Au-O Bonds Research Articles

A core–shell AuNRs@BUT-16 nanocomposite for enhancement SERS detection and efficient removal of deoxynivalenol

IntroductionDeoxynivalenol (DON) is widely found in grains and poses a serious threat to human health, so there is an urgent need to develop methods for its simultaneous removal and detection. The novel metal organic framework (MOF) material BUT-16 has a high adsorption capacity (79.8%) for DON. Meanwhile, surface-enhanced Raman spectroscopy (SERS) has been widely used for rapid detection of analytes. ObjectivesThe aim of this work is to prepare a material that can be used for enhancement SERS detection and efficient removal of DON. MethodsAuNRs@BUT-16 was prepared by in-situ solvothermal method and characterized using a series of characterization tools. AuNRs@BUT-16 was used as an adsorbent and SERS substrate for the removal and detection of DON, and some factors affecting the adsorption and SERS detection were investigated. The adsorption mechanism between DON and AuNRs@BUT-16 was investigated using molecular docking. The proposed SERS method was used to detect DON contamination in real samples. ResultsThe prepared core–shell AuNRs@BUT-16 showed a synergistic effect in improving DON adsorption and SERS response. 97.6 % of DON was removed by AuNRs@BUT-16 in aqueous solution, and 70 % in 80 % methanol. The pre-enrichment effect of BUT-16 could trap more DON molecules in the “hot spots” of AuNRs, thus the proposed SERS sensor based on AuNRs@BUT-16 substrate displayed outstanding SERS response and the limit of detection of DON was 3.87 × 10−3 μg/mL. Molecular docking revealed that hydrogen bond and π-alkyl interaction were the main reasons for high affinity between BUT-16 and DON, and Au-O bonds facilitated the adsorption of DON on AuNRs. ConclusionsAuNRs@BUT-16 with superior enrichment and SERS detection capabilities has been used for simultaneous removal and SERS detection of DON, and it also has great potential to realize sensitive and selective detection and removal of DON in multiple disciplines.

Polyhydroxy Fullerene for Synthesizing Noble Metal Nanoparticles

The redox properties of fullerene and its derivatives enable them to function as electron relays through either oxidation or reduction. Polyhydroxy fullerene (PHF), also known as fullerol or fullerenol, is highly water-soluble derivative and a well-known antioxidant. We have found that PHF is also very effective in reducing noble metal precursors and stabilizing metal nanoparticles by forming covalent M-O bonds. Traditionally, noble metal nanoparticles, such as gold are synthesized using a reducing agent like sodium borohydride (NaBH4) or sodium citrate at elevated temperatures and alkaline pH, with stabilizers added during nucleation to control the size and dispersion of the nanoparticles. While sodium citrate is sometimes used as a single reducing and stabilizing agent, it is not an effective stabilizer and is often replaced with polymers to improve colloidal dispersion. In contrast, PHF can be used as a single reducing and stabilizing agent for metal nanoparticles. The nanoparticles were synthesized by simply mixing the metal precursors with PHF in an aqueous phase and stirring at room temperature. The size and morphology of synthesized nanoparticles were characterized with high-resolution transmission electron microscopy, aberration corrected TEM, and dynamic light scattering. These nanoparticles were monodisperse with size ranging from 2 nm to 100 nm in diameter, which could be controlled by changing the concentration of the two reactants. We utilized multiple tools to investigate the mechanism of reduction and nanoparticle formation. We propose a kinetic agglomerate-dissociation mechanism to describe the formation of gold nanoparticles using PHF. Kinetic agglomerates form instantaneously upon mixing the two reactants due to electrostatic interactions. During the reduction process reactive carbonyl peaks were observed with electron energy loss spectroscopy. The carbonyl moieties were observed only after agglomerate formation and subsequently changed to Au-O bonds between PHF and synthesized gold nanoparticles, which was confirmed with Fourier-transform infrared spectroscopy and density-functional theory calculations, and X-ray photoelectron spectroscopy. Further, we demonstrated that the synthesis of gold nanoparticles using PHF can be affected by pH due to changes in hemiketal structure of PHF. Compared to citrate ions, PHF-coated gold nanoparticles are more stable, remaining so for over two years. The novel PHF-mediated synthesis method can also be applied to other noble metals like platinum, palladium, and silver and create bimetallic nanoparticles. The surface PHF coating on noble metal nanoparticles integrates the properties of both metal and carbon nanoparticles, resulting in superior functional applications. Our findings open up new opportunities for metallic nanoparticle preparation methods, properties, and applications.

Study of the effect of high-temperature activation of quartz on the adsorption of gold cyanide

• Gold cyanide ions are chemically adsorbed onto the quartz surface after high-temperature activation. • After high-temperature activation, Au-Si is adsorbed together, and the valence state of Au is + 1. • The N-Si bonding is enhanced after high-temperature activation. • The Au-O interaction on the surface of Au(CN) 2 − and quartz is transformed into a Au-Si interaction. The leaching rates for gold in the roasting oxidation pretreatment-cyanidation gold leaching process are generally low. An adsorption experiment for quartz baking slag (roasting time of 3 h, roasting temperature of 650℃) and gold cyanide solution was carried out under different stirring time conditions. It is found that after high temperature activation of the quartz surface, gold cyanide can be adsorbed. When the stirring time reaches 50 min, the adsorption rate is 6.08%, and the unit adsorption amount is 0.05 mg·g −1 . The EDS spectrum clearly shows that the Au element is distributed among Si and O, and 0.02% Au is adsorbed on the surface of the quartz baking slag. To further prove the adsorption mechanism for the gold cyanide solution and quartz slag, infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface of quartz slags. Infrared spectroscopy analysis shows that new peaks emerge at 1429 cm −1 , 1878 cm −1 , and 1996 cm −1 after the elution experiment, and the characteristic peaks for inorganic cyanide are observed at 1878 cm −1 and 1996 cm −1 . This shows that after high-temperature activation of the quartz baking slag, chemical adsorption of gold cyanide occurs. The results obtained from X-ray photoelectron spectroscopy (XPS) show that during the adsorption process, Au-Si is adsorbed together, and Au shows a valence of + 1, which indicates that Au(CN) 2− ions are adsorbed onto the quartz activated at high temperature. Density functional theory (DFT) was used to determine the adsorption mechanism for Au(CN) 2− ions and different sites on the surface of quartz (1 0 1) before and after high-temperature activation (three Au-O, Au-Si, and N-Si adsorption sites). During the high-temperature activation process, the Au-O bonds and interactions on the surface of gold cyanide and quartz gradually transform into Au-Si bonds and interactions, and the N-Si bonds are also enhanced. This shows that the adsorption of gold cyanide onto quartz baking slag is enhanced after high-temperature activation. The simulation calculation result is consistent with the experimental results.

A DFT study on the adsorption of nucleobases with Au20.

The interactions of nucleobase with Au20 cluster were studied utilizing density functional theory (DFT). We find that the nucleobases were attached with Au20 through Au-N or Au-O bonds. The orders of calculated adsorption energy are cytosine > adenine > guanine > thymine. The maximum adsorption energy is - 0.787eV which belongs to the adsorption of cytosine on Au20 cluster. Our calculations show that the HOMO-LUMO gaps of Au20 are significantly decreased by the nucleobase adsorption, suggesting the potential detection for nucleobases. The interactions between Au20 and nucleobases are revealed through the QTAIM analysis. The NBO analysis was investigated for the electron transfer of donor-acceptor of all types of complexes. The IR and UV-vis spectra were simulated for the further identification in the experiment.

Plasmon-induced hot electron transfer in Au-ZnO heterogeneous nanorods for enhanced SERS.

Colloid-synthesized matchstick-shaped Au-ZnO heterogeneous nanorods are found to have the Zn ion terminated plane in the ZnO-Au interface without the formation of Au-O bonds based on the atomic-resolution observation of their interfacial structure and electronic states, which is greatly different from the other reported results. The Au-ZnO heterogeneous nanorods with a good expitaxial interface have shown a stronger surface-enhanced Raman scattering (SERS) signal of the dopamine molecules than Au nanoscale seeds alone, which is attributed to the enhanced charge transfer (CT) effect of ZnO which is greatly improved by the plasmon-induced hot electron from Au nanostructures. The enhanced CT effect has also been proved by a higher photocatalysis efficiency. Furthermore, the plasmon-induced hot electron transfer mechanism in Au-ZnO heterogeneous nanorods has been confirmed by a slow rise time of electrons in the transient absorption measurements. These findings suggest the dependency of the plasmon-induced hot electron transfer mechanism on the different mixing of the metal and semiconductor band levels.

Behavior of hybrid thermosensitive nanosystem dextran-graft-PNIPAM/gold nanoparticles: characterization within LCTS

Thermally responsive polymers based on poly (N-isopropylacrylamide) (PNIPAM) with lower critical solution temperature (LCST) in the region of physiological temperatures become a subject of a study as promising material for biomedical application. In the present work, the star-like dextran-graft-PNIPAM with incorporated gold nanoparticles (AuNPs) in water suspension were studied by quasi-elastic light scattering (QELS), transmission electron microscopy (TEM), and spectroscopic methods in the temperature range around LCST. The analysis of QELS results revealed the individual polymer macromolecules with incorporated inside AuNPs and their aggregates, which demonstrate the LCST near 34 °C with the collapse of individual macromolecules and polymer aggregates too. The absorption spectroscopy measurements showed that the polymer structure is reversible at LCST and heating of polymer/AuNP nanosystems from room temperatures to LCTS does not accompany with a release of AuNPs from the polymer matrix. Raman spectra of nanohybrids were measured below and above LCTS and analyzed in details by employing DFT calculations. Two intense bands in the region 50–300-cm−1 region were observed for the first time. They were assigned to the stretching vibrations of Au-Au and Au-O (NIPAM) bonds. The presented work shows that the synthesized polymer with AuNPs has high stability going through LCST encouraging their use as a perspective system for drug delivery strategy.

Reviewing Gold(III) complexes as effective biological operators

Cancer treatment using platinum has vastly been associated with numerous side effects and resistance generation. As a result, current medicinal chemistry is now emphasized on the development of novel metal based drugs bearing diverse pharmacological profile. There has been an increasing interest in the drafting of Au(III) complexes as new metal based drugs. Au(III) complexes have been observed to be especially steady under different physicochemical situations. Dithiocarbamato-Au(III) complexes show antiproliferative property against specific human tumor cells because of their inhibiting effect towards the tumor cell augmentation. The review focuses on the results obtained in the field of anticancer and antimicrobial efficacy of Au(III) complexes. The represented Au(III) complexes as anticancer agents have been classified as (a) complexes containing Au-N bonds, (b) complexes containing Au-S bonds and (c) complexes containing Au-O bonds. Au(III) complexes as antimicrobial agents have been subdivided as (a) antibacterial and antifungal (b) antimalarial and (c) antitrypanosomial Au(III) complexes. The results obtained from the analysis of anticancer and antimicrobial Au(III) complexes could possibly lead to the design and development of novel potent Au(III) complexes exhibiting enhanced activities.

Exchange bias effect inAu-Fe3O4dumbbell nanoparticles induced by the charge transfer from gold

We have studied the origin of the exchange bias effect in the ${\text{Au-Fe}}_{3}{\mathrm{O}}_{4}$ dumbbell nanoparticles in two samples with different sizes of the Au seed nanoparticles (4.1 and 2.7 nm) and same size of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanoparticles (9.8 nm). The magnetization, small-angle neutron-scattering, synchrotron x-ray diffraction, and scanning transmission electron microscope measurements determined the antiferromagnetic FeO w\"ustite phase within ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanoparticles, originating at the interface with the Au nanoparticles. The interface between antiferromagnetic FeO and ferrimagnetic ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is giving rise to the exchange bias effect. The strength of the exchange bias fields depends on the interfacial area and lattice mismatch between both phases. We propose that the charge transfer from the Au nanoparticles is responsible for a partial reduction of the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ into the FeO phase at the interface with Au nanoparticles. The Au-O bonds are formed, presumably across the interface to accommodate an excess of oxygen released during the reduction of magnetite.

Adsorption of small molecules on helical gold nanorods: a relativistic density functional study.

We study the adsorption of a variety of small molecules on helical gold nanorods using relativistic density functional theory. We focus on Au40 which consists of a central linear strand of five gold atoms with seven helical strands of five gold atoms on a coaxial tube. All molecules preferentially adsorb at a single low-coordinated gold atom on the coaxial tube at an end of Au40. In most cases, there is significant charge transfer (CT) between Au40 and the adsorbate, for CO and NO2, there is CT from the Au40 to adsorbate while for all other molecules there is CT from the adsorbate to Au40. Thus, Au40-adsorbate can be described as a donor-accepter complex and we use charge decomposition analysis to better understand the adsorption process. We determine the adsorption energy order to be C5H5N >NO2 > CO > NH3 > CH2=CH2 > CH2=CH-CHO > NO > HC≡CH > H2S > SO2 > HCN > CH3OH > H2C=O > O2 > H2O > CH4 > N2. We find that the Au-C, Au-N, Au-S, and Au-O bonds are surprisingly strong, with clear implications for reactivity enhancement of the adsorbate. The Au-H bond is relatively weak but, for interactions via an H atom that is bonded to a carbon atom (e.g., CH4), we find that there is large charge polarization of the Au-H-C moiety and partial activation of the inert C-H bond. Although the Au-S and Au-O bonds are generally weaker than the Au-C and Au-N bonds, we find that adsorption of H2S or H2O causes greater distortion of Au40 in the binding region. However, the degree of distortion is small and the helical structure is retained, demonstrating the stability of the helical Au40 nanorod under perturbations.

Electronic structure of the L-cysteine films on dental alloys studied by ultraviolet photoelectron spectroscopy

The valence electronic structures of the dental alloys, type 1, type 3, K14, and MC12 and their interaction with L-cysteine have been studied by ultraviolet photoelectron spectroscopy with synchrotron radiation. It was found that the electronic structures of the type-1 and type-3 dental alloys are similar to that of polycrystalline Au, while that of the K14 dental alloy is much affected by Cu. The electronic states of the MC12 dental alloy originate dominantly from Cu 3d states and Pd 4d states around the top of the valence bands, while the 4~7-eV electronic structure of MC12 originates from the Ag 4d states. The peak shift and the change in shape due to alloying are observed in all the dental alloys. For the L-cysteine thin films, new peak or structure observed around 2 eV on all the dental alloys is suggested to be due to the bonding of S 3sp orbitals with the dental alloy surfaces. The Cu-S bond as well as the Au-S and Au-O bonds may cause the change in the electronic structure of the L-cysteine on type 1, type 3 and K14. For MC12, the interaction with L-cysteine may be dominantly due to the Pd-S, Cu-S, and Ag-O bonds, while the contribution of the Ag-S bond is small.

First-principle modeling of gold adsorption on BeO (0001)

Gold nanowire chains are considered a good candidate for nanoelectronic devices since they exhibit remarkable structural and electrical properties. For practical engineering devices, α-wurtzite BeO may be a useful platform for supporting these nanowires, since the atom separation of the BeO (0001) surface is compatible with the Au–Au atom spacing. However, its influence on the nanowire conductivity is unknown. Here, ab initio simulations of adsorption of one gold atom on cleaved BeO (0001) surfaces have been performed to find the most favorable adsorption site. An attractive adsorption was obtained for all the studied sites, but the most favorable site was above the oxygen for the O-terminated surface and above the Be–Be bridge for the Be-terminated surface. A relatively high electron density is observed in the AuO and AuBe bonds, and the local density of states exhibits high peaks in the vicinity of the Fermi energy.

ZnO–Au nanohybrids by rapid microwave-assisted synthesis for CO oxidation

We present a green method for the synthesis of ZnO-Au hybrids using an ultrafast microwave-based technique. This method provides good control over the nucleation of the metal nanoparticles on the oxide support, which governs the morphology and microstructure of the hybrids. The hybrids exhibit good catalytic activity for CO oxidation compared to similar hybrids reported in the literature. Detailed XPS investigation reveals the presence of Au-Zn and Au-O bonds at the interface. This surface doping leads to the formation of anionic and cationic Au sites that contribute to the enhanced activity. Our method is general and can be applied for designing other supported catalysts with controlled interfaces.

Effect of Gold Oxide in Measurements of Colloidal Force

Atomic force microscopy, contact-angle, and spectroscopic ellipsometry measurements were employed to investigate the presence and properties of gold oxide on the surface of gold metal. It was found that, in agreement with available literature, unoxidized gold surfaces were hydrophobic, whereas oxidation rendered the surface highly hydrophilic. The oxide could be removed with ethanol or base but appeared to be stable over long periods in water or salt solutions between pH 3 and 7. After oxidation, the oxide layer thickness, determined using ellipsometry, was consistent with an approximate monolayer of Au-O bonds at the gold surface. The presence of gold oxide was found to alter significantly the electrical double-layer characteristics of the gold surface below pH 6 and may explain the apparent inconsistencies in observed force behavior where gold is employed as well as aiding in design of future microfluidic systems which incorporate gold as a coating.

Structure and stability of Au rods onTiO2(110)surfaces by first-principles calculations

In order to examine atomic and electronic structures of the perimeter of Au-particle/${\text{TiO}}_{2}$ systems related to the catalytic activity, we perform density-functional theory calculations of Au-rod/${\text{TiO}}_{2}(110)$ models with the relationship of $\text{Au}(111)[0\char21{}11]\ensuremath{\parallel}{\text{TiO}}_{2}(110)[001]$ observed by electron microscope. We deal with models of different interface stoichiometry with various rigid-body translations. For the stoichiometric systems, stable configurations have Au-O bonds between edge-Au and bridging-O atoms at the perimeter. For the Ti-rich systems with bridging-O atoms removed under a Au rod, rather strong Au-Ti bonds with covalent nature are formed and appear at the perimeter as well as weak Au-O bonds. For the O-rich systems formed by digging a pit under a Au rod, quite strong Au-O bonds with significant $\text{Au}\text{ }5d\text{-O}\text{ }2p$ hybridization and charge transfer like gold-oxides are formed and appear at the perimeter as well as rather weak Au-O bonds. About the relative stability among the systems with different stoichiometry, ab initio atomistic thermodynamics has shown that the O-rich $\text{Au}/{\text{TiO}}_{2}$ systems can be the most stable in usual atmosphere of the catalyst formation, indicating the importance of the perimeter of O-rich interfaces.

Oxidative Addition of Small Molecules to a Dinuclear Au(I) Amidinate Complex, Au2[(2,6-Me2Ph)2N2CH]2. Syntheses and Characterization of Au(II) Amidinate Complexes Including One Which Possesses Au(II)−Oxygen Bonds

The dinuclear Au(I) amidinate complex Au2(2,6-Me2Ph-form)2 (1) is isolated in quantitative yield by the reaction of (THT)AuCl and the potassium salt of 2,6-Me2Ph-form in a 1:1 stoichiometric ratio. Various reagents such as Cl2, Br2, I2, CH3I, and benzoyl peroxide add to the dinuclear Au(I)amidinate complex Au2(2,6-Me2Ph-form)2 to form oxidative-addition Au(II) metal-metal-bonded complexes 2, 3, 4, 5, and 6. The Au(II) amidinate complexes are stable as solids at room temperature. The structures of the dinuclear Au2(2,6-Me2Ph-form)2 and the Au(II) oxidative-addition products Au2(2,6-Me2Ph-form)2X2, X=Cl, Br, I, are reported. Crystalline products with an equal amount of oxidized and unoxidized complexes in the same unit cell, [Au2(2,6-Me2Ph-form)2X2][Au2(2,6-Me2Ph-form)2], X=Cl, 2m, or Br, 3m, are isolated and their structures are presented. The structure of [Au2(2,6-Me2Ph-form)2X2][Au2(2,6-Me2Ph-form)2], X=Cl has a Au(II)-Au(II) distance slightly longer, 0.05A, than that observed in the fully oxidized product Au2(2,6-Me2-form)2Cl2, 2. The gold-gold distance in the dinuclear complex decreases upon oxidative addition with halogens from 2.7 to 2.5 A, similar to observations made with the Au(I) dithiolates and ylides. The oxidative addition of benzoyl peroxide leads to the isolation of the first stable dinuclear Au(II) nitrogen complex possessing Au-O bonds, Au2(2,6-Me2Ph-form)2(PhCOO)2, 6, with the shortest Au-Au distance known for Au(II) amidinate complexes, 2.48 A. The structure consists of unidentate benzoate units linked through oxygen to the Au(II) centers. The replacement of the bromide in 3 by chloride, and the benzoate groups in 6 by chloride or bromide also occurs readily. The unit cell dimensions are, for 1, a=7.354(6) A, b=9.661(7) A, c=11.421(10) A, alpha=81.74(5) degrees, beta=71.23(5) degrees, and gamma=86.07(9) degrees (space group P, Z=1), for 2.1.5C6H12, a=11.012(2) A, b=18.464(4) A, c=19.467(4) A, alpha=90 degrees, beta=94.86(3) degrees, and gamma=90 degrees (space group P21/c, Z=4), for 2m.ClCH2CH2Cl, a=16.597(3) A, b=10.606(2) A, c=19.809(3) A, alpha=90 degrees, beta=94.155(6) degrees, and gamma=90 degrees (space group P21/n, Z=2), for 3m, a=16.967(3) A, b=10.783(2) A, c=20.060(4) A, alpha=90 degrees, beta=93.77(3) degrees, and gamma=90 degrees (space group P21/n, Z=2), for 4.THF, a=8.0611(12) A, b=10.956(16) A, c=11.352(17) A, alpha=84.815(2) degrees, beta=78.352(2) degrees, and gamma=88.577(2) degrees (space group P, Z=1), for 5, a=16.688 A, b=10.672(4) A, c=19.953(7) A, alpha=90.00 (6) degrees, beta=94.565(7) degrees, and gamma=90.00 degrees (space group P21/n, Z=4), for 6.0.5C7H8, a=11.160(3) A, b=12.112(3) A, c=12.364(3) A, alpha=115.168(4) degrees, beta=161.112(4) degrees, and gamma=106.253(5) degrees (space group P, Z=1).

Density functional study of the interaction between small Au clusters, Aun (n=1–7) and the rutile TiO2 surface. I. Adsorption on the stoichiometric surface

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.

Role ofAu+in Supporting and ActivatingAu7onTiO2(110)

The adhesion properties and catalytic activity of rutile TiO(2)(110)-supported Au(7) nanoclusters in different oxidation states are investigated by means of density functional theory. The calculations cover both surface science conditions of reduced TiO(2) and real catalyst conditions of oxidized (alkaline) TiO(2) supports. Large adhesion energies of Au(7) are found only when modeling real catalysts where the cluster becomes cationic with Au(+) ions in Au-O or Au-OH bonds. The full catalytic cycle for oxidation of CO by O(2) over Au(7) on alkaline TiO(2)(110) is calculated and found to involve only small activation barriers. In the presence of the CO reductant, the Au(+) sites are capable of cycling between bonding of atomic and molecular oxygen. We confirm our findings by comparison of calculated and experimental infrared stretch frequency data for adsorbed CO.

Partial Oxidation of Propylene to Propylene Oxide over a Neutral Gold Trimer in the Gas Phase: A Density Functional Theory Study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.

Synthesis and Characterization of Gold(III) Halide Complexes of Quinaldic Acid (Hqd), Methyl Quinaldate and Ethyl Quinaldate, and X-Ray Structure of [(HQd)2H] [AuBr4].H2O

Complexes of the types HAuX4.2HQd, where X = Cl or Br and HQd = quinaldic acid, and AuX3L2, where L is methyl or ethyl quinaldate , have been prepared and characterized. Quinaldic acid as well as methyl and ethyl quinaldates function as monodentate ligands in these complexes, whose stereochemistries are discussed in relation to the number of gold-halogen stretching frequencies observed in their far-infrared spectra. The measured conductivities of these complexes are also discussed. Single-crystal X-ray analysis of monohydrated HAuBr4.2HQd revealed that it should be formulated as [( HQd )2H][AuBr4].H2O, in which a pair of zwitterionic HQd moieties are connected by a strong O...H...O hydrogen bond, and the gold(III) atom is in an elongated octahedral coordination environment with two long Au-O bonds of 3.388(8) and 3.440(8)Ǻ.