Chengbu Liu

Mechanism Study of the Gold-Catalyzed Cycloisomerization of α-Aminoallenes: Oxidation State of Active Species and Influence of Counterion

A computational study with the B3LYP density functional theory was carried out to study the reaction mechanism for the cycloisomerization of allenes catalyzed by Au(I) and Au(III) complexes. The catalytic performance of Au complexes in different oxidation states as well as the effects of the counterion on the catalytic activities has been studied in detail. Our calculations show that the catalytic reaction is initiated by coordination of the Au(I) or Au(III) catalyst to the distal double bond of allene and activation of allene toward facile nucleophilic attack, then 3-pyrroline obtained via two-step proton shift, followed by demetalation. On the basis of our calculations, H shifts are key steps of the catalytic cycle, which are significantly affected by the gold oxidation state, counterion, ligands, and assistant catalyst. AuCl is found to be more reactive than AuCl(3); however, the Au(III)-catalyzed path does not involve an oxidation state change from Au(III) to Au(I). Our calculated results rationalize the experimental findings well and overthrow the previous conjecture about Au(I) serving as the catalytically active species for Au(III)-catalyzed cycloisomerization.

Molecular Dynamics Study of the Effect of Calcium Ions on the Monolayer of SDC and SDSn Surfactants at the Vapor/Liquid Interface

The effect of Ca(2+) ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. For each surfactant, two different surface concentrations were used to perform the simulations, and the aggregation morphologies and structural details have been reported. The results showed that the aggregation structures relate to both the surface coverage and the calcium ions. The divalent ions can screen the interaction between the polar head and Na(+) ions. Thus, Ca(2+) ions locate near the vapor/liquid interface to bind to the headgroup, making the aggregations much more compact via the salt bridge. The potential of mean force (PMF) between Ca(2+) and the headgroups shows that the interaction is decided by a stabilizing solvent-separated minimum in the PMF. To bind to the headgroup, Ca(2+) should overcome the energy barrier. Among contributions to the PMF, the major repulsive interaction was due to the rearrangement of the hydration shell after the calcium ions entered into the hydration shell of the headgroup. The PMFs between the headgroup and Ca(2+) in the SDSn systems showed higher energy barriers than those in the SDC systems. This result indicated that SDSn binds the divalent ions with more difficulty compared with SDC, so the ions have a strong effect on the hydration shell of SDC. That is why sulfonate surfactants have better efficiency in salt solutions with Ca(2+) ions for enhanced oil recovery.

A theoretical study of silicon-doped boron nitride nanotubes serving as a potential chemical sensor for hydrogen cyanide

In order to search for a novel sensor to detect and control exposure to hydrogen cyanide (HCN) pollutant molecule in environments, the reactivities of pristine and silicon-doped (Si-doped) (8, 0) single-walled boron nitride nanotubes (BNNTs) towards the HCN molecule are investigated by performing density functional theory (DFT) calculations. The HCN molecule presents strong chemisorption on both the silicon-substituted boron defect site and the silicon-substituted nitrogen defect site of the BNNT, which is in sharp contrast to its weak physisorption on pristine BNNT. A remarkable charge transfer occurs between the HCN molecule and the Si-doped BNNT as proved by the electronic charge densities. The calculated data for the electronic density of states (DOSs) further indicate that the doping of the Si atom improves the electronic transport property of the BNNT, and increases its adsorption sensitivity towards the HCN molecule. Based on calculated results, the Si-doped BNNT is expected to be a potential resource for detecting the presence of toxic HCN.

Influence of structural parameters of Cu2O2 core on magnetic coupling in alkoxo-bridged Cu(II) binuclear system

Abstract The influence of the structural parameters of Cu 2 O 2 core on magnetic coupling is studied in model alkoxo-bridged Cu(II) dimers using the density functional method and the broken symmetry approach. The effect of the hinge distortion of the bridge and the out-of-plane displacement of alkoxo group is also analyzed. A small Cu–O–Cu angle, a large Cu–O distance, the hinge distortion and the out-of-plane displacement are all the factors that can enhance the ferromagnetic character. The asymmetry at the Cu 2 O 2 backbone affects the magnetic coupling slightly. Spin polarization may exist in weak antiferro- or ferromagnetic systems.

Molecular dynamics simulation of pyrene solubilized in a sodium dodecyl sulfate micelle.

In the present work, the structural and dynamical aspects of the solubilization process of pyrene within a sodium dodecyl sulfate micelle were studied using molecular dynamics simulations. Our results showed that free pyrene as the fluorescence probe can be spontaneously solubilized into the micelle and prefers to be located in the hydrophobic core region. As the local concentration of pyrene increased, two molecular probes could enter into the core hydrophobic region and the excited dimer of pyrene molecules was formed, showing a stacking mode of π-π conjugation. Since the π-π stacking interaction between the two pyrene molecules was very weak, formation of the excimer was a dynamic process with the two pyrene molecules alternately separating and associating with each other. In this case, the two pyrene molecules were found to be mainly distributed in the palisade layer of the micelle due to the balance between the weak π-π stacking interaction and the hydrophobic interaction of probe molecules with the surfactant tails.

Stabilization of Amino Acid Zwitterions with Varieties of Anionic Species : The Intrinsic Mechanism

With high-level ab initio theoretical methods, varieties of novel anions (two monoanions and dianions with two binding sites) were explored to stabilize the glycine zwitterions. Unlike the malonic and oxalic dianions (J. Am. Chem. Soc. 2005, 127, 13098.), the presently found anions are self-stable and widely available, ensuring the direct and convenient applications to stabilize the zwitterions. Some of the complexes formed with the anions and glycine zwitterions have very large vertical dissociation energies (>500.0 kJ mol (-1)), implying that the contained anions can be used to stabilize much more unstable zwitterions than the glycine zwitterions. Further studies revealed that the stabilization effects are closely related with the proton-capturing capacities of the anions. In order to stabilize the glycine zwitterions, the proton affinity (PA) of the anionic species should fall within the range of 1567.0-1983.6 kJ mol (-1) and, meanwhile, the proton-affinity differences of the two binding sites (DeltaPA) should be less than 104.2 kJ mol (-1). The present results can be used to direct the efficient designs of the stabilizers to other amino acid zwitterions as well as other types of zwitterions. In addition, the density functional theory was used and compared with the default MP2 theory, with the details given in the discussions.

Theoretical elucidation of Au(I)-catalyzed cycloisomerizations of cycloalkyl-substituted 1,5-enynes: 1,2-alkyl shift versus C-H bond insertion products.

The Au(I)-catalyzed cycloisomerization reactions of cycloalkyl-substituted 1,5-enynes (A) have been investigated by performing density functional theory (DFT) calculations. Theoretical calculations suggest that the reaction proceeds via a stepwise mechanism by the initial formation of a Au(I)-carbene intermediate (B), followed by a 1,2-alkyl shift or C-H insertion reaction to form the ring-expanded three-cyclic product (C) or ring-closed four-cyclic product (D) depending on the size of cycloalkyl substitutions. It is found that the formation of intermediate B is the rate-determining step, and the formation of products C or D is controlled by the size (n) of cycloalkyl substitutions in 1,5-enynes. In the situations with n = 1 and 2, the calculated relative free energies and the barriers consistently indicate that the 1,5-enynes prefer to evolve into product C to product D. In contrast, for the situation of n = 4, the barrier forming product C is found to be higher than that forming product D, supporting the experimental observation that a range of the 1,5-enynes with n = 4 isomerize into product D, although it is thermodynamically less favorable than product C. The present theoretical results provide insight into the mechanism details of the catalytic rearrangement of 1,5-enynes and rationalize the early experimental observations well.

Microscopic Wetting of Self-Assembled Monolayers with Different Surfaces: A Combined Molecular Dynamics and Quantum Mechanics Study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.

A THEORETICAL INVESTIGATION OF THE INTERACTIONS BETWEEN CELLULOSE AND 1-BUTYL-3-METHYLIMIDAZOLIUM CHLORIDE

To better understand the interactions between cellulose and imidazolium-based ionic liquids (ILs), quantum chemistry calculations have been performed on the systems composed of one cellulose unit with the anion, cation, and the ion pair of 1-butyl-3-methylimidazolium chloride ([bmim]Cl) by the density functional method. The relevant geometries, energies, electronic properties and IR characteristics have been systematically discussed. It is found that H-bond interaction is essential for the systems under consideration. The hydroxyls in cellulose bind to chloride anions strongly through H-bonds, which could be predominant to cellulose dissolution in ILs. Chloride anion prefers to occur between two adjacent hydroxyls in cellulose to form bridging OH⋯Cl⋯HO hydrogen bonds. In contrast, weak hydrogen bonds exist between the hydrogen atoms on the imidazolium cation and hydroxyl oxygen atoms of cellulose, which are too much weaker than the hydrogen bonds between the cellulose hydroxyls and chloride anions to be detected by the experiments. The phenomena of cellulose dissolution in ILs should be a result of the joint interactions of chloride anions and [bmim]+ cations with hydroxyls in cellulose.

Theoretical Elucidation of Glucose Dehydration to 5-Hydroxymethylfurfural Catalyzed by a SO3H-Functionalized Ionic Liquid

While the catalytic conversion of glucose to 5-hydroxymethyl furfural (HMF) catalyzed by SO3H-functioned ionic liquids (ILs) has been achieved successfully, the relevant molecular mechanism is still not understood well. Choosing 1-butyl-3-methylimidazolium chloride [C4SO3HmimCl] as a representative of SO3H-functioned IL, this work presents a density functional theory (DFT) study on the catalytic mechanism for conversion of glucose into HMF. It is found that the conversion may proceed via two potential pathways and that throughout most of elementary steps, the cation of the IL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bonding, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol, respectively, which are compatible with the observed catalytic performance of the IL under mild conditions (100 °C). The present results provide help for rationalizing the effective conversion of glucose to HMF catalyzed by SO3H-functionalized ILs and for designing IL catalysts used in biomass conversion chemistry.