ELECTRONIC-STRUCTURE AND ELECTRONIC-SPECTRUM OF 5-MEMBERED HETEROAROMATIC-COMPOUNDS

The electronic structure and electronic spectrum of five-membered aromatic heterocycles (furan, thiophene, selenophene, tellurophene, pyrrole and 42 of their substituted derivatives) have been investigated by photoelectron spectroscopy, ultraviolet absorption spectroscopy and semiempirical quantum-chemical calculations. The photoelectron spectrum of all the investigated derivatives could be successfully interpreted using Koopmans' theorem. In order to describe the effect of the substituents on the aromatic pi-system a simple model was introduced, that accounted well for the observed shifts on the ionization energies for each substituent investigated. The aromatic electronic structure has only been perturbed, but not significantly altered by the substituents, in accordance with the well known aromatic character of the molecules investigated. In the near ultraviolet spectrum of the five membered heteroaromatic systems considered here Rydberg- and valence-electronic transitions have been assigned. To assign Rydberg-transitions comparison of the band profile of the gas-phase absorption spectrum and that of the photoelectron spectrum have been used. The assignment have been further supported by the disappearance of the corresponding bands from the solution-phase spectrum. Rydberg-term values for transitions terminating at ''s''-type orbitals are 21000-25000 cm(-1), while for transitions terminating at ''p''-type orbitals are between 17000-21000 cm(-1). Among Rydberg-transitions two R(p) <-- pi(3) and one R(s) <-- pi(3) transition could assigned in case of the furan, thiophene and pyrrole derivatives. For selenophene and tellurophene both R(s) <-- pi(3) and R(s) <-- pi(2) transitions were observable. In case of thiophene two valence transitions have been assigned with B-2 <-- A(1) and A(1) <-- A(1) symmetry in order of their increasing transition energy. The ordering of the above two transitions is the same for all the substituted derivatives investigated. Earlier theoretical suggestions about the presence of two valence transitions in the near UV spectrum of furan are in accordance with the spectra of substituted furan derivatives too.