Photoelectron spectra, electronic structures, and conformational properties of stilbene-like molecules, 2. Photoelectron spectra, electronic structures, and conformational properties of substituted 2-styrylpyrroles.
The UV photoelectron spectra of three pairs of cis/trans-isomers of 2-(2-methylstyryl)pyrroles (1-6) have been recorded and analyzed, making use of d. functional theory (DFT) calcns. at the B3LYP level. Compds. 1 and 2 have no further substituents in the pyrrole ring. In the other compds. the pyrrole ring is substituted by a Me group in the 1- (3, 4) or 5-position (5, 6). Twisted conformations were calcd. for 1-6 in which the toluene ring is generally much more distorted from the plane of the central C=C double bond than the pyrrole ring. In the cis-isomers (2, 4, 6) the torsion angle of the toluene groups amts. to about 50 Deg, while in the trans-isomers (1, 3, 5) the torsional angle is only about half as large. The energy difference DIP of the ionization bands assocd. with the MOs (MOs) p7 and p3 is related to this torsion. Calcd. DIP values are generally 5-10% larger than the exptl. values, indicating that torsional vibrations contribute to the apparent distortion of the mols. The results clearly indicate that PE spectroscopy - in combination with high-level quantum chem. calcns. - is a versatile method to distinguish E/Z isomers of heterocyclic stilbene-like mols.
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