EFFECT OF PH ON MUONIUM REACTIONS IN AQUEOUS-SOLUTIONS OF P-AMINOBENZOIC ACID

p-Aminobenzoic acid was studied as a solute in water to determine the effect of altering the pH from 1 to 10 on the rate of reaction and on the site of addition of muonium atoms. This solute exists in three dif ferent acid-base states ranging from the fully protonated, through neu tral, to anionic, and its substituents change from electron-accepters to electron-donors with increasing pH. The overall rate of reaction is barely affected by pH, but there is nearly a 2-fold switch in the sit e at which muonium adds to the ring. There is also a considerable swit ch in the magnetic field at which muon level crossing resonance occurs , the radical with muonium attached at C(2) moving upfield by 125 mT a nd the C(3) one moving downfield by a similar amount. If this compound is representative of aromatic systems, then the differences in pH whi ch usually exist between hydrogen and muonium studies would account fo r only a small factor in most reported kinetic isotope effects.