Effects of High Pressure on Isotope Effects and Hydrogen Tunneling

Abstract

Kinetic isotope effects arising from transition state phenomena such as tunneling are sensitive to changes in pressure, whereas those arising from differences in zero point energies in the reactant state are not (Isaacs, N. S. Isotope Effects in Organic Chemistry; Buncel, E., Lee, C. C., Eds.; 1984; Vol. 6, pp 67−105). A new equation is derived which differentiates between the two origins as a function of pressure. Fitting published isotope effects on hydride transfer between chloranil and leucocrystal violet (Isaacs, N. S.; Javaid, K.; Rannala, E. J. Chem. Soc., Perkin Trans. 2 1978, 709−711) to this equation yields ΔVQ = 36.5 ± 3.0 mL/mol for the apparent volume difference between the transition states of hydride versus deuteride transfer, kH/kD = 7.8 ± 0.1 for the semiclassical isotope effect due to differences in zero point energies, and QH/QD = 1.44 ± 0.02 for the transition state effect. Thus, tunneling accounts for 33 ± 1% of the observed deuterium isotope effect at atmospheric pressure. Incorporat...