Superacid Chemistry on Mildly Acidic Water

The mechanism of proton transfer across water-hydrophobic media boundaries is investigated in experiments in which the protonation of gaseous n-hexanoic acid (PCOOH) upon collision with liquid water microjets is monitored by online electrospray mass spectrometry as a function of pH. Although PCOOH(aq) is a very weak base (pK(BH+) < -3), PCOOH(g) is converted to PC(OH)(2+) on pH < 4 water via a process that ostensibly retains some of the exoergicity of its gas-phase counterpart, PCOOH + H3O+ = PC(OH)(2)(+) + H2O, Delta G < -22 kcal mol(-1). The large kinetic isotope effects observed on H2O/D2O microjets, PC(OH)(2)(+)/PC(OH)OD+ = 88 and PC(OH)OD+/PC(OD)(2)(+) = 156 at pD = 2, and their inverse dependences on pH indicate that PCOOH(g) hydronation on water (1) involves tunneling, (2) is faster than H-isotope exchange, and (3) is progressively confined to the outermost layers as water becomes more acidic. Proton transfers across steep water density gradients appear to be promoted by both dynamic and thermodynamic factors.