OPTICAL SPECTROSCOPY OF THE ARYL CATION .3. SUBSTITUENT EFFECTS ON THE PRODUCTION AND ELECTRONIC-SPECTRA OF INTERMEDIATES IN THE PHOTODECOMPOSITION OF ARN2+ - OPTICAL CHARACTERIZATION OF THE REACTION AR++N2[-ARN2+

The extreme reactivity of the aryl cation is confirmed by the first optical detection of the reversibility of the heterolysis of the diazonium cation at temperatures slightly above 77 K: Seven differently substituted diazonium salts have been found to produce the aryl cation and/or radical species following low temperature photolysis in highly acidic and hydroxylic matrices and the UV-visible spectra of these intermediates have been determined. Generally the radical species absorb within the UV range and the aryl cations in the visible region. The energy of the electronic transition of the radical is matrix dependent, in contrast to the aryl cation for which lambda(max) is matrix independent. The 3,4,5-trimethoxybenzenediazonium salt, which gives no triplet electron spin resonance (ESR) spectrum, also fails to produce an optically detectable aryl cation following photolysis, in contrast to the 2,4,5-trimethoxy compound which provides a triplet ESR spectrum and easily identified optical absorption bands of 3Ar+.