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Updated: Aug 31, 2026

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
Published on: July 19, 2019
Linear free energy relationship and kinetic isotope effects as measures for the transition state variation. A
Salai Cheettu Ammal1, Masaaki Mishima, Hiroshi Yamataka
1Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047 Japan.
Abstract:
Linear free energy relationship (LFER) and kinetic isotope effects (KIEs) are frequently used experimental means to study reaction mechanisms, in particular the nature of transition states (TSs). Density functional theory (B3LYP/6-311+G**) calculations were carried out on a model reaction, acid-catalyzed ionization of phenylethyl alcohol, to analyze how experimentally observable properties, such as nonlinearity in the Hammett and Brønsted relations and variation in KIE, are related to a variation of the transition state structure and the mechanism. Several conclusions and insights were obtained: (1) Linear Hammett plots with a dual parameter treatment may not be evidence for an invariable TS structure for a series of reactions. (2) Variations of KIEs indeed reflect the variations of TS structures. (3) Nonlinear Brønsted plots cannot always be taken as evidence for a stepwise mechanism. (4) A TS structure in the gas phase may change much more easily than a TS structure in solution.
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