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WIREs Comput Mol Sci
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External electric field effects on chemical structure and reactivity

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Abstract In recent years, external electric fields (EEFs) have captured some spotlight as novel effectors of chemical change. EEFs directly impact the structure of molecular systems. For example, aligning an electric field along a specific bond‐axis leads to either shortening or elongation of the bond (and ultimately bond breaking). Furthermore, EEFs enable unprecedented control over chemical reactivity. Orienting an electric field along the so‐called “reaction‐axis,” the direction in which the electrons reorganize during the conversion from reactant to product, leads to catalysis or inhibition and can induce mechanistic crossover from concerted to stepwise reactions. Off‐reaction‐axis orientation enables control over the stereoselectivity of reactions and disables forbidden–orbital mixing. Two‐directional fields enable control over both reactivity and selectivity. In this advanced review, we offer an overview of this rapidly evolving research field with a focus on the valence bond modeling of EEF effects and the insight it offers. A wide variety of examples will be considered and a link to the experiment will be made throughout. We end by offering some perspectives in which we postulate that, as experimental techniques continue to mature, EEFs could potentially become a generally applicable “zapping” tool to facilitate elaborate chemical syntheses. This article is categorized under: Structure and Mechanism > Reaction Mechanisms and Catalysis
VB structures contributing to the state‐wave function throughout the H‐exchange reaction. 1R and 2P are the covalent structures describing the H─H bonds in the reactants (R) and products (P), respectively. The remaining structures (38) are ionic‐ and CT‐structures, which mix into the wave function at a varying extent throughout the transformation. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). VB, valence bond
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(a) The effect of protonation/deprotonation on the BDE‐values of the O─CH3 bond in TEMPO derivatives; (b) stabilization of the nitroxyl radical caused by an enhancement of resonance structure II due to the electric field generated by the negative charge. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). BDE, bond‐dissociation enthalpy
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Plots of the weights (W) of the VB structures contributing to (a) the H─H and (b) the H─Cl bond‐wave function versus FZ. The weights have been calculated at VBSCF/6‐311G(d,p) level of theory. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). VB, valence bond
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Homonuclear bonds and their influence by the Z‐OEEF. For each bond we show from top‐to‐bottom: The bond lengthening (R in å), the corresponding range of FZ (in a.u., 1 a.u. = 51.4 V/å) responsible for this lengthening, the field value FZ(dis), that leads to dissociation, and the dissociation products. The calculations were performed at B3LYP/cc‐pVTZ level of theory. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). OEEF, oriented external electric field
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An orbital mixing diagram showing the mixing between the 2s and 2pz in the presence of a z‐directed OEEF, represented by the electric field operator FZ. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). OEEF, oriented external electric field
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A graphical depiction of the electric field conventions in Gaussian (right) and most other quantum chemistry packages (left). (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry)
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IEFs enable regioselectivity control of a carbene rearrangement catalyzed by a rhodium–porphyrin complex. The complex is linked to an electrode coated by an insulator that blocks faradic current flow. (Reprinted with permission from Reference . Copyright 2013 American Chemical Society). IEF, interfacial electric field
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Use of an EEF to catalyze a Diels–Alder reaction employing an STM tip and a gold surface to orient the reactants along the field vector of the electric field. (Reprinted with permission from Reference . Copyright 2016 Nature Publishing Group). EEF, external electric field; STM, scanning tunneling microscopy
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The energy barriers (in kcal/mol) as a function of the applied OEEF (in a.u.) along the x‐, y‐, and z‐axes for the Menschutkin reaction of pyridine and methyl iodide. The convention of FZ > 0 is shown in the inset. (Reprinted with permission from Reference . Copyright 2018 American Chemical Society). OEEF, oriented external electric field
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The relative stabilization (kcal/mol) compared to the reactant cluster (RC) of the H‐abstraction intermediate (IM1) formed during the dopamine formation under different conditions: (A) in the gas phase, (B) in the gas phase, but with the residue Asp 301 included in the model, (C) in the actual enzyme, and (D) the same as Model B, but then with the inclusion of the point charges of the protein residues within 3.0 å of the tyramine. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry)
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The energy profile (kcal/mol−1) of the mechanism of the tyramine to dopamine conversion selected by the CYP2D6 protein (blue). The red profile shows the H‐abstraction in the gas phase. (Reprinted with permission from Reference . Copyright 2011 American Chemical Society)
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Tyramine (left) and dopamine (right). (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry)
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(a) The VB diagram for a Menschutkin reaction between pyridine and methyl iodide. (b) Definitions of the reaction‐axis (labeled as Z), and of FZ > 0. (Reprinted with permission from Reference . Copyright 2018 American Chemical Society). VB, valence bond
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VB diagrams for a generic Diels–Alder reaction, leading to the prediction of Z‐OEEF‐induced catalysis/inhibition and mechanistic crossovers. (a) The field‐free situation in which the TS arises by the mixing of the reactant and the product states (Φ(R) and Φ(P)) and the CT state (ΨCT) along the concerted pathway. The extent to which the CT state mixes into the bold adiabatic curve (denoted in brown), depends on the donor–acceptor properties of the two reactants. The inset graphically shows the FZ < 0 that will stabilise ΨCT (based on the charges on this state in part (a)), leading to catalysis of the reaction. (b) At some critically large FZ < 0, one can expect ΨCT to descend well below the crossing point of the Φ(R) and Φ(P) curves, thus mediating a crossover to a stepwise mechanism with the formation of an intermediate (Izwitterionic). (Reprinted with permission from Reference . Copyright 2010 Wiley‐VCH). CT, charge‐transfer; OEEF, oriented external electric field; TS, transition state; VB, valence bond
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Generic VB diagrams, depicting the energy curves along the reaction coordinate connecting the reactants (R) to the products (P). (a) The covalent structures (Φcov, i.e., 1R and 2P) describe the covalent bonds of R and P. The mixing of these two VB structures leads to the VB state shown in the bold curve, in which ΨTS is the transition state. (b) The same VB diagram but with the ionic‐ and CT‐structures included (Φion/ΨCT); the downward arrow indicates the lowering of ΨTS due to the admixture of these structures. CT, charge‐transfer; VB, valence bond
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(a) A correlation between the applied electric field (FZ in 10−3 a.u.; 1 a.u. = 51.4 V/å) and the energy barriers (kcal/mol) for the endo‐ and exo‐cycloadditions. The numerical values on the graph are the calculated dipole moments (in Debye) of the TSs at the corresponding FZ values. The inset shows the conventions for FZ, μz, and QT. (b) The two C···C bond lengths (å) in asynchronous‐TS geometries at different field strengths. (Reprinted with permission from Reference . Copyright 2010 Wiley‐VCH). TS, transition state
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Illustration of the effect of FZ < 0 along the reaction‐axis for the H‐abstraction reaction of the nonheme oxoiron(IV) complex TMCS‐FeO+ in its two spin states. The upright 5TSH is stabilized by 4.7 kcal/mol more than the bent 3TSH, resulting in field‐induced spin‐state selectivity. (Reprinted with permission from Reference . Copyright 2016 Nature Publishing Group)
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An OEEF oriented along the z‐axis causes selective‐bond‐activation of propene by Compound I, such that either C─H hydroxylation or C═C epoxidation can be induced. (a) The epoxidation and hydroxylation pathways during functionalization by Compound I (the adopted FZ‐convention is depicted alongside Compound I). (b) Plots of the spin‐averaged transition state‐energy differences, E(TS1H) − E(TS1C) for hydroxylation versus epoxidation, as a function of the OEEF's strengths and directions along the three axes. From the graph, one can clearly conclude that only an OEEF along the z‐direction leads to significant effects; FZ < 0 favors the epoxidation reaction (E(TS1H) − E(TS1C) > 0), whereas FZ > 0 favors the hydroxylation reaction (E(TS1H) − E(TS1C) < 0). On the right‐hand side, the dipole moments for TS1H and TS1C are shown in absence of an electric field (black), FZ > 0 (red), and FZ < 0 (blue). One can straightforwardly conclude that the evolution of the dipole moments is in line with the calculated trends in the TS stabilization; μz is bigger for TS1C in the presence of a positive (FZ > 0) field, corresponding to a more pronounced stabilization of the TS (and thus a lower barrier), whereas the opposite is true for FZ < 0. (Reprinted with permission from Reference . Copyright 2016 Nature Publishing Group). OEEF, oriented external electric field; TS, transition state
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(a) The active species of P450, Compound I, with a Z‐OEEF applied along the S─Fe─O axis. (b) Distribution of the group‐spin‐densities in Compound I (for the quartet state) in the absence of an electric field (FZ = 0; black), with an applied FZ > 0 (red) and with an applied Fz < 0 (blue). The porphyrin ligand is represented as a circle around Fe. (Reprinted with permission from Reference . Copyright 2016 Nature Publishing Group). OEEF, oriented external electric field
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(a) Click reactions catalyzed by OEEFs oriented along the reaction‐axis. (b) Diels–Alder‐type reactions catalyzed by pH‐switchable D‐LEFs. (Reprinted with permission from Reference . Copyright 2017 The Royal Society of Chemistry). D‐LEF, designed local electric field; OEEF, oriented external electric field
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