OEEF-Driven Intramolecular Self-Redox of Superalkali Rb3BeB6Be'Rb'3: A High-Performance Candidate for NLO Molecular Switch

To provide a novel intramolecular self-redox switch, a boron-based sandwich-like complex Rb3BeB6Be'Rb'3 is achieved by using theoretical computations. An applicable oriented external electric field (OEEF) can result in the occurrence of intramolecular self-redox (IMSR) with a long-range electron transfer from tetrahedral Be'Rb'3 to Rb3Be and subsequently [Rb3Be]3+[B6]6-[Be'Rb'3]3+ (D3d) changes to [Rb3Be]2+[B6]6-[Be'Rb'3]4+ (C3v), accompanying high-performance NLO switchable effect for both static and dynamic first hyperpolarizability (β0). [Rb3Be]3+[B6]6-[Be'Rb'3]3+ (off-form) owns zero of dipole moment (μ0) and β0, while [Rb3Be]2+[B6]6-[Be'Rb'3]4+ (on-form) exhibits a μ0 of 3.36 D and a β0e of 2.18 × 105 au. The different dynamic first hyperpolarizabilities between [Rb3Be]3+[B6]6-[Be'Rb'3]3+ and [Rb3Be]2+[B6]6-[Be'Rb'3]4+ are also significant. This indicates that Rb3BeB6Be'Rb'3 is a potential candidate for an IMSR NLO switch.

Electromagnetic bioeffects: a multiscale molecular simulation perspective

Electromagnetic bioeffects remain an enigma from both the experimental and theoretical perspectives despite the ubiquitous presence of related technologies in contemporary life. Multiscale computational modelling can provide valuable insights into biochemical systems and predict how they will be perturbed by external stimuli. At a microscopic level, it can be used to determine what (sub)molecular scale reactions various stimuli might induce; at a macroscopic level, it can be used to examine how these changes affect dynamic behaviour of essential molecules within the crowded biomolecular milieu in living tissues. In this review, we summarise and evaluate recent computational studies that examined the impact of externally applied electric and electromagnetic fields on biologically relevant molecular systems. First, we briefly outline the various methodological approaches that have been employed to study static and oscillating field effects across different time and length scales. The practical value of such modelling is then illustrated through representative case-studies that showcase the diverse effects of electric and electromagnetic field on the main physiological solvent - water, and the essential biomolecules - DNA, proteins, lipids, as well as some novel biomedically relevant nanomaterials. The implications and relevance of the theoretical multiscale modelling to practical applications in therapeutic medicine are also discussed. Finally, we summarise ongoing challenges and potential opportunities for theoretical modelling to advance the current understanding of electromagnetic bioeffects for their modulation and/or beneficial exploitation in biomedicine and industry.

How Oriented External Electric Fields Modulate Reactivity

A judiciously oriented external electric field (OEEF) can catalyze a wide range of reactions and can even induce endo/exo stereoselectivity of cycloaddition reactions. The Diels-Alder reaction between cyclopentadiene and maleic anhydride is studied by using quantitative activation strain and Kohn-Sham molecular orbital theory to pinpoint the origin of these catalytic and stereoselective effects. Our quantitative model reveals that an OEEF along the reaction axis induces an enhanced electrostatic and orbital interaction between the reactants, which in turn lowers the reaction barrier. The stronger electrostatic interaction originates from an increased electron density difference between the reactants at the reactive center, and the enhanced orbital interaction arises from the promoted normal electron demand donor-acceptor interaction driven by the OEEF. An OEEF perpendicular to the plane of the reaction axis solely stabilizes the exo pathway of this reaction, whereas the endo pathway remains unaltered and efficiently steers the endo/exo stereoselectivity. The influence of the OEEF on the inverse electron demand Diels-Alder reaction is also investigated; unexpectedly, it inhibits the reaction, as the electric field now suppresses the critical inverse electron demand donor-acceptor interaction.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.

Energetics for Proton Reduction in FeFe Hydrogenase

The energetics for proton reduction in FeFe-hydrogenase has been reinvestigated by theoretical modeling, in light of recent experiments. Two different mechanisms have been considered. In the first one, the bridging hydride position was blocked by the enzyme, which is the mechanism that has been supported by a recent spectroscopic study by Cramer et al. A major difficulty in the present study to agree with experimental energetics was to find the right position for the added proton in the first reduction step. It was eventually found that the best position was as a terminal hydride on the distal iron, which has not been suggested in any of the recent, experimentally based mechanisms. The lowest transition state was surprisingly found to be a bond formation between a proton on a cysteine and the terminal hydride. This type of TS is similar to the one for heterolytic H₂ cleavage in NiFe hydrogenase. The second mechanism investigated here is not supported by the present calculations or the recent experiments by Cramer et al., but was still studied as an interesting comparison. In that mechanism, the formation of the bridging hydride was allowed. The H–H formation barrier is only 3.6 kcal/mol higher than for the first mechanism, but there are severe problems concerning the motion of the protons.

Switch chemistry at cryogenic conditions: quantum tunnelling under electric fields

While the influence of intramolecular electric fields is a known feature in enzymes, the use of oriented external electric fields (EEF) to enhance or inhibit molecular reactivity is a promising topic still in its infancy. Herein we will explore computationally the effects that EEF can provoke in simple molecules close to the absolute zero, where quantum tunnelling (QT) is the sole mechanistic option. We studied three exemplary systems, each one with different reactivity features and known QT kinetics: π bond-shifting in pentalene, Cope rearrangement in semibullvalene, and cycloreversion of diazabicyclohexadiene. The kinetics of these cases depend both on the field strength and its direction, usually giving subtle but remarkable changes. However, for the cycloreversion, which suffers large changes on the dipole through the reaction, we also observed striking results. Between the effects caused by the EEF on the QT we observed an inversion of the Arrhenius equation, deactivation of the molecular fluxionality, and stabilization or instantaneous decomposition of the system. All these effects may well be achieved, literally, at the flick of a switch.

Manipulation of Diatomic Molecules with Oriented External Electric Fields: Linear Correlations in Atomic Properties Lead to Nonlinear Molecular Responses

Oriented external electric fields (OEEFs) have been shown to have great potential in being able to provide unprecedented control of chemical reactions, catalysis, and selectivity with applications ranging from H₂ storage to molecular machines. We report a theoretical study of the atomic origins of molecular changes because of OEEFs since understanding the characteristics of OEEF-induced couplings between atomic and molecular properties is an important step toward comprehensive understanding of the effects of strong external fields on the molecular structure, stability, and reactivity. We focus on the atomic and molecular (bond) properties of a set of homo- (H₂, N₂, O₂, F₂, and Cl₂) and heterodiatomic (HF, HCl, CO, and NO) molecules under intense external electric fields in the context of quantum theory of atoms in molecules (QTAIM). It is shown that the atomic properties (atomic charges, energies, and localization indices) correlate linearly with the field strengths, but molecular properties (bond length, electron density at the bond critical point, and electron delocalization index) exhibit nonlinear responses to the imposed fields. In particular, the changes in the electron density distribution alter the shapes and locations of the zero-flux surfaces, atomic volumes, atomic electron population, and localization/delocalization indices. The topography and topology of the molecular electrostatic potential undergo dramatic changes. External fields also perturb the covalent-polar-ionic characteristic of the studied chemical bonds, hallmarking the impact of electric fields on the stability and reactivity of chemical compounds. The findings are well-rationalized within the framework of the QTAIM and form a coherent conceptual understanding of these effects in prototypical diatomic molecules.

The electrostatic potential as a descriptor for the protonation propensity in automated exploration of reaction mechanisms

We discuss the possibility of exploiting local minima of the molecular electrostatic potential for locating protonation sites in molecules in a fully automated manner. We implement and apply this concept to exploring the mechanism of proton reduction catalyzed by a hydrogenase model complex [Orthaber et al., Dalton Trans., 2014, 43, 4537]. A large number of distinct structures arising already in the early stages of the hydrogen evolution mechanism demonstrates the need for reliable, automated algorithms for the thorough analysis of catalytic processes.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Computational Methods for Modeling Metalloproteins

Metalloproteins are challenging objects if we want to investigate their chemical reactivity with theoretical approaches such as density functional theory (DFT). The complexity of these biomolecules often requires us to find a compromise between accuracy and feasibility, one that is tailored to the questions we set out to answer. In this chapter, we discuss computational approaches to studying chemical reactions in metalloproteins and how to utilize the information hidden in homologous proteins.

Structure and reactivity/selectivity control by oriented-external electric fields

This is a tutorial on use of external-electric-fields (EEFs) as effectors of chemical change. The tutorial instructs readers how to conceptualize and design electric-field effects on bonds, structures, and reactions. Most effects can be comprehended as the field-induced stabilization of ionic structures. Thus, orienting the field along the "bond axis" will facilitate bond breaking. Similarly, orienting the field along the "reaction axis", the direction in which "electron pairs transform" from reactants- to products-like, will catalyse the reaction. Flipping the field's orientation along the reaction-axis will cause inhibition. Orienting the field off-reaction-axis will control stereo-selectivity and remove forbidden-orbital mixing. Two-directional fields may control both reactivity and selectivity. Increasing the field strength for concerted reactions (e.g., Diels-Alder's) will cause mechanistic-switchover to stepwise mechanisms with ionic intermediates. Examples of bond breaking and control of reactivity/selectivity and mechanisms are presented and analysed from the "ionic perspective". The tutorial projects the unity of EEF effects, "giving insight and numbers".

Toward functional type III [Fe]-hydrogenase biomimics for H2 activation: insights from computation

The chemistry of [Fe]-hydrogenase has attracted significant interest due to its ability to activate molecular hydrogen. The intriguing properties of this enzyme have prompted the synthesis of numerous small molecule mimics aimed at activating H2. Despite considerable effort, a majority of these compounds remain nonfunctional for hydrogenation reactions. By using a recently synthesized model as an entry point, seven biomimetic complexes have been examined through DFT computations to probe the influence of ligand environment on the ability of a mimic to bind and split H2. One mimic, featuring a bidentate diphosphine group incorporating an internal nitrogen base, was found to have particularly attractive energetics, prompting a study of the role played by the proton/hydride acceptor necessary to complete the catalytic cycle. Computations revealed an experimentally accessible energetic pathway involving a benzaldehyde proton/hydride acceptor and the most promising catalyst.