Effects of high pressure on isotope effects and hydrogen tunneling

Kinetic isotope effects arising from transition state phenomena such as tun neling an sensitive to changes in pressure, whereas those arising from diff erences 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 effect s on hydride transfer between chloranil and leucocrystal violet (Isaacs, N. S,; Javaid, K.; Rannala, E. J. Chem. Sec., Perkin Trans. 2 1978, 709-711) to this equation yields Delta V-Q = 36.5 +/- 3.0 mL/mol for the apparent vo lume difference between the transition states of hydride versus deuteride t ransfer, k(H)/k(D) = 7.8 +/- 0.1 for the semiclassical isotope effect due t o differences in zero point energies, and Q(H)/Q(D) = 1.44 +/- 0.02 for the transition state effect. Thus, tunneling accounts for 33 +/- 1% of the obs erved deuterium isotope effect at atmospheric pressure. Incorporating the t ransition state effect into a pair of Bell tunneling correction equations ( Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall; London and Ne w York, 1980) allows the precise estimation of the reaction frequency as up silon(H)double dagger = 797 +/- 12 cm(-1) with Q(H) = 1.97 +/- 0.05. Quanti fying hydrogen tunneling in this way provides a powerful new tool for probi ng transition state chemistry.