Pseudo Jahn-Teller Effect in Transition States of Redox Processes

Introduction

The role of the chemical reactions' transition (activated) states in environmental processes is crucial for revealing the latter's nature and their possible manipulation. Most of these reactions involve either electron, or proton transfer, or both. On the other hand, the vibronic coupling theory, mostly the Jahn-Teller effect (JTE) and the pseudo JTE (PJTE), were shown to be the only source of spontaneous symmetry breaking of high-symmetry configurations of polyatomic systems, see e.g. (Bersuker & Polinger, 1989); (Bersuker, 2006); (Bersuker, 2013); (Bersuker, 2016); (Bersuker, 2021). In the application of this fundamental principle to chemical-reaction transition states, it was shown that the instability of their ground states at the barrier point is due to the PJTE interaction with an appropriate low-lying excited state (Bersuker & Polinger, 1989); (Bersuker, 1984). This new understanding allowed for further developments of the theory of the PJTE origin of instability of the transition states of chemical reactions. As the only source of instability and spontaneous symmetry breaking of high-symmetry configurations of molecular systems and solids, the JTE and PJTE may be related to a huge variety of physical, chemical, and biological phenomena, but so far, only a limited number of such possibilities were explored.

In this Chapter we demonstrate the role of the JTE and PJTE in forming the transition or activated states of chemical reactions relevant to environmental problems, as well as some examples from spectroscopy and photochemistry. As a whole, this Chapter also contributes significantly to the development of the general theory of the PJTE in chemical reactivity.

In Section 2 we introduce the PJTE in the form applicable to the redox problems under consideration. It includes, in particular, modeling the polarizing effect of the solvent, responsible for an additional stabilizing effect on intramolecular H-bond. In Section 3, a simple case of the reaction, H + H2 ® H2 + H in a linear collision (Bersuker, 1987) is used to show the feasibility of the idea about the PJTE origin of the negative curvature of the potential energy surface at the point of the transition state, and the existence in this nuclear configuration in the stable excited states that cause the instability of the activated complex of the reaction (Bersuker, 1980). It is also shown that in some cases of intermediate states of chemical reactions, it is necessary to take into account the JTE in the degenerate excited state. We also show an example of applying the PJTE approach to the problem of transition states of photochemical reactions (Wang, 2019), which is also important in environmental problems.

In some more recent works, the PJTE was shown to control the proton transfer between two molecular systems, often termed as the Hydrogen (H) bond (Gorinchoy, 2021); (Geru, 2012), (García-Fernández, 2008). This approach, in turn, allowed us to estimate the main parameter of the H-bond, the energy barrier for the proton transfer between the interacting molecular systems. In Section 4, several examples illustrate this methodology by showing how the PJTE influences the energy barrier for the proton transfer. Illustrative examples include the proton transfer in deprotonated H₃O₂^- and protonated H₅O₂⁺(H₂O)₄ water clusters, which involve a strong low-barrier hydrogen bond - a phenomenon often introduced to explain the surprisingly high rates of some enzyme-catalyzed reactions; several proton-bound dimers MH⁺ + M « M + HM⁺, and other similar systems are also considered. Section 5 of this Chapter analyzes the role of the JTE and PJTE is analyzed in the charge-transfer processes, when common bonding and antibonding molecular orbitals are formed between the interacting reactants, leading to a redistribution of the electronic density in the activated complex.