Pyranyl Radicals and Their Dimers

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<jats:title>Abstract</jats:title> <jats:p>Photochemical and chemical reduction of pyrylium salts in THF solution gave their corresponding radicals which were in equilibrium with dimers. The quantum yields for raidcal formation and the standard enthalpy change ΔH0 for dimerization decrease with increasing the electron-donating character of the substituting groups in the pyrylium ring. Specially, ΔH0 for pentaphenylpyranyl and pentaphenylthiopyranyl radicals, in which all carbons in the ring are substituted by phenyl groups, become possitive. The absorption maxima and their molar extinction coefficients of pyranyl radicals, and concurrently the equilibrium constants for the dimerization were determined spectrophotometrically.</jats:p>

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