Kinetic and Equilibrium Isotope Effects of Proton Exchange and Autoprotolysis of Pure Methanol Studied by Dynamic NMR Spectroscopy

Abstract

AbstractThe rate constants of the intermolecular proton exchange in pure methanol, i.e. the reciprocal proton lifetimes, τ0−1, have been determined as a function of the temperature by total lineshape analysis of the 1H‐NMR spectra. Since CH3OH is an AB3 spin system of high order the quantum mechanical density matrix formalism was employed for the simulation of the spectra. The neglect of high order effects as well as the presence of impurities had led to inconsistencies in previous studies. For the first time, the primary kinetic isotope effects were determined indirectly by simulation of the 1H‐NMR spectra of CH3OD samples containing 1 vol‐% CH3OH. The results are given by for CH3OH, and for CH3OD, with a kinetic isotope effect of 3.2 ± 0.4 at 298 K. ‐ The data cannot be explained by a cyclic exchange mechanism. However, they can be quantitatively related to the autoprotolysis constant of methanol and to Grunwald's kinetic data on proton transfer in buffered methanol solutions. It is concluded that the proton lifetimes in pure methanol are determined by the natural amount of free solvated CH3OH2+ and CH3O− ions generated by autoprotolysis. The observed energy of activation is then the sum of two terms, namely the energy of activation of the proton jumps between the ions and a methanol molecule, and half the enthalpy of methanol selfdissociation. In the presence of acid or basic impurities the second term becomes negligible. We propose a method for the quantitative determination of these impurities in the 10−9 to 10−7 mol 1−1 range. From our results we derive an equilibrium isotrope effect of (KH/KD)298K = 6 ± 3 for the autoprotolysis of pure methanol, which has not been reported before. It is shown that the literature data on proton exchange in pure water and other protoc systems can be explained in a similar way. The mechanism of the neutralisation in water and methanol is discussed. For the neutralisation of solutions containing 1 mol of each ion it is found that the mean number of proton transfer steps necessary is 5.3 for water and 2.5 for methanol. These data correlate well with the molar solvent density.