Rongxiu Zhu

A Density Functional Theory Study on the Ring-Opening Polymerization of D-Lactide Catalyzed by a Bifunctional-Thiourea Catalyst

The thiourea-catalyzed methanolysis of d-lactide, a model system for the initiation and propagation of the organocatalyzed ring-opening polymerization (ROP) of lactide, has been studied by performing density functional theory calculations. Both the catalyzed and uncatalyzed reactions are explored along two possible pathways: one involves the stepwise addition–elimination pathway and the other is related to the concerted pathway. It is found that the reaction without the presence of the catalyst is difficult because the barrier involved is as high as 176 kJ mol–1. With the aid of a thiourea catalyst, the barrier is reduced to 88 kJ mol–1 with a preference for the stepwise addition–elimination mechanism over the concerted one. The role of the catalyst has been rationalized by analyzing the frontier molecular orbital interactions between the catalyst and substrates and by performing natural population analysis. Finally, another mechanism involving acyl transfer is discussed for the thiourea-catalyzed ROP.

Assembly and stabilization of multi-amino acid zwitterions by the Zn(II) ion: a computational exploration.

Density functional calculations were employed to stabilize multi-amino acid zwitterions by metal ions as well as to understand the stabilizing mechanism. This work distinguishes from the previous metalations and stabilizations of a single amino acid zwitterion, where the metal ions are coordination-unsaturated. It was found that up to three Gly zwitterions can be stabilized by the Zn(2+) ion at the same time. The addition of the fourth Gly zwitterion to the Zn(Gly)(3) system causes the spontaneous transformation of one zwitterion to the neutral isomer. The single mutations of Gly in Zn(Gly)(4) with Arg or Asp render the Zn(2+)-combined four amino acid zwitterions geometrically stable. The double mutation of Gly with Arg further stabilizes the structure with four amino acid zwitterions, with its energy being lower than the one containing one neutral and three zwitterionic isomers. The systematic studies on a series of M-combined amino acid systems (M = Zn(2+), Zn(+), Li(+), Na(+)) indicated that enough charge transfers from the metal ions are essential to stabilize the amino acid zwitterions. There exist critical values of charge transfers, and for Zn(2+)-combined amino acid systems, an estimation of 0.20 |e| is required to stabilize each Gly zwitterion. Further analysis revealed that the charge transfers rather than the formations of direct M-O bonds play a more important role in the stabilizations of amino acid zwitterions.

Density functional theory study on the geometrical and electronic structures of a new thinnest boron nanotube

Single-walled boron nanotubes (BNTs), which has been synthesized successfully recently, were imagined as duals (hexagonal pyramidal structures) of carbon nanotubes (CNTs) in the literature. In this work, we call attention to the fact that BNTs are not limited to hexagonal pyramidal structures constructed from the so-called Aufbau principle, and alternatively, we propose that the thinnest BNT may be a geometrical analog of the corresponding CNT. As shown by our density functional theory calculations, both the tubular open-end cluster models and the infinitely long tube possess high structural, dynamic, and thermal stability, which should be of interest for attempts at its synthesis. Compared to the energetically most stable isomers of the corresponding clusters, the thinnest BNT might be a metastable structure, and from an electronic view of point, it was predicted to have metallic conductivity like hexagonal pyramidal BNTs predicted previously, in contrast to semiconducting crystalline rhombohedral α- and β-boron.

Theoretical Investigation on the Chiral Diamine-Catalyzed Epoxidation of Cyclic Enones: Mechanism and Effects of Cocatalyst

The asymmetric epoxidation of 2-cyclohexen-1-one with aqueous H(2)O(2) as oxidant, 1,2-diaminocyclohexane as catalyst, and a Brønsted acid trifluoroacetic acid (TFA) as cocatalyst has been studied by performing density functional theory calculations. It is confirmed that the catalyzed epoxidation proceeds via sequential nucleophilic addition and ring-closure processes involving a ketiminium intermediate. Four possible pathways associated with two Z isomers and two E isomers of ketiminium have been explored in detail. Our calculation indicates that these four pathways have high barriers and a small energy gap between two more favorable R and S pathways. We have analyzed the effects of the TFA anion and H(2)O on the activity and enantioselectivity of catalytic epoxidation. It is found that the TFA anion acts as a counterion to stabilize the transition states of the catalytic epoxidation by hydrogen-bond acceptance, leading to decreases in the barriers of the nucleophilic addition and ring-closure processes. The most significant decrease occurred in the ring-closure step of the Z-R-pathway, resulting in H-bond-induced enantioselectivity. Our calculations also show that water cooperates with TFA to further increase the reaction rate significantly.

Theoretical study of the adsorption of pentachlorophenol on the pristine and Fe-doped boron nitride nanotubes.

AbstractTo explore the novel application of boron nitride nanotubes (BNNTs), we investigated the interaction of pentachlorophenol (PCP) pollutant with the pristine and Fe doped (Fe-doped) (8, 0) single-walled BNNTs by performing density functional theory calculations. Compared with the weak physisorption on the pristine BNNT, PCP molecule presents strong chemisorption on the Fe-doped BNNT. The calculated data for the electronic properties indicate that doping Fe atom into the BNNT significantly improves the electronic transport property of BNNT, induces magnetism in the BNNT, and increases its adsorption sensitivity toward PCP molecule. It is suggested that doping BNNTs with Fe is an available strategy for improving the properties of BNNTs, and that Fe-doped BNNT would be a potential resource for adsorbing PCP pollutant in environments. FigureThe Fe-doped BNNT presents high sensitivity to PCP pollutant and is expected to be a potential resource for adsorbing toxic pentachlorophenol molecule.

Revised Role of Selectfluor in Homogeneous Au‐Catalyzed Oxidative CO Bond Formations

The pairing of transition metal catalysis with the reagent Selectfluor (F-TEDA-BF4) has attracted considerable attention due to its utility in myriad C-C and C-heteroatom bond-forming reactions. However, little mechanistic information is available for Selectfluor-mediated transition metal-catalyzed reactions and controversy surrounds the precise role of Selectfluor in these processes. We present herein a systematic investigation of homogeneous Au-catalyzed oxidative C-O bond-forming reactions using density functional theory calculations. Currently, Selectfluor is thought to serve as an external oxidant in Au(I)/Au(III) catalysis. However, our investigations suggest that these reactions follow a newly proposed mechanism in which Selectfluor functions as an electrophilic fluorinating reagent involved in a fluorination/defluorination cycle. We have also explored Selectfluor-mediated gold-catalyzed homocoupling reactions, which, when cyclopropyl propargylbenzoate is used as a substrate, lead to an unexpected byproduct.

Theoretical Insight into the Mechansim and Origin of Ligand-Controlled Regioselectivity in Homogenous Gold-Catalyzed Intramolecular Hydroarylation of Alkynes

This work aims at understanding the mechanism and regioselectivity in ligand-controlled gold-catalyzed divergent intramolecular hydroarylation of alkynes reported by Jiang et al. ( J. Am. Chem. Soc. 2016 , 138 , 5218 - 5221 ). Focusing on a representative alkyne, N-propargyl-N-tosylaniline, we conducted a detailed computational study on the ortho- and para-position hydroarylation of the alkyne catalyzed by gold(I) catalysts with different ligands. Both the ortho- and para-position hydroarylation reactions are found to follow a similar three-stage mechanism: electrophilic cyclization, proton loss, and protiodeauration. The initial electrophilic cyclization was identified as the rate- and regiochemistry-determining step. With the flexible electron-deficient phosphite ligand, the ortho-position cyclization is identified as the energetically more favorable pathway, while with the rigid electron-abundant phosphine (Xphos) ligand, the dominant pathway turns to the para-position cyclization. The theoretical results are in good agreement with the experimental observations. The π-π interaction between alkynyl phenyl and the directing acylamino group are found to be mainly responsible for the observed ortho-selectivity, while a combination of favorable noncovalent CH···π interaction and steric repulsion between Xphos ligand and alkynyl group contributes to the observed exclusive para-selectivity. The present calculations provide deeper insight into the mechanism and origin of regioselectivity of the title reaction.

Interactions of Zn(II) with single and multiple amino acids. Insights from density functional and ab initio calculations

Calculations were performed to study the interactions of metal ions (M) with (multiple) amino acids (AA) and fill the gap between single AA and proteins. A complete conformational search results in nine and eleven ZnGly isomers at B3P86 and MP2 levels, respectively, and four populated conformers of glycine are responsible for production of these isomers. For all M, the isomers via the OO and NO binding modes are the main constituents, and the OO mode is favored by stronger electrostatic interactions. Binding with more glycines causes larger structural distortions, improves relative stabilities of monodentate binding isomers and generates new binding modes (e.g. ZnB(III) via only the hydroxyl group). The scaling factor of Zn(Gly)(n) structures, the ratio of its binding affinity versus the sum of comprising ZnGly isomers, is linear with glycine number (n), and the linear relationship may not be altered by mutations of glycines and M. It thus allows to estimate M(AA)(n) binding affinities (n ≥ 2) from the comprising MAA structures and analyze their structures with kinetic methods. The DFT and MP2 results become comparable by increasing metal coordination, e.g. the ZnB(III) versus ZnA(I) (zwitterionic) relative energy differs by 41.9 kcal mol(-1) at B3P86 and MP2 levels and is close by addition of three water molecules (4.1 kcal mol(-1)). The presence of water solvent improves the relative stabilities of monodentate binding isomers and results in a broader conformational distribution.

Theoretical Elucidation of the Mechanism and Kinetic Experimental Phenomena on the Esterification of α-Tocopherol with Succinic Anhydride: Catalysis of a Histidine Derivative vs an Imidazolium-Based Ionic Liquid

DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of α-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazolium-based ionic liquid (IL). The two catalytic reactions involve an intrinsically consistent molecular mechanism: a rate-determining, concerted nucleophilic substitution followed by a facile proton-transfer process. The histidine derivative or the IL anion is shown to play a decisive role, acting as a Brönsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated free energy barriers of two reactions (15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction) together with their respective characteristic features, the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL, rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time.

Adsorption behaviors of monomer and dimer of formic acid on Pt (111) in the absence and presence of water

AbstractBy performing density functional theory (DFT) theory calculations, we studied the adsorption behaviors of the monomer and dimer of formic acid (HCOOH, FA) on the Pt(111) surface with and without the presence of water molecules. The monomer prefers to stand on the surface of Pt(111), and in the most stable adsorption configuration the carbonyl O of HCOOH binds to the atop site of a Pt atom and the hydroxyl H points asymmetrically to two neighboring Pt atoms. The dimer of HCOOH not only exists in the gas-phase but also on Pt(111) surface, and the eight-membered ring dimer is identified as the energetically most favorable dimeric structure of HCOOH both in gas-phase and on Pt(111) surface. With the presence of water molecules, both the monomer and dimer of HCOOH prefer to lie parallel to the surface so as to maximize the number of H-bonds to adjacent water molecules. These results indicate that water molecules significantly influence the initial adsorption manner of HCOOH and further its decomposition reactivity on Pt(111) surface. The present work shows the adsorption behavior of HCOOH dimer on Pt(111) for the first time and also gives several new adsorption configurations of the monomer that are not reported in literature. The theoretical results are expected to provide a valuable input to understand the reactivity of HCOOH on Pt(111). FigureThe most stable monomer and dimer of HCOOH adsorbed on Pt(111) in the absence and presence of water