Proton Conductance and the Existence of the H3O· Ion

Abstract

Measurements of the conductance of HCl in water, methanol, ethanol, n‐propanol, ethylene glycol, glycerol, 1:4 dioxane, acetone, formic, and acetic acids and the respective aqueous‐nonaqueous systems have been made as a function of concentration of water and HCl. The degree of anomalous proton conductance decreases with increasing n in C_nH_2n+1OH, and conductance minima occur in aqueous alcoholic HCl solutions of low H₂O content. Former theories of proton conductance have permitted no quantitative distinction to be made between the rates of classical proton transfer, quantum‐mechanical tunneling transfer, and water rotation as rate determining stages. If resonance in H₃O^· is set up sufficiently quickly after arrival of a given proton at a water molecule, rotation of H₃O^· cannot be a rate determining step. Detailed calculations of the rates of classical proton transfer, tunnel transfer, and the rate of rotation of water are made. Classical transfer is much slower and tunnel transfer much faster than corresponds to the observed proton mobility; rotation of hydrogen bonded water molecules near the H₃O^· ion is the rate determining reaction (preceded and succeeded by fast tunneling of protons). This conclusion is quantitatively consistent with the OH′ mobility in water, and the H^·/D^· mobility ratio and qualitatively consistent with pressure and temperature effects and with the absence of abnormal mobility of NH₄^· and NH₂′ in liquid NH₃. A detailed interpretation of the dependence of abnormal conductance upon chain length in ROH gives fair quantitative agreement with experiment. The conductance minima observed in aqueous alcoholic HCl are quasi‐quantitatively predicted by the theory. The proton spends about 1% of its time undergoing tunnel transfers; the effective existence of the H₃O^· ion is thus confirmed.