Calculation of Deuterium Isotope Effects in Proton Transfer Reactions

Various levels of theory are tested for the purpose of computing the rate constant for proton transfer reactions. Standard transition state theory is applied to a series of molecules with a progressively more bent intramolecular hydrogen bond. The systems all display similar deuterium isotope effects (DIEs); the larger DIE at low temperature is attributed to zero-point vibrational effects. However, when tunneling is incorporated via a microcanonical approach, a dramatically enhanced effect is observed for the most distorted H-bond. The energy barrier for proton transfer between carbon atoms involved in triple bonds is smaller than for carbons with lesser multiplicity. The DIE displays a sensitivity to temperature that is least for the carbon atoms with the greatest multiplicity of bonding. The tunneling obtained by following the minimum energy reaction path along the potential energy surface is similar to that when the potential is approximated by an Eckart barrier. However, significant discrepancies are observed at temperatures below about 250 K