ENTROPY BARRIERS TO PROTON-TRANSFER

Proton transfer between sterically hindered pyridines and amines proceeds through locked-rotor, low-entropy intermediates. The reactions exhibit slow kinetics (efficiencies of 0.1-0.0001) and large negative temperature coefficients (up to k = CT8.7). The rates become slower and the temperature dependencies steeper with increasing steric hindrance. The observations are reproduced by a multiple complex-switching RRKM model that allows several alternative complexes to be rate controlling: a series of loose complexes, a locked-rotor tight complex that occurs before the formation of a hydrogen-bonded complex, and a complex located at the central barrier. The rate-limiting transition state shifts from the loose to the tight and central-barrier complexes with increasing temperature. The model suggests that at elevated temperatures, above 1000 K, ion-molecule reactions will become slow even for unhindered, small reactants. Ion kinetics may then become similar to neutral radical kinetics.