Photoelectron spectroscopic study of N-aryl- and N-heteroaryl-pyrroles

Abstract

HeI photoelectron (PE) spectra of 13 N-aryl- and N-heteroaryl-pyrroles have been measured. The low energy region of the spectra has been analysed using semiempirical MNDO SCF MO calculations [assuming the validity of Koopmans’ theorem (Physica, 1934, 1, 104)] and the composite molecule method (CMM) which has proven to be particularly useful for electronic structure elucidation of complex molecules. Electronic structure, especially in terms of the distribution of the outer valence electrons over the molecule, has been shown previously to be mostly responsible for their chemical properties.

Attention has been paid to the effect which particular constituents, i.e. either pyrrole or N-aryl- and N-heteroaryl-pyrrole, respectively, may play in the pharmacological activity of arylpyrroles. In all these compounds, the highest occupied MO is of pyrrole-type character, (a₂ symmetry, with a node on the N-atom). Consequently, the same ionisation process yields the lowest energy band in all PE spectra.

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