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
| Autor: | RW Havenith, AL Spek, LW Jenneskens, Albert W. Marsman, van Lenthe Jh, Sabine Bethke, Rolf Gleiter, Martin Lutz |
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| Rok vydání: | 2000 |
| Předmět: | |
| Zdroj: | The Journal of Organic Chemistry. 65:4584-4592 |
| ISSN: | 1520-6904 0022-3263 |
| DOI: | 10.1021/jo000199y |
| Popis: | Long-range sigma-pi interactions in tetrahydro4H-thiopyran end-capped oligo(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G* calculations. The vertical ionization energies Ivj of the highest occupied molecular orbitals (MO's) were assigned using Koopmans' theorem (Iv,j = -epsilonj) 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 pi-bonds splittings deltaIv,j of the pi-bands in the range from approximately 0.5 to 0.7 eV (delta-epsilonj approximately 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 [Lppi(S)] split significantly by deltaIvj approximately 0.3 to 0.4 eV (delta-epsilonj approximately 0.3 to 0.4 eV), i.e. sigma-pi interactions over distances of ca. 8 and 12 A, 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 Hax-C-C-Hax precanonical MO's (PCMO's) centered on the cyclohexyl-type rings are paramount for the relay of the through-bond sigma-pi interactions; no through-space sigma-pi interactions were identified. |
| Databáze: | OpenAIRE |
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