K. Venkateswaran et al., EFFECT OF PH ON MUONIUM REACTIONS IN AQUEOUS-SOLUTIONS OF P-AMINOBENZOIC ACID, Journal of physical chemistry, 100(9), 1996, pp. 3569-3572
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.