Extreme ‘Hydrogen Isotopes’ Illuminate Reaction Kinetics

Using extremely light and heavy hydrogen isotope mimics, a team of chemists has illustrated the kinetic isotope effect in a more dramatic fashion than ever before, making it possible to explore reaction mechanisms with finely detailed comparisons between theory and experiment (Science, DOI: 10.1126/science.1199421). Exploiting the kinetic isotope effect has been a long-standing strategy for chemists to probe chemical kinetics. Typically, a heavier isotope, such as deuterium, is substituted for a lighter one, such as hydrogen. But a research team including Donald G. Fleming of the University of British Columbia and Donald G. Truhlar of the University of Minnesota, Twin Cities, tried something different. Previously, scientists created an ersatz, ultralight “hydrogen isotope” by replacing hydrogen’s proton nucleus with a positive muon—an elementary particle that is similar to an electron. The group created a superheavy mimic by replacing one helium electron with a negative muon. A muon is about nine times lighter than a proton and 207 times heavier than an electron. The researchers used the light and heavy isotope mimics to study the simple reaction H + H₂ → H₂ + H with excellent theoretical agreement with their experiments.