A pictorial representation of zero‐point energy and tunnelling contributions to primary hydrogen isotope effects

Abstract

AbstractThe interpretation of primary hydrogen isotope effects in terms of isotopic sensitivities of zero‐point energies in the reactants and transition state is reviewed. The reader is reminded that the transition state for a hydrogen transfer reaction corresponds to the position of maximum energy on the minimum energy path defined by the motion of the heavy atoms between which hydrogen is transferred. Tunnelling occurs through the potential energy barrier separating the reactant and product channels of the potential energy surface for this reaction. It is a consequence of rapid vibrational motion of the hydrogen within a collision complex and the overlap of vibrational wave functions between reactant and product sides of the barrier. Comparing zero‐point energies of a tunnelling complex and a hydrogen bond confirms results of calculations indicating that the zero‐point energy of the complex is likely to be less than that of the reactants. It follows that there should be a compensation between contributions from zero‐point energy changes and tunnelling which may explain why tunnelling rarely leads to very large isotope effects at ambient temperatures. The isotopic sensitivity of the zero‐point energy of the transition state is discussed in terms of variations in force constants of reacting bonds within a three‐centre model. A shift in isotopic sensitivity from the reaction coordinate mode to real stretching vibration as the structure of the transition state becomes more reactant‐ or product‐like, which provides a basis for the Westheimer effect, contrasts with the behaviour of hydrogen bonds, for which there is no change in isotopic sensitivity of the two stretching modes. Copyright © 2010 John Wiley & Sons, Ltd.