The energy spectra of three classes of polybenzenoid hydrocarbons with a large number N (N ≈ 10 3 ) of carbon atoms have been studied theoretically. It is shown that in the asymptotic case N f ∞ the energy gap (EG) ∆E(Nf∞) is different from zero if the electron correlation is taken into account; that is, the π systems calculated should possess semiconductor properties. The results for the ∆E(Nf∞) * 0 of the hydrocarbons are in qualitative agreement with the results calculated for the EG of three classes of 1-D ladder polymers, which can be considered as models of quasi-1-D graphite. With increasing width (L) of the polymers, the band gap ∆E(Lf∞) approaches a value different from zero. The problem of the existence of defect states of hydrocarbons with vacancies is briefly discussed.

We report on a comparative quantum-chemical study of oligo@-and m-phenyleneviny1ene)s (OPPV and OMPV, respectively) isomers and their radical anions in order to explain their optical absorption spectra. The AM1 semiempirical approach is employed to assess the structural changes from neutral to monocharged states.The optimized geometries are further used in PPP-CI calculations of the optical transitions for the closed and open shell n-systems. With increasing number of repeat units, OPPV exhibits significant splitting of the frontier n-states, which remain almost degenerate for OMPV. As a consequence, reduction is found to result in an extended charged defect for the former oligomers. In contrast, self-trapping of the excess charge into a single stilbene unit is predicted for OMPV. This result serves further as a basis to reproduce and interpret in detail the several characteristic absorptions of the title compounds.

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The structure and energy spectra of four classes of polybenzenoid hydrocarbons with different edge structures, a large number N (N ∼ 10 4 ) of carbon atoms, and different types of defect states (Tamm, Schottky and chemisorption states) have been studied theoretically. Several types of (probably) stable monoradicals and triplet biradicals with singly occupied MOs in the energy gap (molecular analogues of semiconductors with defect states) are characterized which could be used in the molecular electronics. A new concept for viewing high-spin π systems with ferromagnetically coupled electrons within the half-filled band of NBMOs in term of point defects (vacancies) was developed. The NBMOs can be considered as surface states (Tamm or Schottky).

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COMMUNICATIONS three-membered Zr. 0, C ring, followed by a I .2-alkyl shift from the zirconium atom to the neighboring former alkene carbon atom (see Scheme 1 ) readily explains the formation of the cyclopropanation product 4 with the observed stereochemistry.These reactions add to the examples of small organic units which approach each other and react at the molecular Zr-O-Zr metal oxide template stabilized by cyclopentadienyl ligands.l3, Perhaps the chemistry of such molecular compounds can indeed shed some light on the possible individual steps of reactions at metal oxide phases in heterogeneous catalysis.['o] Received: May 19, 1994 [Z26948IE] German versioii: Ai~geiv. Cirein. 1994, 106. 2043 [l] Fischer Tropsch chemistry parallels the chemistry ofmethylene on the metal: metal oxide catalyst surface in significant aspects: F. Fischer. H. Tropach, BrcJnii.w Cho7i. 1926. 7. 97: Cheni B P~. 1926, 59. 830: R. C. Brddy 111. R. Pettit,

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