CONFORMATIONAL AND UV PHOTOELECTRON-SPECTROSCOPY ANALYSIS OF THE CHALCOGENOBISPYRIDINES

It is now possible to review the conformational and electronic behavio urs of the isomers of chalcogenobispyridines and related compounds, so providing information on the link between the geometric and electroni c properties, and providing some insight into their observed and antic ipated physical and biological activities. Moreover, observations can now be made on the effect of the bridging atoms (O, S, Se, Te) and pos itional isomerism on conformation and electronic distributions; on the balance between resonance and inductive effects; and on the energetic s of conformer interconversion. As a result of the non-rigid nature of the chalcogenobispyridines and related compounds, it is necessary to determine the preferred geometries to calculate the ionization energie s (Ifs). For the entire family of the chalcogenobispyridines, the mini mum energy structures predict the pyridine ring planes to be nearly pe rpendicular, allowing maximum overlap of the chalcogen valence orbital s with the pi system of only one of the pyridine rings. Rotational bar rier height calculations do not support the fully conjugated planar fo rms on steric grounds, but suggest that a range of near-isoenergetic c onformers surround the minimum structure. Ab initio methods (such as r estricted Hartree-Fock (HF) methods) were found to assign incorrectly the UV photoelectron spectra of these pyridine-based compounds. While HF methods were shown to be reasonably successful for the prediction o f the Ifs of the pi-type molecular orbitals (MOs), they failed to posi tion the Ifs correctly for the lone-pair nitrogen MOs (n(N)), so requi ring the use of computer-intensive configuration interaction (CI) meth ods. Bands caused by distinct conformers could not be resolved in the UV photoelectron spectra of the chalcogenobispyridines. Conformational analyses predicted low energy pathways for concerted disrotatory moti on, permitting a smooth transition between near isoenergetic rotamers. The calculated Ifs of the most populated conformers were found to be within the instrumental resolution, so were difficult to resolve. This was supported by the production of 'synthetic' spectra. Within the co mposite-molecule model, correlations with the Ifs of pyridine and the methylchalcogeno-pyridines assisted in the spectral assignment of the chalcogenobispyridines. For each congener, the first IE was assigned t o ionization from an anti-bonding pi-n(X) combination in which the con tribution of pyridine pi character was found to decrease down the grou p. The spectra recorded with different radiation sources were valuable in identifying bands associated primarily with the chalcogen characte r. (C) 1997 Elsevier Science S.A.