Large protein macromolecules in enzymatic catalysis have been shown to exert a specific electric field that reduces the reorganization energy upon barrier crossing and thus reduces the reaction free energy barrier. In this work we suggest that the charge density in the active site of an enzyme investigated using formalisms embodied by the quantum theory of atoms in molecules (QTAIM) provides a sensitive and quantum field-reactant state charge density-reaction barrier correlation. Hence, QTAIM can be used for the analysis of electric field in enzyme active sites, and further investigations and exploitations of the found correlations may prove useful in enzyme design where preorganization is optimized.

The abundant yet widely distributed methane resources require efficient conversion of methane into liquid chemicals, whereas an ambient selective process with minimal infrastructure support remains to be demonstrated. Here we report selective electrochemical oxidation of CH 4 to methyl bisulfate (CH 3 OSO 3 H) at ambient pressure and room temperature with a molecular catalyst of vanadium (V)-oxo dimer. This water-tolerant, earthabundant catalyst possesses a low activation energy (10.8 kcal mol-1) and a high turnover frequency (483 and 1336 hr −1 at 1-bar and 3-bar pure CH 4 , respectively). The catalytic system electrochemically converts natural gas mixture into liquid products under ambient conditions over 240 h with a Faradaic efficiency of 90% and turnover numbers exceeding 100,000. This tentatively proposed mechanism is applicable to other d 0 early transition metal species and represents a new scalable approach that helps mitigate the flaring or direct emission of natural gas at remote locations.

Experimental and computational study quantifying internal electric fields in synthetic systems using transition metal Schiff base complexes functionalized with a crown ether unit containing a mono- or dicationic alkali or alkaline earth metal ion.

Computational design of molecular homogeneous organometallic catalysts followed by experimental realization remains a significant challenge. Here, we report the development and use of a density functional theory transition-state model that provided quantitative prediction of molecular Cr catalysts for controllable selective ethylene trimerization and tetramerization. This computational model identified a general class of phosphine monocyclic imine (P,N)-ligand Cr catalysts where changes in the ligand structure control 1-hexene versus 1-octene selectivity. Experimental ligand and catalyst synthesis as well as reaction testing quantitatively confirmed predictions.

Although there is evidence that catalytic active sites can restructure under reaction conditions, their optimal reconstruction to provide the lowest activation barrier is still unclear. Here, we show with methane activation on supported Pt clusters and by an explicit sampling of cluster configurations at the transition state that important restructuring is required to reach the most active transition state. The capability of the cluster to reconstruct, simultaneously with the C–H dissociation, is a key aspect for catalytic activity. We underline two types of reconstructions, concomitant or independent, depending on whether they emerge in or off the course of reactive trajectories. The concomitant reconstructions depict the significant rearrangement of the catalyst itself as part of the reaction coordinate and play critical roles in the most competitive pathways. The best active sites can only be found in the course of the reactive trajectories and not from the equilibrium geometry prior to the reaction.

Table of Contents SMD solvated (U)M06/def2-TZVPPD//(U)M06/cc-pwCVDZ-PP[aug-cc-pVDZ] xyz coordinates and energies (hartree) …………..

Activation and functionalization of alkane C–H bonds has historically been dominated by transition-metal complexes. Light alkanes can also be partially oxidized by sixth-row main-group compounds, such as TlIII(TFA)3 (TFA = trifluoroacetate). Here we present density-functional calculations which demonstrate that TlIII(TFA)3 oxidizes alkanes by closed-shell C–H activation and Tl–alkyl functionalization mechanisms. The discovery of a C–H activation pathway is surprising, because TlIII often oxidizes arene C–H bonds through an electron transfer mechanism and the transition-metal complex CoIII(TFA)3, with similar oxidation state and ligand coordination, oxidizes alkanes via an open-shell radical mechanism. Comparison of TlIII(TFA)3 to the transition-metal analogue IrIII(TFA)3 reveals that key to TlIII oxidation of alkanes is a moderate barrier for C–H bond activation that is lower in energy than open-shell pathways and a subsequent metal–alkyl functionalization reaction step with a very low barrier. Our calculations suggest that the high-spin ground state of CoIII(TFA)3 provides a low-energy open-shell decarboxylation pathway that leads to radical oxidation of alkanes, which is not available for the d10 TlIII(TFA)3 complex. The C–H activation pathway and transition state model provide a straightforward explanation for why TlIII(TFA)3 promotes alkane C–H bond activation but HgII(TFA)2 does not.

We describe the results of our combined experimental and computational investigation of structurally analogous (N-phosphinoamidinate)metal(N(SiMe 3 ) 2 ) precatalysts ((PN)M; M = Mn 2+ , Fe 2+ , Co 2+ , and Ni 2+ ; d 5 −d 8 ) in the isomerization−hydroboration of 1-octene, cis-4-octene, or trans-4-octene (1a−c) with HBPin. As part of this investigation, the synthesis and crystallographic characterization of diamagnetic (PN)Ni, ((PN)NiH) 2 , (PN)NiH(L) (L = pyridine or DMAP), and (PN)Ni(NHdipp) (dipp = 2,6-iPr 2 C 6 H 3 ) are reported. Divergent catalytic reactivity and selectivity was noted for members of the (PN)M series; (PN)Mn and (PN)Ni afforded poor hydroboration yields, whereas the use of (PN)Fe or (PN)Co afforded high conversion and selectivity for the terminal borylation product, (n-octyl)BPin (2a). DFT calculations involving (PN)M as well as stoichiometric reactivity studies featuring (PN)Ni confirmed that (PN)MH intermediates generated upon reaction of (PN)M with HBPin represent viable catalytic species whereby formation of putative (PN)Ni(H 2 BPin) is reversible. Conversely, poor catalytic performance was noted for ((PN)NiH) 2 and (PN)NiH(L) (L = pyridine or DMAP).Using DFT calculations, the relative reactivity of (PN)M precatalysts was found to be a function of their spin-state energy gaps. For reaction of (PN)MnH with trans-4-octene (1c) there is no viable spin crossover mechanism and migratory insertion is slow, resulting in poor reaction yields. In contrast, (PN)FeH can access a lower barrier through spin crossover, whereas (PN)CoH has a very low migratory insertion barrier from its low spin state. While (PN)NiH has a reasonable migratory insertion barrier, it is plausible that off-catalytic cycle intermediates are responsible for the diminished reaction rate and product yields that are observed experimentally. On the basis of the computed isomerization and borylation energy landscapes, a Curtin−Hammett-type scenario with fast isomerization through β-hydride elimination and migratory insertion steps is proposed, giving rise to a catalytic equilibrium of isomeric (PN)M(octyl) resting states, followed by slow product-forming borylation. The significantly lower barriers calculated for borylation of terminal (PN)M(n-octyl) species versus isomeric internal (PN)M(CHR 2 ) intermediates provides a rationale for the experimentally observed terminal isomerization−hydroboration selectivity.