Oriented Electric Fields─Universal Catalysts.

Abstract

ConspectusThis Account outlines principles of electric-field-mediated chemistry, whereby oriented-external electric fields (OEEFs) function as universal "reagents" that control reactivity/selectivity and structures of molecules/clusters. The TOC graphics illustrate the rate-enhancing OEEFs for two different reactions. For the Diels-Alder reaction, we also mark the corresponding reaction-axis (RA). The RA arrow specifies the directional-flow, of the electron density and bond-coupling, from reactants to products (RC→PC). Determining the RA direction, for a given process, involves curly arrow-pushing in the charge-transfer direction. By convention, the arrowhead of the RA signifies the direction of the negative charge flow (Scheme 2). The arrowhead of the F_Z (OEEF) vector is marked as positive, hence corroborating the direction of negative charge flow, which will be induced by F_Z.Thus, as the Account demonstrates, the impact of OEEFs on reactions and structural transformations is unique. Energy-barriers-lowering generally occurs along a single direction in space, specified by the RA. Furthermore, the OEEF also catalyzes reactions in the presence of solvents! For example, the computed OEEF lowers the barrier of the Menshutkin reaction (pyridine/CH₃-I) by 10.6-12.6 kcal/mol in the three polar solvents. Thus, solvent screening of the OEEF is imperfect (see F_{solvent-induced} in the conspectus art), and hence, chemical reactions are not limited to gas- or solid-phases. As the main text elaborates, this imperfect screening-effect in solution is fundamental, and applicable to reactions and to OEEF-induced structural changes. As such, the OEEF is a universal enhancer of chemical change.The Account starts with conceptual principles for understanding and predicting the theoretically computed and/or experimentally observed OEEF effects on chemical reactions as well as structural transformations. These principles highlight the role of OEEFs as tweezers that orient molecular species along the respective RAs, and accelerate their transformation to products.Subsequently, the paper describes experimental support of the theoretical results and guidelines. Some of the applications also use continuous-flow setups, which will eventually scale-up product yields to Molar concentrations, and render OEEFs as practical tools in chemistry. Evidence is presented for the potential existence of OEEF/thermal dichotomy, wherein the OEEF-induced products differ from those which are produced corresponding thermal-only reactions (see later work by Matile et al.).The paper addresses also an important structural process; on the type of EEF (oscillating vs static), which carries out most effectively the decomposition of peptide-plaques (e.g., as those which are found in brains during Alzheimer's disease). We show that in accord with experimental results, the most efficient decomposition is incurred with oscillating EEFs in the frequency range that is smaller than or equal to 1 GHz.The article concludes with a vision that in the near future, OEEF usage will change chemical education, chemical practice, and the art of making molecules.