conformations of the oligo(cyclohexyliden e) hydrocarbon skeletons. RHF/6-31G* Natural Bond Orbital analyses reveal t hat 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 sigma-pi intera ctions; no through-space sigma-pi interactions were identified.