A STUDY OF PHOTOELECTRON-SPECTRA IN THE FRAMEWORK OF THE SEMIEMPIRICAL AM1 METHOD .5. HALOGENATED DERIVATIVES OF 2-PHENYLPYRROLE AND BIPHENYL

The characteristics of photoelectron (PE) spectra of 2-phenylpyrroles and biphenyls have been correlated with the UV spectroscopy data for their radical cations. The geometric and electronic structures of the compounds and radical cations have been calculated using the AM1 method. The absorption bands of the radical cations observed in the visible region of spectrum have been found to pertain to the quasi-Koopmans type, i.e., they are realized between the doubly and partially occupied pi-MO, but their energies differ strongly from those calculated on the basis of PE spectra. This is explained by the distinction between the spatial structure of a molecule and that of the respective radical cation. The second cause of the discrepancy between the data of the two spectral methods consists in orbital sigma-pi interactions, which are most conspicuous in the case of 4(4')-halogen-substituted compounds.