Experimental and theoretical approach to the electronic structure and the molecular conformation of azabiphenyls. Assymetric bipyridines

• Published in: Chemical physics 1985-07, Vol.96 (3), p.435-445
• Main Authors: Barone, Vincenzo, Lelj, Francesco, Commisso, Luigi, Russo, Nino, Cauletti, Carla, Piancastelli, M.Novella
• Format: Article
• Language: English
• ISSN: 0301-0104
• DOI: 10.1016/0301-0104(85)85105-3

The geometric and eletronic structure of asymmetric bipyridines has been investigated by the combined use of photoelectron spectroscopy and quantum mechanical computations. This study, together with the results previously reported for symmetric compounds, has allowed a comprehensive analysis of general trends along the whole series of bipyridines, and, more generally, of azabiphenyls. Conformational energies obtained by ab initio-STO-3G computations show a good internal coherence and are in aggreement with the available experimental data. They show that all the isomers except 2,2′-bipyridine have non-planar equilibrium conformations and that the angle of twisting increases with the number of interactions between ortho CH-groups of the two rings Koopmans' approximation is not adequate to reproduce the correct sequence of ionic states while the HAM/3 method allows a reliable assignment of the photoelectron spectra. The ionic level (especially lone pairs), are scarcely sensitive to conformational variations and their splitting is dominated by the direct coupling between atomic orbitals of the inter-ring carbon atoms.