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".

Oriented external electric fields (OEEFs) as 'smart reagents' are no longer a theoretical dream. Here, we discuss the wide-ranging potential of using OEEFs to catalyse and control a variety of non-redox reactions and impart selectivity at will. An OEEF along the direction of electron reorganization (the so-called reaction axis) will catalyse nonpolar reactions by orders of magnitude, control regioselectivity and induce spin-state selectivity. Simply flipping the direction of the OEEF or orienting it off of the reaction axis, will control at will the endo/exo ratio in Diels-Alder reactions and steps in enzymatic cycles. This Perspective highlights these outcomes using theoretical results for hydrogen abstraction reactions, epoxidation of double bonds, C-C bond forming reactions, proton transfers and the cycle of the enzyme cytochrome P450, as well as recent experimental data. We postulate that, as experimental techniques mature, chemical syntheses may become an exercise in zapping oriented molecules with OEEFs.

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This article addresses the intriguing hydrogen-abstraction (H-abstraction) and oxygen-transfer (O-transfer) reactivity of a series of nonheme [Fe(IV)(O)(TMC)(Lax)](z+) complexes, with a tetramethyl cyclam ligand and a variable axial ligand (Lax), toward three substrates: 1,4-cyclohexadiene, 9,10-dihydroanthracene, and triphenyl phosphine. Experimentally, O-transfer-reactivity follows the relative electrophilicity of the complexes, whereas the corresponding H-abstraction-reactivity generally increases as the axial ligand becomes a better electron donor, hence exhibiting an antielectrophilic trend. Our theoretical results show that the antielectrophilic trend in H-abstraction is affected by tunneling contributions. Room-temperature tunneling increases with increase of the electron donation power of the axial-ligand, and this reverses the natural electrophilic trend, as revealed through calculations without tunneling, and leads to the observed antielectrophilic trend. By contrast, O-transfer-reactivity, not being subject to tunneling, retains an electrophilic-dependent reactivity trend, as revealed experimentally and computationally. Tunneling-corrected kinetic-isotope effect (KIE) calculations matched the experimental KIE values only if all of the H-abstraction reactions proceeded on the quintet state (S = 2) surface. As such, the present results corroborate the initially predicted two-state reactivity (TSR) scenario for these reactions. The increase of tunneling with the electron-releasing power of the axial ligand, and the reversal of the "natural" reactivity pattern, support the "tunneling control" hypothesis (Schreiner et al., ref 19). Should these predictions be corroborated, the entire field of C-H bond activation in bioinorganic chemistry would lay open to reinvestigation.

This Account outlines interplay of theory and experiment in the quest to identify the reactive-spin-state in chemical reactions that possess a few spin-dependent routes. Metalloenzymes and synthetic models have forged in recent decades an area of increasing appeal, in which oxometal species bring about functionalization of hydrocarbons under mild conditions and via intriguing mechanisms that provide a glimpse of Nature's designs to harness these reactions. Prominent among these are oxoiron(IV) complexes, which are potent H-abstractors. One of the key properties of oxoirons is the presence of close-lying spin-states, which can mediate H-abstractions. As such, these complexes form a fascinating chapter of spin-state chemistry, in which chemical reactivity involves spin-state interchange, so-called two-state reactivity (TSR) and multistate reactivity (MSR). TSR and MSR pose mechanistic challenges. How can one determine the structure of the reactive transition state (TS) and its spin state for these mechanisms? Calculations can do it for us, but the challenge is to find experimental probes. There are, however, no clear kinetic signatures for the reactive-spin-state in such reactions. This is the paucity that our group has been trying to fill for sometime. Hence, it is timely to demonstrate how theory joins experiment in realizing this quest. This Account uses a set of the H-abstraction reactions of 24 synthetic oxoiron(IV) complexes and 11 hydrocarbons, together undergoing H-abstraction reactions with TSR/MSR options, which provide experimentally determined kinetic isotope effect (KIE) data. For this set, we demonstrate that comparing KIE results with calculated tunneling-augmented KIE (KIE) data leads to a clear identification of the reactive spin-state during H-abstraction reactions. In addition, generating KIE data for a reaction of interest, and comparing these to KIE values, provides the mechanistic chemist with a powerful capability to identify the reactive-TS in terms of not only its spin state but also its geometry and ligand-sphere constitution. Since tunneling "cuts through" barriers, it serves as a chemical selectivity factor. Thus, we show that in a family of oxoirons reacting with one hydrocarbon, the tunneling efficiency increases as the ligands become better electron donors. This generates counterintuitive-reactivity patterns, like antielectrophilic reactivity, and induces spin-state reactivity reversals because of differing steric demands of the corresponding TS species, etc. Finally, for the same series, the Account reaches intuitive understanding of tunneling trends. It is shown that the increase of ligand's donicity results in electrostatic narrowing of the barrier, while the decrease of donicity and increase of bond-order asymmetry in the TS (inter alia due to Bell-Evans-Polanyi effects) broadens the barrier. Predictions are made that usage of powerful electron-donating ligands may train H-abstractors to activate the strongest C-H bond in a molecule. The concepts developed here are likely to be ap...