This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O). Whenever the radical is delocalized, e.g., in [MgO] the HAT barrier increases due to the penalty of radical localization. Adding a dopant (GaO) to [MgO] localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [AlO] the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO] by AlO enables HAT and PCET to compete. Similarly, [ZnO] activates methane by PCET generating many products. Adding a CHCN ligand to form [(CHCN)ZnO] leads to a single HAT product. The CHCN dipole acts as an OEF that switches off PCET. [MC] cations (M = Au, Cu) act by different mechanisms, dictated by the M-C bond covalence. For example, Cu, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu.
A mechanistically unique, simultaneous activation of two C-H bonds of methane has been identified during the course of its reaction with the cationic copper carbide, [Cu-C]. Detailed high-level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT-ICR mass spectrometry. The behavior of [Cu-C]/CH contrasts that of [Au-C]/CH, for which a stepwise bond-activation scenario prevails. An explanation for the distinct mechanistic differences of the two coinage metal complexes is given. It is demonstrated that the coupling of [Cu-C] with methane to form ethylene and Cu is modeled very well by the reaction of a carbon atom with methane mediated by an oriented external electric field of a positive point charge.
The oxygen-atom transfer reaction catalyzed by the mononuclear molybdenum enzyme dimethyl sulfoxide reductase (DMSOR) has attracted considerable attention by both experimental and theoretical studies. We show here that this reaction is more sensitive to details of quantum mechanical calculations that what has previously been appreciated. Basis sets of at least triplezeta quality are needed to obtain qualitatively correct results. Dispersion has an appreciable effect on the reaction, in particular the binding of the substrate or the dissociation of the product (up to 34 kJ/mol). Polar and non-polar solvation effects are also significant, especially if the enzyme can avoid cavitation effects by using a pre-formed active-site cavity. Relativistic effects are considerable (up to 22 kJ/mol), but they are reasonably well treated by a relativistic effective core potential. Various density-functional methods give widely different results for the activation and reaction energy (differences of over 100 kJ/mol), mainly reflecting the amount of exact exchange in the functional, owing to the oxidation of Mo from +IV to +VI. By calibration towards local CCSD(T0) calculations, we show that none of eight tested functionals (TPSS, BP86, BLYP, B97-D, TPSSH, B3LYP, PBE0, and BHLYP) give accurate energies for all states in the reaction. Instead, B3LYP gives the best activation barrier, whereas pure functionals give more accurate energies for the other states. Our best results indicate that the enzyme follows a two-step associative reaction mechanism with an overall activation enthalpy of 63 kJ/mol, which is in excellent agreement with the experimental results.
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