A new form of kinetic isotope effect. Dynamic effects on isotopic selectivity and regioselectivity.

The intramolecular H/D kinetic isotope effect in the ene reaction of singlet oxygen with tetramethylethylene is studied using quasiclassical direct dynamics calculations on a B3LYP/6-31G* potential energy surface. Starting from the area of the energy surface around a valley-ridge inflection point, random trajectories lead to predominantly H abstraction over D abstraction, despite the symmetry of the surface and the absence of a barrier to either reaction. This demonstrates a new form of kinetic isotope effect, unrelated to the usual effect of zero-point energies on barriers. Dynamics calculations on the reaction of cis-2-pentene predict the experimentally observed mixture of regioisomeric products, while the minimum-energy path leads to only one product. For energy surfaces containing two adjacent saddle points, dynamics effects are important for understanding both product and isotopic selectivity, and this should be considered in the interpretation of experimental results.