Inference of Transition State Geometries from Kinetic Isotope Effects: An Ab Initio Study of an E2 Model System

The corrections to rate constants for an harmonicity of vibrational excited states have been evaluated over the temperature range of 200–1100 K. The reaction O2 + X, where X is H or D, has been chosen as the model system. Only the influence of vibrational anharmonicity of the triatomic transition state has been determined. Two geometric shapes for the transition state, bent and isosceles configurations, have been investigated in detail by the bond order method.It is found that the correction can be large, depending upon the geometry and force field of the transition state and the temperature. The magnitude of the correction for anharmonicity of the vibrational excited states depends mainly, at a particular temperature, on the strength of the O—X bond in the transition state. In the case of a large correction, anharmonicity may lead to a nonlinear Arrhenius plot.Because of cancellation effects, the correction for anharmonicity of the excited vibrational states in kinetic isotope effects can be ignored in the lower temperature region. It has also been found that anharmonicity of the vibrational groundstate can explain unexpected large isotope effects.

Download Full-text

Kinetic isotope effects in the reduction of 2- and 3-alkyl-substituted-indanone-(tricarbonyl)chromium complexes by NaBH4 and NaBD4. Evidence for displacement of the transition state depending on the substrate

Journal of the Chemical Society Chemical Communications ◽

10.1039/c39760000655 ◽

1976 ◽

pp. 655 ◽

Cited By ~ 11

Author(s):

Bertrand Caro ◽

G�rard Jaouen

Keyword(s):

Transition State ◽

Isotope Effects ◽

Kinetic Isotope Effects ◽

Chromium Complexes ◽

Kinetic Isotope

Download Full-text

Kinetics and Mechanism of the Aminolysis of S-Aryl O-Ethyl Dithiocarbonates in Acetonitrile

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc20042174 ◽

2004 ◽

Vol 69 (12) ◽

pp. 2174-2182 ◽

Cited By ~ 9

Author(s):

Hyuck Keun Oh ◽

Ji Young Oh ◽

Dae Dong Sung ◽

Ikchoon Lee

Keyword(s):

Transition State ◽

Isotope Effects ◽

Kinetic Isotope Effects ◽

Interaction Constant ◽

Kinetics And Mechanism ◽

Cyclic Structure ◽

Concerted Mechanism ◽

Tetrahedral Intermediate ◽

Kinetic Isotope ◽

Hydrogen Bonded

The aminolysis of S-aryl O-ethyl dithiocarbonates with benzylamines are studied in acetonitrile at -25.0 °C. The βX (βnuc) values are in the range 0.67-0.77 with a negative cross-interaction constant, ρXZ = -0.24, which are interpreted to indicate a concerted mechanism. The kinetic isotope effects involving deuterated benzylamine nucleophiles (XC6H4CH2ND2) are large, kH/kD = 1.41-1.97, suggesting that the N-H(D) bond is partially broken in the transition state by forming a hydrogen-bonded four-center cyclic structure. The concerted mechanism is enforced by the strong push provided by the EtO group which enhances the nucleofugalities of both benzylamine and arenethiolate from the putative zwitterionic tetrahedral intermediate.

Download Full-text