Search

In this study, we present a new hybrid functional denoted as xOPBE, which is optimized at the 6-311+G(2d,p) basis set and designed with a specific aim of providing accurate 13C chemical shifts. By mixing the Hartree-Fock exchange into the OPBE functional, xOPBE provides a significantly improved overall performance as compared to its parent OPBE functional, while OPBE was shown previously as an excellent functional for 13C chemical shifts. Even in the case of the 1-adamantyl cation, for which OPBE completely fails in reproducing the experimental results, xOPBE still performs very well with similar accuracy as the standard CCSD(T) method with a large basis set. Our results also demonstrate that xOPBE not only can improve quantitatively the description of the correct assignments given by OPBE but also can revert OPBE's incorrect assignments qualitatively. Thus, we would like to recommend the use of xOPBE for routine evaluations of 13C chemical shifts.

Storing hydrogen efficiently in condensed materials is a key technical challenge. Tremendous efforts have been given to inorganic hydrides containing B–H, Al–H and/or N–H bonds, while organic compounds with a great variety and rich chemistry in manipulating C–H and unsaturated bonds, however, are undervalued mainly because of their unfavourable thermodynamics and selectivity in dehydrogenation. Here, we developed a new family of hydrogen storage material spanning across the domain of inorganic and organic hydrogenous compounds, namely metallo-N-heterocycles, utilizing the electron donating nature of alkali or alkaline earth metals to tune the electron densities of N-heterocyclic molecules to be suitable for hydrogen storage in terms of thermodynamic properties. Theoretical calculations reveal that the enthalpies of dehydrogenation (ΔHd) of these metallo-N-heterocycles are dependent on the electronegativity of the metals. In line with our calculation results, sodium and lithium analogues of pyrrolides, imidazolides and carbazolides of distinct structures were synthesized and characterized for the first time, where the cation-π interaction was identified. More importantly, a reversible hydrogen absorption and desorption can be achieved over lithium carbazolide which has a hydrogen capacity as high as 6.5 ​wt% and a suitable enthalpy of dehydrogenation of 34.2 ​kJ ​mol−1-H2 for on-board hydrogen storage.

Hydrogen uptake and release in arene-cycloalkane pairs provide an attractive opportunity for on-board and off-board hydrogen storage. However, the efficiency of arene-cycloalkane pairs currently is limited by unfavorable thermodynamics for hydrogen release. It is shown here that the thermodynamics can be optimized by replacement of H in the -OH group of cyclohexanol and phenol with alkali or alkaline earth metals. The enthalpy change upon dehydrogenation decreases substantially, which correlates with the delocalization of the oxygen electron to the benzene ring in phenoxides. Theoretical calculations reveal that replacement of H with a metal leads to a reduction of the HOMO-LUMO energy gap and elongation of the C-H bond in the α site in cyclohexanolate, which indicates that the cyclohexanol is activated upon metal substitution. The experimental results demonstrate that sodium phenoxide-cyclohexanolate, an air- and water-stable pair, can desorb hydrogen at ca. 413 K and 373 K in the solid form and in an aqueous solution, respectively. Hydrogenation, on the other hand, is accomplished at temperatures as low as 303 K.

Gold-catalyzed intermolecular alkyne oxidation has attracted much synthetic attention, but mostly suffering undesired over-oxidation. Recent experiments demonstrated that over-oxidation could be dramatically suppressed in zinc(II)-catalyzed intermolecular alkyne oxidation/CH functionalization. By means of first-principle density functional theory calculations, we explored the mechanism of the M-catalyzed intermolecular alkyne oxidations (M = Zn(OTf)2 and Au+ PR3 ) as well as the effects of oxidants, temperature, and metal catalysts on chemoselectivity, in an effort to disclose the origin of the extraordinary chemoselectivity pertaining to zinc catalysis. Our calculations indicate that the Zn-catalyzed intermolecular alkyne oxidation/CH functionalization proceeds by a Friedel-Crafts alkylation mechanism rather than metal carbene insertion mechanism. The chemoselectivity of CH functionalization against over-oxidation in Zn catalysis, in comparison with gold catalysis, can be jointly controlled by four factors: (1) the use of less nucleophilic N-oxide, (2) the enhanced electrophilicity and carbocationic nature of the carbenic site in the α-oxo metal carbenoid intermediate, (3) enhanced steric repulsion to incoming oxidant exerted by bulky ancillary ligand in the close nearby of the carbenic site to disfavor intermolecular over-oxidation and (4) the large negative value of activation entropy in the intermolecular over-oxidation pathway, that jointly give rise to lower activation free energy for the intramolecular cyclization/CH functionalization pathway than for the intermolecular over-oxidation pathway. © 2018 Wiley Periodicals, Inc.

The development of cost-effective and highly efficient catalysts is of scientific importance and practical need in the conversion and utilization of clean energy. One of the strategies fulfilling that demand is to achieve high exposure of a catalytically functional noble metal to reactants to maximize its utilization efficiency. We report herein that the single-atom alloy (SAA) made of atomically dispersed Pt on the surface of Ni particles (Pt is surrounded by Ni atoms) exhibits improved catalytic activity on the hydrolytic dehydrogenation of ammonia–borane, a promising hydrogen storage method for onboard applications. Specifically, an addition of 160 ppm of Pt leads to ca. 3-fold activity improvement in comparison to that of pristine Ni/CNT catalyst. The turnover frequency based on the isolated Pt is 12000 molH2 molPt–1 min–1, which is about 21 times the value of the best Pt-based catalyst ever reported. Our simulation results indicate that the high activity achieved stems from the synergistic effect bet...

In this work, we present an alternative mechanism without the initial coordination of reactant and catalyst for the asymmetric Henry reaction over Cu(II) catalyst. Our calculations show that the re-coordination of acetate and Cu center is essential for the enantioselectivity. Thus, any effect of the re-coordination process would affect the enantioselectivity for this reaction.

Au-catalyzed propargyl ester reactions have been investigated by a comprehensive density functional theory (DFT) study. Our calculations explain the experimental observed chemoselectivity of Au-catalyzed propargyl ester reactions very well by considering all possible pathways both in the absence and presence of 1,2,3-triazole (TA). The “X-factor” of TA is disclosed to have triple effects on this reaction. First of all, it can stabilize and prevent rapid decomposition of the Au catalyst. Secondly, the existence of TA promotes the nucleophilic attack and alters the chemoselectivity of this reaction. Moreover, TA acts as a “relay” to promote the proton transfer.

Fatty acid photodecarboxylase (FAP), was reported recently that can catalyze fatty acid decarboxylation to afford n-alkanes in blue light. A trimolecular photocycle mechanism involving an unidentified proton donor HX was proposed, which is seemingly incompatible with the observed high quantum yield (>80%). Herein, we present an energetic preferred bimolecular mechanism by DFT that involves FADH-based intermediates. Our present work suggests that the light-capturing organic molecule itself, lumiflavin (FI) of FAD, may act as a metal-free photocatalyst for the photodecarboxylation of fatty acids to afford hydrocarbons in organic synthesis.

A series of chiral and achiral neutral vanadyl complexes with N - heterocycle chelated ligands [V 2 O 2 ( S -lact) 2 (bpy) 2 ] ( 1 ), [V 2 O 2 ( S -lact) 2 (phen) 2 ] ( 2 ), [V 2 O 2 ( R -lact)( S -lact)(bpy) 2 ] ( 3 ), [V 2 O 2 ( R -lact)( S -lact)(phen) 2 ] ( 4 ) (H 2 lact = lactic acid, phen = 1,10-phenanthroline, and bpy = 2,2′-bipyridine) have been obtained under hydrothermal condition. The complexes feature bidentate lactate that chelates to vanadium atom through their α-alkoxido and carboxylato groups, while the other coordination sites are occupied by N - heterocycle ligand and terminal oxygen without coordinated or crystallized water molecule. Unusual short bond distances are observed for the coordinated lactates with V (IV) –O α-alkoxy and V (IV) –O carboxy 1.986(7) A in average, which imply the influence of full deprotonation in the chelated ring compared with those of 2.15 A in iron vanadium cofactor of V-nitrogenase. Enhanced activity and the red-shifts of vibrational circular dichroism (VCD) have been observed for the chiral neutral vanadyl complexes 1 and 2 and supported by B3LYP calculation. The carboxylato vibrations in VCD are observed in 1660, 1649, 1600, 1474, 1443 cm −1 and 1664, 1653, 1636, 1483, 1431 cm −1 for 1 and 2 respectively. Their absolute configurations of chiral metal center are assigned as Λ S . The EPR spectra of 1 and 3 with bpy ligands appear to be isotropic, while 2 and 4 with phen ligands display textbook rhombic EPR spectra, and the g values obtained are g xx 2.4727, g yy 1.9975, g zz 1.6456 for 2 ; and g xx 2.4585, g yy 1.9976, g zz 1.6412 for 4 respectively.

Previously, we have proposed a method, FPA-M, for focal-point analysis of magnetic parameter calculations [Sun et al., J. Chem. Phys. 138, 124113 (2013)], where the shielding constants at equilibrium geometries σe are calculated with the second order Moller-Plesset perturbation (MP2) approach, which are extrapolated to the complete basis set (CBS) limit and then augmented by the [σe(CCSD(T)) − σe(MP2)] difference at a valence triple-ζ (VTZ) basis set, where CCSD(T) stands for the coupled cluster singles and doubles model with a perturbative correction for triple excitations. This FPA-M(MP2) method provides satisfactory results to approach to the corresponding CCSD(T)/CBS values for elements of the first two rows in the periodic tables. A series of extensions have been explored here, which replace the MP2/CBS with the Hartree-Fock (HF)/CBS for efficiency. In particular, the [σe(CCSD(T)) − σe(MP2)] VTZ difference is replaced by a step-wise correction from the [σe(CCSD(T)) − σe(MP2)] difference at a valence double-ζ basis set plus the [σe(MP2) − σe(HF)] VTZ difference, leading to a new scheme, denoted here as FPA-M(HF′). A systematical comparison has demonstrated that the FPA-M(HF′) method provides an excellent balance between accuracy and efficiency, which makes routinely accurate calculations of the shielding constants for medium-sized organic molecules and biomolecules feasible.

Measured one-bond spin-spin coupling constants (SSCC) 1J(CC) can be used to describe the nature of the C-C bond, provided one is able to separate the various coupling mechanisms leading to 1J(CC). The Fermi-contact (FC) term probes the first-order density at the positions of the coupling nuclei, whereas the noncontact terms (the paramagnetic spin orbit (PSO) and the spin-dipole (SD) terms) probe the pi character of the C-C bond (the diamagnetic spin orbit (DSO) term can mostly be neglected). A model is tested, in which the value of the FC(CC) term is estimated with the help of measured SSCCs 1J(CH). The difference between the measured J(CC) and the estimated FC(CC) values, delta(CC) = PSO(CC) + SD(CC) + DSO(CC), provides a semiquantitative measure of the pi character of a C-C multiple bond. The applicability and limitations of this approach are discussed by partitioning the four Ramsey terms of the SSCC 1J(CC) into one- and two-orbital contributions. The FC, PSO, and SD terms of 1J(CC) are explained and analyzed with regard to their relationship to other C-C bond properties. It is shown that empirical relationships between measured SSCCs and the s character of a bond need reconsideration.