Model calculations of hydrogen isotope effects for non-linear transition states

Abstract

Calculations are reported showing that primary hydrogen isotope effects depend smoothly on transition-state geometry and yield consistently low values, k_H/k_D= 1–2, for sufficiently non-linear configurations. Specific calculations for 1,2-hydride shifts, borane hydrolyses, and E2C eliminations are compared with available measurements for these and for other hydrogen-transfer reactions proceeding through 3-, 4-, 5-, and 6-membered cyclic transition states. A unique temperature-independent k_H/k_D is noted. The low isotope effects are ascribed to the presence of a large isotopically sensitive vibration ν_R‡(e.g., 3500 cm.^–1) in the transition state. The results are fairly insensitive to reasonable force-constant changes. The usefulness of k_H/k_D in defining the geometry of transition states for hydride transfer reactions is discussed.