Catalysis of Methyl Transfer Reactions by Oriented External Electric Fields: Are Gold–Thiolate Linkers Innocent?

Exploring the effect of external electric fields on the hydrogen bonding of a methanol dimer and fluorinated methanol dimers: a quantum chemical investigation

It is well established that external electric fields (EEFs) influence the H-bond network of liquid methanol and methanol clusters and induce proton transfer reactions. In this paper, a detailed ab initio study of the structural changes and energetics in the methanol dimer and its fluorine substituted dimers under the influence of EEFs of varying strengths is performed by employing density functional theory (DFT) and coupled-cluster with singles and doubles (CCSD) theory. The geometries are optimized using a wide variety of DFT functionals, spanning from the GGA (revPBE) functional to hybrid GGA (B3LYP, revPBE0) to meta-GGA (M06-2X) and double-hybrid functionals (B2PLYP). The structures and energy values from these calculations are benchmarked against the CCSD results. Furthermore, QTAIM calculation is performed on the methanol dimer to get insights into the nature of the H-bonding interactions. The H-bonds are observed to strengthen in the presence of EEFs, as demonstrated by other studies. It is shown that all DFT functionals overestimate the H-bond distance by 18-20%. This shows a strong dependence of the methanol dimer under the electric field at the DFT level. Furthermore, QTAIM predicts an increase in H-bonding energy under the influence of the electric field. The fluorine-substituted methanol dimers exhibit the same picture of structure and energetics. Overall, this study provides a deeper understanding of the H-bonding interaction under the electric field of methanol and its substituted systems.

PyCPET─Computing Heterogeneous 3D Protein Electric Fields and Their Dynamics

Electrostatic preorganization is an exciting mode to understand the catalytic function of enzymes, yet limited tools exist to computationally analyze it. In particular, no methods exist to interpret the geometry, dynamics, and fundamental components of 3D electric fields, E⃗(r), in protein active sites. To address this, we present PyCPET (Python Computation of Electric Field Topologies), a comprehensive, open-source toolbox to analyze E⃗(r) in enzymes. We designed it around computational efficiency and user friendliness with both CPU- and GPU-accelerated codes. Our aim is to provide a set of functions for rich, descriptive analysis of enzyme systems including dynamics, benchmarking, distribution of streamlines analysis in 3D E⃗(r), computation of point E⃗(r), principal component analysis, and 3D E⃗(r) visualization. Finally, we demonstrate its versatility by exploring the nature of electrostatic preorganization and dynamics in three cases: Cytochrome C, Co-substituted Liver Alcohol Dehydrogenase, and HIV Protease. These test systems, along with previous work, establish PyCPET as an essential toolkit for the in-depth analysis and visualization of electric fields in enzymes, unlocking new avenues for understanding electrostatic contributions to enzyme catalysis.

Harnessing Halide Ligands and External Electric Fields in Cobalt-Catalyzed Oxidative Cyclometalation: Mechanistic Insights and Reactivity Modulation

This study explores the roles of halide ligands and external electric fields (EEFs) in tuning the reactivity of cobalt-catalyzed oxidative cyclometalation (OCM) of 1,6-enynes, focusing on the concerted mechanism. Using density functional theory (DFT), we investigate how these factors influence key processes in the OCM step, particularly the cleavage of π bonds, the formation of M-C bonds, and the creation of a new C-C bond. Our findings show that polar solvents lower activation barriers, while halide ligands increase them, inhibiting the reaction by weakening π back-donation and reducing orbital overlap. However, strategic application of EEFs counteracts this inhibition, enhancing electron back-donation, stabilizing the transition state, and facilitating bond formation. The Dewar-Chatt-Duncanson (DCD) model, distortion/interaction analysis, and quantum theory of atoms in molecules (QTAIM) delocalization index (DI) calculation reveal that halide ligands reduce electron density on the cobalt center, weakening π-back-donation and raising energy barriers. This work provides key insights into how electronic and geometric factors can be manipulated to optimize the catalytic performance in cobalt-catalyzed synthetic transformations.

Unique catalytic role of intermolecular electric fields that emanate from Lewis acids in a ring closing carbonyl olefin metathesis reaction

Electric field (EF) catalysis has evolved as an effective tool for controlling reactivity and selectivity of reactions. While EF catalysis brings precise control over reactivity, it also challenges the concept's practical realization due to the difficulties in juxtaposing reactants with directional EF. The present density functional theory (DFT) studies demonstrate the catalytic role of the inherent intermolecular EFs that originate from Lewis acids (LA) during a ring-closing carbonyl-olefin metathesis (RCCOM) reaction. The specificity of LA coordination to reactants generates specifically oriented intermolecular EF components along the reaction axis which is defined parallel to the direction of flow of electrons wherein the influence of the EF would be at maximum. By examining the thermal [2+2] cycloaddition and carbonyl-ene reaction steps in a RCCOM reaction as model systems, the results revealed the pivotal role of intermolecular EF in mixing some of the dormant ionic structures into normal covalent structures and facilitating a partial rotation of the nonbonding orbitals at the carbonyl oxygen to enhance an ionic pseudo-pericyclic pathway. The unique role of intermolecular EF is further verified by modelling the pristine reaction, in the absence of LAs, under oriented external EFs. The conspicuous intermolecular EF component adds to other modes of catalysis, such as conventional Lewis acidity, to result in the gross catalytic effect. The findings offer insights into the practical realization of EF catalysis by harnessing the intermolecular EFs and point out the need to include intermolecular EF as an inevitable factor for a holistic explanation of any catalytic mechanism.

Piezoelectrostatic Catalysis of the Azide-Alkyne Huisgen Cycloaddition

Electric fields are increasingly recognized for their role as 'smart reagents' that can trigger or accelerate chemical reactions. Expanding upon this concept, our research introduces an innovative method that exploits electric fields induced by ultrasound on piezoelectric nanoparticles to facilitate the azide-alkyne Huisgen cycloaddition in nonaqueous environments. The intense electric field generated around the BaTiO3 nanoparticles, as supported by density functional theory calculations, provides the suitable conditions necessary to trigger the cycloaddition of the alkyne-functionalized nanoparticles and the azide present in the solution. To quantitatively assess the occurrence of the click cycloaddition reaction at the nanoparticle surface interface, we tacked the azide with either an electroactive ferrocene moiety or with gold nanoparticles, which act as surface Raman enhancers. These experiments not only provide experimental validation of our approach, but also highlights the potential of piezoelectrostatic catalysts in enhancing the scalability of electrostatic catalysis.

Utilization of Electric Fields to Modulate Molecular Activities on the Nanoscale: From Physical Properties to Chemical Reactions

As a primary energy source, electricity drives broad fields from everyday electronic circuits to industrial chemical catalysis. From a chemistry viewpoint, studying electric field effects on chemical reactivity is highly important for revealing the intrinsic mechanisms of molecular behaviors and mastering chemical reactions. Recently, manipulating the molecular activity using electric fields has emerged as a new research field. In addition, because integration of molecules into electronic devices has the natural complementary metal-oxide-semiconductor compatibility, electric field-driven molecular devices meet the requirements for both electronic device miniaturization and precise regulation of chemical reactions. This Review provides a timely and comprehensive overview of recent state-of-the-art advances, including theoretical models and prototype devices for electric field-based manipulation of molecular activities. First, we summarize the main approaches to providing electric fields for molecules. Then, we introduce several methods to measure their strengths in different systems quantitatively. Subsequently, we provide detailed discussions of electric field-regulated photophysics, electron transport, molecular movements, and chemical reactions. This review intends to provide a technical manual for precise molecular control in devices via electric fields. This could lead to development of new optoelectronic functions, more efficient logic processing units, more precise bond-selective control, new catalytic paradigms, and new chemical reactions.

Toward the Electrostatic Catalysis of Nucleophilic Substitutions: A Surface Chemistry Study of the Menshutkin Reaction

The catalysis of nonredox reactions by external electric fields is one of the most rapidly expanding areas of chemistry. The Menshutkin reaction, a classic example of bimolecular nucleophilic substitution (SN2), involves the conversion of a tertiary amine to a quaternary ammonium salt by coupling it with an alkyl halide. The reaction barrier of the Menshutkin reaction is theoretically predicted to be highly sensitive to the magnitude and direction of an external electric field experienced by the transition state. In this study, we investigate how near-surface electric fields can drive this prototypical nucleophilic substitution by examining the coupling of a diffusive redox-tagged tertiary amine with an electrode-tethered alkyl bromide under a variable external bias. Our findings reveal a competition between electrostatically assisted reactions, solvent effects, and electrochemically triggered side reactions involving radical intermediates. We estimate that only about 5% of the coupling events are attributable to the external field, while the majority of the reaction products originate from electrochemically generated radical intermediates.

Revisiting the Enhanced Chemical Reactivity in Water Microdroplets: The Case of a Diels-Alder Reaction

Often, chemical reactions are markedly accelerated in microdroplets compared with the corresponding bulk phase. While identifying the precise causative factors remains challenging, the interfacial electric field (IEF) and partial solvation are the two widely proposed factors, accounting for the acceleration or turning on of many reactions in microdroplets. In sharp contrast, this combined computational and experimental study demonstrates that these two critical factors have a negligible effect on promoting a model Diels-Alder (DA) reaction between cyclopentadiene and acrylonitrile in water microdroplets. Instead, the acceleration of the DA reaction appears to be driven by the effect of confinement and the concentration increase caused by evaporation. Quantum chemical calculations and ab initio molecular dynamics simulations coupled with enhanced sampling techniques predict that the air-water interface exhibits a higher free-energy barrier of this reaction than the bulk, while external electric fields marginally reduce the barrier. Remarkably, the catalytic capability of the IEF at the water microdroplet surface is largely hampered by its fluctuating character. Mass spectrometric assessment of the microdroplet reaction corroborates these findings, suggesting that the DA reaction is not facilitated by the IEF as increasing the spray potential suppresses the DA products by promoting substrate oxidation. While the DA reaction exhibits a surface preference in water microdroplets, the same reaction tends to occur mainly within the core of the acetonitrile microdroplet, suggesting that the partial solvation is not necessarily a critical factor for accelerating this reaction in microdroplets. Moreover, experiments indicate that the rapid evaporation of microdroplets and subsequent reagent enrichment within the accessible confined volume of microdroplets caused the observed acceleration of the DA reaction in water microdroplets.

Evaluating the Ability of External Electric Fields to Accelerate Reactions in Solution

This study investigates the catalytic effects of external electric fields (EEFs) on two reactions in solution: the Menshutkin reaction and the Chapman rearrangement. Utilizing a scanning tunneling microscope-based break-junction (STM-BJ) setup and monitoring reaction rates through high-performance liquid chromatography connected to a UV detector (HPLC-UV) and ultraperformance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-q-ToF-MS), we observed no rate enhancement for either reaction under ambient conditions. Density functional theory (DFT) calculations indicate that electric field-induced changes in reactant orientation and the minimization of activation energy are crucial factors in determining the efficacy of EEF-driven catalysis. Our findings suggest that the current experimental setups and field strengths are insufficient to catalyze these reactions, underscoring the importance of these criteria in assessing the reaction candidates.

Massive acceleration of SN2 reaction using the oriented external electric field

Nucleophilic substitution is one of the most fundamental chemical reactions, and the pursuit of high reaction rates of the reaction is one of the ultimate goals in catalytic and organic chemistry. The reaction barrier of the nucleophilic substitution originates from the highly polar nature of the transition state that can be stabilized under the electric field created by the solvent environment. However, the intensity of the induced solvent-electric field is relatively small due to the random orientation of solvent molecules, which hinders the catalytic effects and restricts the reaction rates. This work shows that oriented external electric fields applied within a confined nanogap between two nanoscopic tips could accelerate the Menshutkin reaction by more than four orders of magnitude (over 39 000 times). The theoretical calculations reveal that the electric field inside the nanogap reduces the energy barrier to increase the reaction rate. Our work suggests the great potential of electrostatic catalysis for green synthesis in the future.

My Vision of Electric-Field-Aided Chemistry in 2050

This manuscript outlines my outlook on the development of electric-field (EF)-mediated-chemistry and the vision of its state by 2050. I discuss applications of oriented-external electric-fields (OEEFs) on chemical reactions and proceed with relevant experimental verifications. Subsequently, the Perspective outlines other ways of generating EFs, e.g., by use of pH-switchable charges, ionic additives, water droplets, and so on. A special section summarizes conceptual principles for understanding and predicting OEEF effects, e.g., the "reaction-axis rule", the capability of OEEFs to act as tweezers that orient reactants and accelerate their reaction, etc. Finally, I discuss applications of OEEFs in continuous-flow setups, which may, in principle, scale-up to molar concentrations. The Perspective ends with the vision that by 2050, OEEF usage will change chemical education, if not also the art of making new molecules.

Ligand effect on Ru-centered species toward methane activation

Ligands have been known to profoundly affect the chemical transformations of methane, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we demonstrate that the conversion of methane can be regulated by Ru centered cations with a series of ligands (C, CH, CNH, CHCNH). Gas-phase experiments complemented by theoretical dynamic analysis were performed to explore the essences and principles governing the ligand effect. In contrast to the inert Ru+, [RuC]+, and [RuCNH]+ toward CH4, the dehydrogenation dominates the reaction of ligand-regulated systems [RuCH]+/CH4 and [RuCHCNH]+/CH4. In active cases, CH acts as active sites, and regulates the activation of CH4 assisted by the "seemingly inert" CNH ligand.

Optimal Oriented External Electric Fields to Trigger a Barrierless Oxaphosphetane Ring Opening Step of the Wittig Reaction

The Wittig reaction is one of the most important processes in organic chemistry for the asymmetric synthesis of olefinic compounds. In view of the increasingly acknowledged potentiality of the electric fields in promoting reactions, here we will consider the effect of the oriented external electric field (OEEF) on the second step of Wittig reaction (i. e. the ring opening oxaphosphetane) in a model system for non-stabilized ylides. In particular, we have determined the optimal direction and strength of the electric field that should be applied to annihilate the reaction barrier of the ring opening through the polarizable molecular electric dipole (PMED) model that we have recently developed. We conclude that the application of the optimal external electric field for the oxaphosphetane ring opening favours a Bestmann-like mechanism.

Tuning optical properties of π-conjugated double nanohoops under external electric field stimuli-responsiveness

Carbon nanorings have attracted substantial interest from synthetic chemists due to their unique topological structures and distinct physical properties. An intriguing π-conjugated double-nanoring structure, denoted as [8]CPP-[10]cyclacene, was constructed via the integration of [8]cycloparaphenylene ([8]CPP) into [10]cyclacene. Using the external electric field stimuli-responsiveness of [8]CPP-[10]cyclacene, directional charge transfer can be induced, resulting in the emergence of intriguing properties. The effects of the external electric field in three specific directions were explored, vertically in the [8]CPP unit (Fy), vertically in the [10]cyclacene unit (Fz), and horizontally along the double nanorings diameter (Fx). Interestingly, the external electric field vertically to the [10]cyclacene unit significantly enhanced the first hyperpolarizability (βtot) compared to that vertically to the [8]CPP unit. Notably, [8]CPP-[10]cyclacene under Fx exhibited significantly larger the βtot values (1.48 × 105 a.u.) than those of vertical Fy and Fz. This work opens up a wide range of nonlinear optics, making it a compelling area to explore in the field of carbon nanomaterials.

Electrostatic catalysis of a click reaction in a microfluidic cell

Electric fields have been highlighted as a smart reagent in nature's enzymatic machinery, as they can directly trigger or accelerate chemical processes with stereo- and regio-specificity. In enzymatic catalysis, controlled mass transport of chemical species is also key in facilitating the availability of reactants in the active reaction site. However, recent progress in developing a clean catalysis that profits from oriented electric fields is limited to theoretical and experimental studies at the single molecule level, where both the control over mass transport and scalability cannot be tested. Here, we quantify the electrostatic catalysis of a prototypical Huisgen cycloaddition in a large-area electrode surface and directly compare its performance to the conventional Cu(I) catalysis. Our custom-built microfluidic cell enhances reagent transport towards the electrified reactive interface. This continuous-flow microfluidic electrostatic reactor is an example of an electric-field driven platform where clean large-scale electrostatic catalytic processes can be efficiently implemented and regulated.

Harnessing the High Interfacial Electric Fields on Water Microdroplets to Accelerate Menshutkin Reactions

Even though it is still an emerging field, the application of a high external electric field (EEF) as a green and efficient catalyst in synthetic chemistry has recently received significant attention for the ability to deliver remarkable control of reaction selectivity and acceleration of reaction rates. Here, we extend the application of the EEF to Menshutkin reactions by taking advantage of the spontaneous high electric field at the air-water interfaces of sprayed water microdroplets. Experimentally, a series of Menshutkin reactions were accelerated by 7 orders of magnitude. Theoretically, both density functional theory calculations and ab initio molecular dynamics simulations predict that the reaction barrier decreases significantly in the presence of oriented external electric fields, thereby supporting the notion that the electric fields in the water droplets are responsible for the catalysis. In addition, the ordered solvent and reactant molecules oriented by the electric field alleviate the steric effect of solvents and increase the successful collision rates, thus facilitating faster nucleophilic attack. The success of Menshutkin reactions in this study showcases the great potential of microdroplet chemistry for green synthesis.

An algorithm to find the optimal oriented external electrostatic field for annihilating a reaction barrier in a polarizable molecular system

The use of oriented external electric fields (OEEFs) to promote and control chemical reactivity has motivated many theoretical and computational studies in the last decade to model the action of OEEFs on a molecular system and its effects on chemical processes. Given a reaction, a central goal in this research area is to predict the optimal OEEF (oOEEF) required to annihilate the reaction energy barrier with the smallest possible field strength. Here, we present a model rooted in catastrophe and optimum control theories that allows us to find the oOEEF for a given reaction valley in the potential energy surface (PES). In this model, the effective (or perturbed) PES of a polarizable molecular system is constructed by adding to the original, non-perturbed, PES a term accounting for the interaction of the OEEF with the intrinsic electric dipole and polarizability of the molecular system, so called the polarizable molecular electric dipole (PMED) model. We demonstrate that the oOEEF can be established by locating a point in the original PES with unique topological properties: the optimal barrier breakdown or bond-breaking point (oBBP). The essential feature of the oBBP structure is the fact that this point maintains its topological properties for all the applied OEEFs, also for the unperturbed PES, thus becoming much more relevant than the commonly used minima and transition state structures. The PMED model proposed here has been implemented in an open access package and is shown to successfully predict the oOEEF for two processes: an isomerization reaction of a cumulene derivative and the Huisgen cycloaddition reaction.

Two-Way Catalysis in a Diels-Alder Reaction Limits Inhibition Induced by an External Electric Field

Oriented external electric fields (EEFs) act as catalysts that can induce selectivity in chemical reactions. The responses of the Diels-Alder (DA) reaction between butadiene and ethylene (BDE-DA) as well as cyclopentadiene and ethylene (CPDE-DA) towards EEF stimuli are investigated here using density functional theory (B3LYP) calculations. EEF is a vector that catalyzes the reaction in one direction while inhibiting it in the opposite direction. Here we report that the inhibitive direction becomes rate-enhancing after some increase in the EEF. The EEF value that brings about the maximum possible inhibition for the reaction is defined as the electrostatic resistance point (ERP). The possibility of both normal and inverse electron-demand DA reactions causes catalytic activity in both directions of the EEF starting at a unique ERP value. The C5 substituents of cyclopentadiene control the ERP values depending upon the resistance power that the functional group provides against the EEF. The endo and exo diastereomeric transition states of the DA reaction have distinct ERP values and the difference (ΔERP) provides the through-space electrostatic contribution to the stereoselectivity on a relative scale. Thus, the ERP values can be used as a gauge for the electrostatic interactions between substituent groups and external stimuli.

Research on Electric Field-Induced Catalysis Using Single-Molecule Electrical Measurement

The role of catalysis in controlling chemical reactions is crucial. As an important external stimulus regulatory tool, electric field (EF) catalysis enables further possibilities for chemical reaction regulation. To date, the regulation mechanism of electric fields and electrons on chemical reactions has been modeled. The electric field at the single-molecule electronic scale provides a powerful theoretical weapon to explore the dynamics of individual chemical reactions. The combination of electric fields and single-molecule electronic techniques not only uncovers new principles but also results in the regulation of chemical reactions at the single-molecule scale. This perspective focuses on the recent electric field-catalyzed, single-molecule chemical reactions and assembly, and highlights promising outlooks for future work in single-molecule catalysis.

Voltage-driven control of single-molecule keto-enol equilibrium in a two-terminal junction system

Keto-enol tautomerism, describing an equilibrium involving two tautomers with distinctive structures, provides a promising platform for modulating nanoscale charge transport. However, such equilibria are generally dominated by the keto form, while a high isomerization barrier limits the transformation to the enol form, suggesting a considerable challenge to control the tautomerism. Here, we achieve single-molecule control of a keto-enol equilibrium at room temperature by using a strategy that combines redox control and electric field modulation. Based on the control of charge injection in the single-molecule junction, we could access charged potential energy surfaces with opposite thermodynamic driving forces, i.e., exhibiting a preference for the conducting enol form, while the isomerization barrier is also significantly reduced. Thus, we could selectively obtain desired and stable tautomers, which leads to significant modulation of the single-molecule conductance. This work highlights the concept of single-molecule control of chemical reactions on more than one potential energy surface.

Quantum Tunneling in Reactions Modulated by External Electric Fields: Reactivity and Selectivity

Quantum tunneling and external electric fields (EEFs) can promote some reactions. However, the synergetic effect of an EEF on a tunneling-involving reaction and its temperature-dependence is not very clear. In this study, we extensively investigated how EEFs affect three reactions that involve hydrogen- or (ground- and excited-state) carbon-tunneling using reliable DFT, DLPNO-CCSD(T1), and variational transition-state theory methods. Our study revealed that oriented EEFs can significantly reduce the barrier and corresponding barrier width (and vice versa) through more electrostatic stabilization in transition states. These EEF effects enhance the nontunneling and tunneling-involving rates. Such EEF effects also decrease the crossover temperatures and quantum tunneling contribution, albeit with lower and thinner barriers. Moreover, EEFs can modulate and switch on/off the tunneling-driven 1,2-H migration of hydroxycarbenes under cryogenic conditions. Furthermore, our study predicts for the first time that EEF/tunneling synergy can control the chemo- or site-selectivity of one molecule bearing two similar/same reactive sites.

Protonation state control of electric field induced molecular switching mechanisms

Scanning tunneling microscopy tip-induced deprotonation has been demonstrated experimentally and can be used as an additional control mechanism in electric-field induced molecular switching. The goal of the current work is to establish whether (de)protonation can be used to inhibit or enhance the electric field controlled thermal and photoisomerization processes. Dihydroxyazobenzene is used as a model system, where protonation/deprotonation of the free hydroxyl moiety changes the azo bond order, and so modifies the rate of electric field induced isomerization. Through the combined action of deprotonation and applied field, it was found that the cis-to-trans thermal isomerization barrier could be completely removed, changing the isomerization half-life from the order of several months. In addition, due to the presence of multiple isomerization mechanisms, electric fields could modify the isomerization kinetics by increasing the number of energetically viable isomerization pathways, rather than reducing the activation barrier of the lowest energy pathway. Excited state calculations indicated that the protonation state and electric field could be used together to control the presence of electronic degeneracies along the rotation pathway between S₀/S₁, and along all three pathways between S₁/S₂. This work provides insight into the mechanisms that enable the use of protonation state, light, and electric fields in concert to control molecular switches.

A catastrophe theory-based model for optimal control of chemical reactions by means of oriented electric fields

The effect of oriented external electric fields (OEEF) on chemical reactivity has been studied theoretically and computationally in the last decades. A central goal in this research area is to predict the orientation and the smallest amplitude electric field that renders a barrierless chemical process with the smallest possible strength. Recently, a model to find the optimal electric field has been proposed and described (Bofill JM et al., J. Chem. Theory Comput. 18:935, 2022). We here proof that this model is based on catastrophe and optimum control theories. Based on both theories a technical treatment of the model is given and applied to a two-dimensional generic example that provides insight into its nature and capability. Finally, the model is applied to determine the optimal OEEF for the trans-to-cis isomerization of a [3]cumulene derivative.

Effects and Influence of External Electric Fields on the Equilibrium Properties of Tautomeric Molecules

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications.

Chloride-Mediated Alkene Activation Drives Enantioselective Thiourea and Hydrogen Chloride Co-Catalyzed Prins Cyclizations

The mechanism of chiral hydrogen-bond donor (HBD) and hydrogen chloride (HCl) co-catalyzed Prins cyclizations was analyzed through a combination of experimental and computational methods and revealed to involve an unexpected and previously unrecognized mode of alkene activation. Kinetic and spectroscopic studies support the participation of a catalytically active HCl·HBD complex that displays reduced Brønsted acidity relative to HCl alone. Nevertheless, rate acceleration relative to the HCl-catalyzed background reaction as well as high levels of enantioselectivity are achieved. This inverse Brønsted correlation is ascribed to chloride-mediated substrate activation in the rate-limiting and enantiodetermining cyclization transition state. Density functional theory (DFT) calculations, distortion-interaction analysis, and quasiclassical dynamics simulations support a stepwise mechanism in which rate acceleration and enantioselectivity are achieved through the precise positioning of the chloride anion within the active site of the chiral thiourea to enhance the nucleophilicity of the alkene and provide transition-state stabilization through local electric field effects. This mode of selective catalysis through anion positioning likely has general implications for the design of enantioselective Brønsted acid-catalyzed reactions involving π-nucleophiles.

The High-Effective Catalytic Degradation of Benzo[a]pyrene by Mn-Corrolazine Regulated by Oriented External Electric Field: Insight From DFT Study

The degradation of BaP into hydroxybenzo[a]pyrene by Mn-corrolazine and its regulation by an oriented external electronic field (OEEF) were systematically studied using first-principle calculations. Extensive density function calculations showed that the degradation of BaP into hydroxybenzo[a]pyrene by Mn-corrolazine occurs via a three-step process in the absence of OEEF, in which a more toxic and stable epoxide intermediate is generated. However, upon application of OEEF along the intrinsic Mn-O reaction axis, the degradation of BaP into hydroxybenzo[a]pyrene is greatly simplified. The negative charge on the terminal O atom of Mn-OO corrolazine increases with an increase in the OEEF intensity. As the intensity of the OEEF increases over 0.004 a.u., the negatively charged terminal O atom has the ability to directly abstract the positively charged H atom of BaP and the degradation of BaP into hydroxybenzo[a]pyrene can be completed via a one-step process, avoiding the production of more toxic epoxide intermediates.

Influence of an Oriented External Electric Field on the Mechanism of Double Proton Transfer between Pyrazole and Guanidine: from an Asynchronous Plateau Transition State to a Synchronous or Stepwise Mechanism

The double proton transfer (DPT) reaction between pyrazole and guanidine, a concerted reaction but strongly asynchronous and presenting a "plateau transition region", has been theoretically reinvestigated in the presence of an external uniform electric field. First, we computed the reaction path by DFT and proposed a very detailed description of the constitutive electronic events, based on the ELF topology and the bond evolution theory. Then, we studied the effect of an oriented external electric field (OEEF) on the reaction mechanism, for an OEEF oriented along the proton transfer axis. We observe that in one direction, the DPT reaction can be transformed into a stepwise reaction, going through a stabilized single proton transferred intermediate. Contrarily, the two proton transfers occur simultaneously when the electric field is applied in the opposite direction. In the latter case, the order in which the two protons are transferred in the same elementary step can even be reversed if the OEEF is intense enough. Finally, it has been shown that the evolution of the double proton transfer reaction in the presence of an electric field could be quantitatively anticipated by analyzing the ELF value at the bifurcation point between V(A, H) proton donor and V(B) proton acceptor of the double hydrogen bonded complex in the entrance channel.

Green Catalytic Method for Hydrothiolation of Allylamines: An External Electric Field

Based on the idea of environmental friendliness, we first studied the hydrothiolation reactions of thiophenol with allylamine using a green catalyst-an external electric field (EEF). The hydrothiolation reactions could occur through Markovnikov addition (path M) and anti-Markovnikov addition (path AM) pathways. The calculation results demonstrated that when the EEF was oriented along F -X , F -Y , and F +Z directions, path M was accelerated. However, it is favorable for path AM only when the EEF is oriented along the +X and -Y-axes. In addition, the introduction of the EEF further increased and lowered the differences of the reaction barrier as the EEF was oriented along F -X , F -Y , and F +X directions. The solvent effects were also considered in this work. Hopefully, this unprecedented and green catalytic method for the hydrothiolation reactions of allylamine may provide guidance in the lab.

Controlling Chemical Reactivity with Optimally Oriented Electric Fields: A Generalization of the Newton Trajectory Method

The use of oriented external electric fields (OEEF) as a tool to accelerate chemical reactions has recently attracted much interest. A new model to calculate the optimal OEEF of the least intensity to induce a barrierless chemical reaction path is presented. A suitable ansatz is provided by defining an effective potential energy surface (PES), which considers the unperturbed or original PES of the molecular reactive system and the action of a constant OEEF on the overall dipole moment of system. Based on a generalization of the Newton Trajectories (NT) method, it is demonstrated that the optimal OEEF can be determined upon locating a special point of the potential energy surface (PES), the so-called "optimal bond-breaking point" (optimal BBP), for which two different algorithms are proposed. At this point, the gradient of the original or unperturbed PES is an eigenvector of zero eigenvalue of the Hessian matrix of the effective PES. A thorough discussion of the geometrical aspects of the optimal BBP and the optimal OEEF is provided using a two-dimensional model, and numerical calculations of the optimal OEEF for a S_N2 reaction and the 1,3-dipolar retrocycloaddition of isoxazole to fulminic acid plus acetylene reaction serve as a proof of concept. The knowledge of the orientation of optimal OEEF provides a practical way to reduce the effective barrier of a given chemical process.

Hydrogen and Halogen Bonding in Homogeneous External Electric Fields: Modulating the Bond Strengths

Recent years have witnessed various fascinating phenomena arising from the interactions of noncovalent bonds with homogeneous external electric fields (EEFs). Here we performed a computational study to interpret the sensitivity of intrinsic bond strengths to EEFs in terms of steric effect and orbital interactions. The block-localized wavefunction (BLW) method, which combines the advantages of both ab initio valence bond (VB) theory and molecular orbital (MO) theory, and the subsequent energy decomposition (BLW-ED) approach were adopted. The sensitivity was monitored and analyzed using the induced energy term, which is the variation in each energy component along the EEF strength. Systems with single or multiple hydrogen (H) or halogen (X) bond(s) were also examined. It was found that the X-bond strength change to EEFs mainly stems from the covalency change, while generally the steric effect rules the response of H-bonds to EEFs. Furthermore, X-bonds are more sensitive to EEFs, with the key difference between H- and X-bonds lying in the charge transfer interaction. Since phenylboronic acid has been experimentally used as a smart linker in EEFs, switchable sensitivity was scrutinized with the example of the phenylboronic acid dimer, which exhibits two conformations with either antiparallel or parallel H-bonds, thereby, opposite or consistent responses to EEFs. Among the studied systems, the quadruple X-bonds in molecular capsules exhibit remarkable sensitivity, with its interaction energy increased by -95.2 kJ mol^-1 at the EEF strength 0.005 a.u.

Interaction Regulation Between Ionomer Binder and Catalyst: Active Triple-Phase Boundary and High Performance Catalyst Layer for Anion Exchange Membrane Fuel Cells

As one of the most crucial components, the catalyst layer (CL) plays a critical role in the performance of anion exchange membrane fuel cells (AEMFCs). However, the effect of the structural evolution of ionomer binder on the micromorphology and catalytic activity of CL is yet to be clarified. In this study, pyrrolidinum and quaternary ammonium cations are attached to the polyphenylene oxide (PPO) backbone through flexible spacer units (five, seven, or nine carbon atoms) with different terminal alkyl groups. The Van der Waals force and electrostatic repulsion between the ionomer binder and catalyst are regulated through the flexible spacer units and terminal alkyl groups to alleviate the agglomeration of catalyst particles and acquire a high catalytic activity. To evaluate the electrochemical stability of the cationic groups, the alkaline stability of the ionomer binder is tested under a constant voltage to simulate the true operational environment of the fuel cells. The results reveal that the degradation of the cation groups of ionomer binder is accelerated under a constant voltage condition. This phenomenon in neglect earlier, may serve as a useful reference for the synthesis and performance enhancement of ionomer binders.

The SN2 reaction and its relationship with the Walden inversion, the Finkelstein and Menshutkin reactions together with theoretical calculations for the Finkelstein reaction

This communication gives an overview of the relationships between four reactions that although related were not always perceived as such: SN2, Walden, Finkelstein, and Menshutkin. Binary interactions (SN2 & Walden, SN2 & Menshutkin, SN2 & Finkelstein, Walden & Menshutkin, Walden & Finkelstein, Menshutkin & Finkelstein) were reported. Carbon, silicon, nitrogen, and phosphorus as central atoms and fluorides, chlorides, bromides, and iodides as lateral atoms were considered. Theoretical calculations provide Gibbs free energies that were analyzed with linear models to obtain the halide contributions. The M06-2x DFT computational method and the 6-311++G(d,p) basis set have been used for all atoms except for iodine where the effective core potential def2-TZVP basis set was used. Concerning the central atom pairs, carbon/silicon vs. nitrogen/phosphorus, we reported here for the first time that the effect of valence expansion was known for Si but not for P. Concerning the lateral halogen atoms, some empirical models including the interaction between F and I as entering and leaving groups explain the Gibbs free energies.

Water Splitting Induced by Visible Light at a Copper-Based Single-Molecule Junction

Water splitting is an essential process for converting light energy into easily storable energy in the form of hydrogen. As environmentally preferable catalysts, Cu-based materials have attracted attention as water-splitting catalysts. To enhance the efficiency of water splitting, a reaction process should be developed. Single-molecule junctions (SMJs) are attractive structures for developing these reactions because the molecule electronic state is significantly modulated, and characteristic electromagnetic effects can be expected. Here, water splitting is induced at Cu-based SMJ and the produced hydrogen is characterized at a single-molecule scale by employing electron transport measurements. After visible light irradiation, the conductance states originate from Cu/hydrogen molecule/Cu junctions, while before irradiation, only Cu/water molecule/Cu junctions were observed. The vibration spectra obtained from inelastic electron tunneling spectroscopy combined with the first-principles calculations reveal that the water molecule trapped between the Cu electrodes is decomposed and that hydrogen is produced. Time-dependent and wavelength-dependent measurements show that localized-surface plasmon decomposes the water molecule in the vicinity of the junction. These findings indicate the potential ability of Cu-based materials for photocatalysis.

Electron-Catalyzed Dehydrogenation in a Single-Molecule Junction

Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.

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.

External electric field: a new catalytic strategy for the anti-Markovnikov hydrohydrazination of parent hydrazine

The anti-Markovnikov hydroamination reaction is considered to be a particular challenge, and one of the reactants, parent hydrazine, is also regarded as a troubling reagent. In this study, we first studied the hydrohydrazination of parent hydrazine via an effective and green catalyst—external electric field (EEF). The calculation results demonstrated that the anti-Markovnikov and Markovnikov pathways are competitive when there was no catalyst. EEF oriented along the negative direction of the X axis (F_x) accelerated the anti-Markovnikov addition reaction. Moreover, it lowered the barrier height of the first step by 16.0 kcal mol⁻¹ (from 27.8 to 11.8 kcal mol⁻¹) when the field strength was 180 (×10⁻⁴) au. Under the same conditions, the Markovnikov reaction pathway was inhibited, which means that EEF achieved the specificity of hydrohydrazination. The solvents are favorable for the first step addition reaction, particularly the synergy between solvents and F_x lowered the barrier heights by 8.3 (C₆H₆) and 10.7 (DMSO) kcal mol⁻¹ for an F_x = −60 (×10⁻⁴) au. Besides, the introduction of the electron-withdrawing substituent (trifluoromethyl) is also a good strategy to catalyze hydrohydrazination, while the electron-donating group (methoxy) is unfavorable.

The anti-Markovnikov hydrohydrazination of parent hydrazine were catalyzed by external electric field (EEF) to a large extent. Furthermore, the solvent effects and the substituent effects of the hydrohydrazination were enhanced in the presence of EEF.

Selective Fabrication of Single-Molecule Junctions by Interface Engineering

Recent progress in addressing electrically driven single-molecule behaviors has opened up a path toward the controllable fabrication of molecular devices. Herein, the selective fabrication of single-molecule junctions is achieved by employing the external electric field. For molecular junctions with methylthio (-SMe), thioacetate (-SAc), amine (-NH₂ ), and pyridyl (-PY), the evolution of their formation probabilities along with the electric field is extracted from the plateau analysis of individual single-molecule break junction traces. With the increase of the electric field, the SMe-anchored molecules show a different trend in the formation probability compared to the other molecular junctions, which is consistent with the density functional theory calculations. Furthermore, switching from an SMe-anchored junction to an SAc-anchored junction is realized by altering the electric field in a mixed solution. The results in this work provide a new approach to the controllable fabrication and modulation of single-molecule junctions and other bottom-up nanodevices at molecular scales.

Solvent Organization and Rate Regulation of a Menshutkin Reaction by Oriented External Electric Fields are Revealed by Combined MD and QM/MM Calculations

When and how do external electric fields (EEFs) lead to catalysis in the presence of a (polar or nonpolar) solvent? This is the question that is addressed here using a combination of molecular dynamics (MD) simulations, quantum mechanical/molecular mechanical calculations with EEF, and quantum mechanical/(local) electric field calculations. The paper focuses on a model reaction, the Menshutkin reaction between CH3I and pyridine in three solvents of varying polarity. Using MD simulations, we find that the EEF causes the solvent to undergo organization; the solvent molecules gradually align with the applied field as the field strength increases. The collective orientation of the solvent molecules modifies the electrostatic environment around the Menshutkin species and induces a global electric field pointing in the opposite direction of the applied EEF. The combination of these two entangled effects leads to partial or complete screening of the EEF, with the extent of screening being proportional to the polarity/polarizability of the solvent. Nevertheless, we find that catalysis of the Menshutkin reaction inevitably emerges once the EEF exceeds the opposing field of the organizing solvent, i.e., once polarization of the Menshutkin complex is observed to set in. Overall, our analysis provides a lucid and pictorial interpretation of the behavior of solutions in the presence of EEFs and indicates that EEF-mediated catalysis should, in principle, be feasible in bulk setups, especially for nonpolar and mildly polar solvents. By application of the charge-transfer paradigm, it is shown that the emergence of OEEF catalysis in solution can be generalized to other reactions as well.

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.

Globally optimal catalytic fields for a Diels-Alder reaction

In a previous paper [M. Dittner and B. Hartke, J. Chem. Theory Comput. 14, 3547 (2018)], we introduced a preliminary version of our GOCAT (globally optimal catalyst) concept in which electrostatic catalysts are designed for arbitrary reactions by global optimization of distributed point charges that surround the reaction. In this first version, a pre-defined reaction path was kept fixed. This unrealistic assumption allowed for only small catalytic effects. In the present work, we extend our GOCAT framework by a sophisticated and robust on-the-fly reaction path optimization, plus further concomitant algorithm adaptions. This allows smaller and larger excursions from a pre-defined reaction path under the influence of the GOCAT point-charge surrounding, all the way to drastic mechanistic changes. In contrast to the restricted first GOCAT version, this new version is able to address real-life catalysis. We demonstrate this by applying it to the electrostatic catalysis of a prototypical Diels-Alder reaction. Without using any prior information, this procedure re-discovers theoretically and experimentally established features of electrostatic catalysis of this very reaction, including a field-dependent transition from the synchronous, concerted textbook mechanism to a zwitterionic two-step mechanism, and diastereomeric discrimination by suitable electric field components.

The directions of an external electric field control the catalysis of the hydroboration of C-O unsaturated compounds

The orientation directions of an external electric field (EEF) in catalyzing chemical reactions are an important factor because they can significantly accelerate reaction activity. In this study, we explored a new anti-Markovnikov hydroboration reaction of C-O unsaturated compounds (e.g., benzaldehyde and benzophenone) with the aim of revealing the dominant direction of EEF in accelerating the reactions, and pinacolborane (HBpin) was selected as an efficient reductant. The calculation results showed that the EEF oriented along the direction of electron pair transform rather than that of the molecular dipole moment could reduce the barrier of the hydroboration of benzaldehyde by 20 kcal mol-1 when the EEF was up to 150 × 10-4 au. Moreover, the Markovnikov hydroboration of aldehyde and ketone was investigated for obtaining the mechanistic-switchover point. Unsatisfactorily, the EEF could just influence the respective barriers without a promising competition with the anti-Markovnikov hydroboration reactions.

Plasmonic Nanocavities Enable Self-Induced Electrostatic Catalysis

The potential of strong interactions between light and matter remains to be further explored within a chemical context. Towards this end herein we study the electromagnetic interaction between molecules and plasmonic nanocavities. By means of electronic structure calculations, we show that self-induced catalysis emerges without any external stimuli through the interaction of the molecular permanent and fluctuating dipole moments with the plasmonic cavity modes. We also exploit this scheme to modify the transition temperature T1/2 of spin-crossover complexes as an example of how strong light-matter interactions can ultimately be used to control a materials responses.

Electrophilic Aromatic Substitution Reactions: Mechanistic Landscape, Electrostatic and Electric-Field Control of Reaction Rates, and Mechanistic Crossovers

This study investigates the rich mechanistic landscape of the iconic electrophilic aromatic substitution (EAS) reaction class, in the gas phase, in solvents, and under stimulation by oriented external electric fields. The study uses DFT calculations, complemented by a qualitative valence bond (VB) perspective. We construct a comprehensive and unifying framework that elucidates the many surprising mechanistic features, uncovered in recent years, of this class of reactions. For example, one of the puzzling issues which have attracted significant interest recently is the finding of a variety of concerted mechanisms that do not involve the formation of σ-complex intermediates, in apparent contradiction to the generally accepted textbook mechanism. Our VB modeling elucidates the existence of both the concerted and stepwise mechanisms and uncovers the root causes and necessary conditions for the appearance of these intermediates. Furthermore, our VB analysis offers insight into the potential applications of external electric fields as smart, green, and selective catalysts, which can control at will reaction rates, as well as mechanistic crossovers, for this class of reactions. Finally, we highlight how understanding of the electric fields effect on the EAS reaction could lead to the formulation of guiding principles for the design of improved heterogeneous catalysts. Overall, our analysis underscores the powerful synergy offered by combining molecular orbital and VB theory to tackle interesting and challenging mechanistic questions in chemistry.

Exchange-dependent spin polarized transport and phase transition in a triple monomer molecule

Molecular junctions contribute significantly to the fundamental understanding of the quantum information technologies in molecular spintronics. In this paper, with the aid of the state of the art numerical renormalization group method, we find a triple monomer molecule structure with strong electron-electron interactions could be a potential candidate for a multifunctional spin polarizer when an external magnetic field along the z axis is applied. It is demonstrated that the polarizing scenarios depend closely on the inter-orbital exchange couplings, and results in several kinds of spin polarizers, e.g., the unidirectional, the bidirectional, the dual, and the ternary spin polarizers. We show in detail the related phase diagram, and conclude the Zeeman effect and the charge switching for the bonding, anti-bonding and non-bonding orbitals are responsible for the spin polarizing transport. We stress even when the energy levels are chosen beyond the Kondo regime, the structure still shows a promising platform for molecular spintronics components.