Pyrylium electronic spectra

In the early 1960s, it was found that pyrylium ions undergo single-electron reduction, either by chemical or electrochemical means, with the formation of pyranyl radicals which usually dimerize essentially entirely by coupling at the 4-position.Reduction of 2,4,6-triphenylpyrylium salts, conveniently by zinc dust in inert solvents, led to the red radical 29 which persists for several days in a sealed tube. ° The ESR spectrum of 29 was recorded and eventually analyzed by Degani et Their hyperfine splittings... 

There appear to be rather few data available for the pyrylium cation. Balaban and Simon169 have reported a high value of +97 for the aromaticity constant, a reflection of its electrophilicity. The PMR spectrum, as expected, shows absorptions in the 8.5-9.6 ppm region,296 and qualitative agreement between chemical shifts and r-electron density, as calculated by the HMO method, has been reported.297 Further calculations of charge... 

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