The Nature of the Azonium Cation as Evidenced by X‐Ray Photoelectron Spectroscopy

Abstract

The question of the classical or non-classical nature of aryl-substituted azonium cations as well as of the pyridazinium cation has been settled by X-ray photoelectron spectroscopy in favour of the classical structure. The relative magnitude of the observed Nls binding energies is reasonably well reproduced on the basis of the theoretically calculated charge structure of these ions. The relative magnitude of the crucial intramolecular potential terms, appearing in the theoretical treatment, is also suggested from PMR.-investigations on protonated azo-compounds. The electronic structure of these species is compared with that implied by commonly accepted classical valence structures and parallels to carbo-cations are drawn. MINDO/Z calculations of the proton affinities of azo-compounds, considering structurally different conjugate acids, also support a classical azonium cation structure. The gas phase basicity of azobenzene is predicted to be higher than that of azomethane.

Effect of substituents on the absorption spectra andpK values of some azo dyes derived from thiobarbituric acid

“…From the study of protonation equilibria, Yeh and Jaffe (1959) suggested the formation of a symmetrical diazonium cation, 6, but later experimental results supported the classical structure, 7, for the protonated azobenzene. X-ray photoelectron spectroscopy has also supported the latter structure (Haselbach 1973).…”