Radical-enhanced acidity: why bicarbonate, carboxyl, hydroperoxyl, and related radicals are so acidic

Comparison of accepted pK_a values of bicarbonate, carboxyl, and hydroperoxyl radicals, with those of models having the unpaired electron replaced by H atoms, implied the acidity of the radicals was greatly increased. A Density Functional Theory computational method of estimating pK_as was developed and applied to a set of radicals designed to probe the phenomenon of radical-enhanced deprotonation (RED-shift) and its underlying causes. Comparison of the computed pK_a values of 12 acid radicals to those of the corresponding model acids confirmed the intensified acidity of the title radicals and also pin-pointed the carboxy-ethynyl (HO₂CC≡C^•) and the carboxy-aminyl (HO₂CNH^•) radicals as having enhanced acidity. The underlying cause was found to be extensive charge distribution away from the anionic O atoms of the conjugate radical anions, coupled with spin density displaced toward these O atoms. Ethyne spacers, between the radical and carboxylate centers, transmitted the effect extremely efficiently such that measurable enhancement was detectable up to at least six alkyne units. The bicyclo[1.1.1]pent-1-yl-3-carboxylic acid radical also displayed enhanced acidity, but additional cage units drastically attenuated the effect. Nitroxide radicals with suitably situated carboxylic acid substituents also exhibited enhanced acidity. Several families of potentially persistent radicals with enhanced acidity were identified.