Structural chemistry of polycyclic heteroaromatic compounds, 12. Photoelectron spectra, electronic structures, and conformational properties of (E)-stilbene, styrylthiophenes, and (thienylethenyl)pyridines.
The UV photoelectron spectra of two isomeric styrylthiophenes and six isomeric (thienylethenyl)pyridines have been recorded and analyzed, making use of DFT Becke3LYP calcns. The spectra were compared with those of (E)-stilbene (I) and several styrylpyridine isomers. Using quantum chem. analyses, planar mol. structures were obtained for all these compds. However, from the sepn. DIP of the ionization bands assocd. with the p7 and p3 MOs, it is possible to make a distinction between planar and twisted mol. structures. Accordingly in these compds., 2-substituted pyridine rings and 3-substituted thiophene rings are nearly untwisted, whereas Ph rings, 3- and 4-substituted pyridine, and 2-substituted thiophene rings are twisted to an extent similar to that in I. The apparent distortion of the mols. is probably caused by torsional vibrations, so that twisted av. geometries correspond to planar equil. structures. The B3LYP data permit detailed conclusions to be drawn with regard to the conformational preferences of 2- and 3-substituted thiophene and pyridine rings in heterocyclic analogs of I. The results clearly indicate that PE spectroscopy is a powerful method for anal. of conformational properties of stilbene-like mols.
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