An unusually acidic methyl group directly bound to acridinium cation.

9,10-Dimethylacridinium chloride (1: X = Cl) exhibited strong acidity of pH 3.90 (3.4 x 10(-3) M, 20 degrees C) in an aqueous solution. H-D exchange reaction of 1 in D2O indicated that protons in the 9-methyl group dissociated to generated H+ ions. This is a unique example of a methyl proton functioning as an acid. The acidity derives from the wider pi face in acridinium capable of delocalizing the newly formed negative charge upon proton dissociation. PM3 calculations provided stabilization factors (deltaDeltaH(f)o) between proton dissociated and undissociated forms of several N-heterocycles and also confirmed the acidity observed in acridinium.