Electronic spectra of substituted pyridines adsorbed on aluminas

Abstract

Diffuse reflectance spectra of pyridine, 4-methylpyridine and 2,4,6-trimethylpyridine which had been adsorbed on transition aluminas under different conditions, were recorded in the ultraviolet and visible region. The π-π* transition, which was observed in all cases, shifts slightly towards longer wavelengths owing to the coordination of the bases onto Lewis acid sites. Additional bands in the u.v. region are assigned as metal reduction CT bands, the appearance and position of which are in agreement with our previous postulation of the formation of “inner” and “outer” complexes of the pyridine bases. A band near 470 nm suggests the formation of pyridine radical cations at high temperatures, although its assignment is only tentative.