Photoelectron Spectra, Ab Initio SCF MO, and Natural Bond Orbital Studies on Stellenes. Long-Range π/σ Interactions

Abstract

The He I photoelectron spectra of 2,6-dimethylenetricyclo[3.3.01,5.03,7]octane, (stella-2,6-diene) (6), stella-2,6-dione (8), stella-6-en-2-one (10), distella-2,2‘,6,6‘-triene (7), distella-6,6‘-en-2,2‘-dione (9), and distella-2,2‘,6‘-dien-6-one (11) have been recorded. The energy differences between the π ionizations arising from the terminal π bonds amount to 0.9 eV (6) and 0.4 eV (7). An energy difference of 1.1 eV has been found between the 2p lone-pair ionizations of 8. Using Hartree−Fock SCF calculations with a 3-21G basis, the geometries of 6−11 have been calculated. For all six molecules long central σ bonds (1.58−1.60 Å) were predicted. By means of the Weinhold natural bond localization procedure, the interactions between the σ frame and the π orbitals as well as between the σ frame and the n orbitals have been probed. It has been concluded that the large energy splitting between the π MOs mainly localized at the terminal π bonds in 6 and 7 is mediated by the ribbon orbitals of the twisted cyclohexane ring.

Green's function study of the one‐electron and shake‐up ionization spectra of unsaturated hydrocarbon cage compounds

“…They confirm, in particular, the assignment proposed by Gleiter et al 84 for the three outermost and individually resolved ionization lines. On the experimental side, the first two lines (z, y) derived from orbitals 7b 2 and 6b 3 display asymmetric and sharp profiles, with rather clearly apparent vibrational progressions, which is very typical of -levels (see for instance the He I spectra displayed for a variety of polycyclic aromatic hydrocarbons in refs.…”

“…The next three levels (u, t, s) very strongly overlap, and lead to a broad and intense band (A) that culminates at *11.9 eV, in quantitative agreement with the He I measurements by Gleiter et al 84 As the simulation of Figure 3b clearly demonstrates, any attempt to analyze separately the ionization intensities characterizing these three orbitals should be regarded as physically irrelevant, with regard to the limited resolution of the currently available spectrometers and the extent of vibrational broadening. Analysis of the bands recorded at binding energies around *12.5 eV is problematic, because of the presence at these binding energies of a rather intense signal, which Gleiter et al 84 ascribed to water contamination. Without this experimental artefact, the line (q) related to orbital 5b 1 should have normally been individually identified in a He I spectrum.…”

“…These results, obtained using the B3LYP/TZVP geometry, are compared with the UPS ionization energies of ref. 84. Only the largest ionization lines are given here; see Table 4 for further data.…”

“…The only experimental sources of information on the electronic structure of STDE derive from the UPS (He I) measurements by Gleiter et al 84 These are compared in Figure 3 with the results of our ADC(3)/cc-pVDZ calculations. Theoretical analysis of these measurements, so far, was limited to DSCF (HF/6-31G*//HF/6-31G*) and DMP2 (MP2/6-31G*//HF/6-31G*) studies of the three outermost valence bands of this compound, up to electron binding energies of 10 eV.…”

“…Besides UPS (He I) measurements 84 up to electronic binding energies of *16 eV, the outer-valence electronic structure of STDO has also been studied in detail using EMS. 85,86 These UPS and EMS measurements are reproduced in Figure 4 and compared with the results of our ADC(3) calculations.…”