Kinetic and geometrical isotope effects in hydrogen-atom transfer reaction, as calculated by the multi-component molecular orbital method

To estimate the kinetic isotope effect (KIE) for hydrogen (or deuterium) abstraction from H(D)OR (R = H, CH3, and CN) by an OH radical, we have considered the geometrical isotope effect (GIE) induced by the difference of the protonic and deuteronic wave-functions using the multi-component MO method. The difference by the GIE of hydrogen bond was about 0.005 A. The ratio (k(a)(H)/k(a)(D)) of the rate constant of the reaction for R = H, HOR -> HOR HOH + OR and HO + DOR HOD -> OR, is estimated as 4.4 by our calculation, which is reasonable agreement with experimental result of 6.0 +/- 2.0. We have found that the difference of the nuclear wavefunction of the proton and deuteron affects the changes of geometry and electronic charge density, which plays an important role to theoretically determine the effective potential energy surfaces and the corresponding KIE between H and D compounds. (c) 2005 Elsevier B.V. All rights reserved.