A theory of the hydrogen kinetic isotope effect (KIE) is presented. It is based on the quantummechanical theory of the elementary act of chemical reactions in polar media. The theory takes into account both the non-adiabatic and adiabatic ways of proton (deuteron) transitions between various vibrational energy levels and possible unharmonicity of the proton vibrations. The theory describes the dependence of the KIE on various characteristics of the reactants and the medium. Simplified equations for the KIE are presented. * Bell's approach was put to doubt by Saunders et ~1 . ~~ According to Saunders et al., tunnelling makes an important contribution to KIE but, as follows from model evaluations carried out, it does not determine the form of the dependence of KIE on AG. These authors think that a more probable theory explains KIE variability as being due to changes in the zero energies of the transition complex. However, the formal technique used has no physical basis. t In the original paper 23 the Er value is designated as A. 3 But there does exist an opposite point of view [see ref. (27) and (28)].
The adsorption of several molecular and dissociative dihydrogen systems on a Pd-decorated graphene monolayer was studied using the density-functional theory. Our calculations show that the most favorable graphene-supported coordination structure is similar to the PdH 2 complex in vacuum, where the H-H bond is relaxed but not dissociated. We also computed overlap populations corresponding to bonds and atomic orbital interactions in order to study the evolution of the chemical bonding. During the decoration process with Pd, we detected a weakening of C-C bonds close to the adsorption site and the formation of strong C-Pd bonds, coming from interaction between C 2p z and Pd 5s, 5p z , and 4d z 2 orbitals. After H 2 molecule adsorption, the H-Pd bond is formed by the H 1s orbital overlap with the Pd 5s orbital, but this interaction became stronger during the atomic hydrogen adsorption. The objective of this work is to contribute to the understanding of the hydrogen uptake of Pd-doped graphene surfaces.
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