Showing papers in "Journal of Physical Chemistry B in 1998"

All-atom empirical potential for molecular modeling and dynamics studies of proteins.

Abstract: New protein parameters are reported for the all-atom empirical energy function in the CHARMM program. The parameter evaluation was based on a self-consistent approach designed to achieve a balance between the internal (bonding) and interaction (nonbonding) terms of the force field and among the solvent−solvent, solvent−solute, and solute−solute interactions. Optimization of the internal parameters used experimental gas-phase geometries, vibrational spectra, and torsional energy surfaces supplemented with ab initio results. The peptide backbone bonding parameters were optimized with respect to data for N-methylacetamide and the alanine dipeptide. The interaction parameters, particularly the atomic charges, were determined by fitting ab initio interaction energies and geometries of complexes between water and model compounds that represented the backbone and the various side chains. In addition, dipole moments, experimental heats and free energies of vaporization, solvation and sublimation, molecular volume...

COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds

Abstract: A general all-atom force field for atomistic simulation of common organic molecules, inorganic small molecules, and polymers was developed using state-of-the-art ab initio and empirical parametrization techniques. The valence parameters and atomic partial charges were derived by fitting to ab initio data, and the van der Waals (vdW) parameters were derived by conducting MD simulations of molecular liquids and fitting the simulated cohesive energies and equilibrium densities to experimental data. The combined parametrization procedure significantly improves the quality of a general force field. Validation studies based on large number of isolated molecules, molecular liquids and molecular crystals, representing 28 molecular classes, show that the present force field enables accurate and simultaneous prediction of structural, conformational, vibrational, and thermophysical properties for a broad range of molecules in isolation and in condensed phases. Detailed results of the parametrization and validation f...

Structure of Graphite Oxide Revisited

Abstract: Graphite oxide (GO) and its derivatives have been studied using 13C and 1H NMR. NMR spectra of GO derivatives confirm the assignment of the 70 ppm line to C−OH groups and allow us to propose a new structural model for GO. Thus we assign the 60 ppm line to epoxide groups (1,2-ethers) and not to 1,3-ethers, as suggested earlier, and the 130 ppm line to aromatic entities and conjugated double bonds. GO contains two kinds of regions: aromatic regions with unoxidized benzene rings and regions with aliphatic six-membered rings. The relative size of the two regions depends on the degree of oxidation. The carbon grid is nearly flat; only the carbons attached to OH groups have a slightly distorted tetrahedral configuration, resulting in some wrinkling of the layers. The formation of phenol (or aromatic diol) groups during deoxygenation indicates that the epoxide and the C−OH groups are very close to one another. The distribution of functional groups in every oxidized aromatic ring need not be identical, and both ...

Transferable Potentials for Phase Equilibria. 1. United-Atom Description of n-Alkanes

Abstract: A new set of united-atom Lennard-Jones interaction parameters for n-alkanes is proposed from fitting to critical temperatures and saturated liquid densities. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapor−liquid coexistence curves for methane to dodecane using three united-atom force fields: OPLS [Jorgensen, et al. J. Am. Chem. Soc. 1984, 106, 813], SKS [Siepmann, et al. Nature 1993, 365, 330], and TraPPE. Standard specific densities and the high-pressure equation-of-state for the transferable potentials for phase equilibria (TraPPE) model were studied by simulations in the isobaric−isothermal and canonical ensembles, respectively. It is found that one set of methyl and methylene parameters is sufficient to accurately describe the fluid phases of all n-alkanes with two or more carbon atoms. Whereas other n-alkane force fields employ methyl groups that are either equal or larger in size than the methylene groups, it is demonstrated here that usin...

Role of Particle Size in Nanocrystalline TiO2-Based Photocatalysts

Abstract: This paper presents a systematic study on the role of particle size in pure and doped nanocrystalline TiO2 photocatalysts, which was made possible by a versatile wet-chemical process capable of generating near-agglomeration-free TiO2 with well-controlled particle sizes and dopant dispersion. It is shown that particle size is a crucial factor in the dynamics of the electron/hole recombination process. For TiO2 particles with 6 or 11 nm diameter, Fe3+ dopants were added to inhibit the charge carrier surface recombination. The optimal Fe3+ dopant concentration for different particle sizes was identified, and this concentration was found to decrease with increasing particle size. To assist electron and hole separation in TiO2 with 21 nm diameter, Nb5+ dopants were introduced in combination with minor surface Pt dispersion. These carefully engineered nanocrystalline TiO2 catalysts showed higher reactivities than Degussa P25 TiO2 material in photocatalytic decomposition of chloroform.

Molecular Conformation in Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold and Silver Surfaces Determines Their Ability To Resist Protein Adsorption

Abstract: We report data from infrared absorption (FTIR) and X-ray photoelectron spectroscopies that correlate the molecular conformation of oligo(ethylene glycol) (OEG)-terminated self-assembled alkanethiolate monolayers (SAMs) with the ability of these films to resist protein adsorption. We studied three different SAMs of alkanethiolates on both evaporated Au and Ag surfaces. The SAMs were formed from substituted 1-undecanethiols with either a hydroxyl-terminated hexa(ethylene glycol) (EG6-OH) or a methoxy-terminated tri(ethylene glycol) (EG3-OMe) end group, or a substituted 1-tridecanethiol chain with a methoxy-terminated tri(ethylene glycol) end group and a −CH2OCH3 side chain at the C-12 atom (EG[3,1]-OMe). The infrared data of EG6-OH-terminated SAMs on both Au and Ag surfaces reveal the presence of a crystalline helical OEG phase, coexisting with amorphous OEG moieties; the EG[3,1]-OMe-terminated alkanethiolates on Au and Ag show a lower absolute coverage and greater disorder than the two other compounds. The...

Synthesis and Growth of ZnO Nanoparticles

Abstract: ZnO nanoparticles in the size range from 2 to 7 nm were prepared by addition of LiOH to an ethanolic zinc acetate solution. This method [Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826] was modified and extended at several points. The synthesis of very small ZnO nuclei was simplified. It was found that aging of particles was governed by temperature, the water content, and the presence of reaction products. Water and acetate induced considerably accelerated particle growth. Growth could almost be stopped by removal of these species (“washing”). Washing consisted of repeated precipitation of ZnO by addition of alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. The aging characteristics are interpreted in terms of the concentration of dissolved ZnII species and reactions well-known in sol−gel chemistry. These findings present a better-defined and more versatile procedure for production of clean ZnO sols of readily adjustable particle size. Such sols are of partic...

Structure of the amide i band of peptides measured by femtosecond nonlinear-infrared spectroscopy

Abstract: Femtosecond infrared (IR) pump probe and dynamic hole burning experiments were used to examine the ultrafast response of the modes in the 1600−1700 cm-1 region (the so-called amide I modes) of N-methylacetamide (NMA) and three small globular peptides, apamin, scyllatoxin, and bovine pancreatic trypsin inhibitor (BPTI). A value of 16 cm-1 was found for the anharmonicity of the amide I vibration. Vibrational relaxation of the amide I modes of all investigated peptides occurs in ca. 1.2 ps. An even faster value of 450 fs is obtained for NMA, a model for the peptide unit. The vibrational relaxation is dominated by intramolecular energy redistribution (IVR) and reflects an intrinsic property of the peptide group in any environment. Dynamic hole burning experiments with a narrow band pump pulse which selectively excites only a subset of the amide I eigenstates reveal that energy migration between different amide I states is slow compared with vibrational relaxation. Two-dimensional pump−probe (2D-IR) spectra th...

Photoassisted Degradation of Dye Pollutants. V. Self-Photosensitized Oxidative Transformation of Rhodamine B under Visible Light Irradiation in Aqueous TiO2 Dispersions

Abstract: Chemical oxygen demand (CODCr) and proton NMR, UV−vis, and spin trapping EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions. Both de-ethylation and oxidative degradation take place with the former proceeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-ethylated rhodamine species. Intermediates present after each de-ethylation step remain in a fast dynamic equilibrium between the titania particle surface and the bulk solution. The concentration of •OH radicals, formed from the inverse photosensitization process through the superoxide radical anion, increases upon addition of the anionic dodecylbenzene sulfonate surfactant (DBS) because a larger number of RhB excited states are able to inject an electron into the conduction band of the TiO2 particles. Also, intermediates that can no longer absorb the visible light, (i.e., once the dye solution ...

Solvent Structure, Dynamics, and Ion Mobility in Aqueous Solutions at 25 °C

Abstract: We calculate the mobilities ui of the metal cations Li+, Na+, K+, Rb+, Cs+, and Ca2+ and the halides F-, Cl-, Br-, and I- at infinite dilution by molecular dynamics simulation using the SPC/E model for water at 25 °C and a reaction field for the long-range interactions. The ion mobilities show the same trends as the experimental results with distinct maxima for cations and anions. The mobilities (defined by ui = Di/kT) of the corresponding uncharged species are also determined by simulation and are in qualitative agreement with Stokes' law. The mobilities of Li+, Na+, K+, Rb+ and F- increase on discharge, whereas Cl, Br, and I have smaller mobilities than the corresponding anions. The mobility of the fictitious I+ ion, which differs from I- only in its charge, lies between that of I- and I in the order uI < uI+ < uI−. The residence time of water in the first solvation shell of small cations (Li+ and Na+) and Ca2+ decreases when the ions are discharged, while the opposite is observed on neutralizing I-, su...

Hydrogen Storage in Graphite Nanofibers

Abstract: Graphite nanofibers are a novel material that is produced from the dissociation of carbon-containing gases over selected metal surfaces. The solid consists of very small graphite platelets, 30−500 ...

J- and H-aggregates of porphyrin-surfactant complexes: time-resolved fluorescence and other spectroscopic studies

Abstract: The interactions of several water-soluble ionic porphyrins with different ionic or neutral surfactants in aqueous solutions were studied as a function of surfactant concentration. The interaction leads to the formation of porphyrin aggregates and/or micelle-encapsulated monomers with the exception of those porphyrin−surfactant pairs for which the interaction is Coulombically repulsive. The premicellar surfactant−porphyrin aggregate is identified by absorption and fluorescence spectroscopy, fluorescence lifetime and anisotropy, and resonance light scattering. The spectroscopic results are used to characterize the premicellar aggregates as J-type, H-type, or nonspecific aggregates. All premicellar surfactant−porphyrin aggregates dissociate to form micelle-encapsulated monomers when the surfactant concentration approaches cmc (critical micellar concentration). The interaction of tetrakis-(4-sulfanatophenyl)porphine dianion (H4TPPS2-) at pH <3.5 with cetyltrimethylammonium cation (CTAB) is described by the fo...

Strongly Photoluminescent CdTe Nanocrystals by Proper Surface Modification

Abstract: CdTe nanoclusters were prepared in aqueous solution by the reaction between Cd2+ and NaHTe in the presence of thioglycolic acid. Under reflux, the clusters start to crystallize and show a narrow band emission. The photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution. The maximum quantum yield at room temperature is approximately 18% when the pH value of the CdTe solution is brought to 4.5 by using thioglycolic acid. The optical spectroscopy studies imply that the pH-dependent behavior of the CdTe nanocrystals' fluorescence is caused by structural changes on the surface rather than the size of the nanocrystals. Systematic absorption and fluorescence studies on dialyzed samples suggest that in the acidic range a shell of cadmium thiol complexes is formed around the CdTe core. Thus, the fluorescence quantum yield is enhanced dramatically when the solution is made acidic. In contrast, such a shell can also be produced in the alkaline range, but only afte...

Calculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube Method

Abstract: A practical method for accurate evaluation of the Coulombic contribution to the electronic coupling for energy transfer at any donor−acceptor separation is reported. The method involves the exact interaction between transition densities of each chromophore which are calculated ab initio and may include electron correlation. The method is used to calculate coupling strengths between the pigments of the bacterial light-harvesting complex, LH2, and to compare with results using the ideal dipole approximation (IDA). The results suggest that the relatively symmetric transitions of bacteriochlorophyll a (Bchla) pigments are reasonably well described by the IDA for separations >15 A, although deviations are significant at smaller separations. The less symmetric transition of the twisted carotenoid pigment is rather poorly described by the IDA and shows significant deviation even at separations of well over 20 A. The calculated coupling strengths are combined with estimates of the spectral overlap integral to est...

Isolation and Selected Properties of a 10.4 kDa Gold:Glutathione Cluster Compound

Abstract: An unprecedented small thioaurite cluster compound (with metallic Au0 core) has been isolated in high yield by decomposition of polymeric Au(I)SG compounds, where GSH is the ubiquitous tripeptide glutathione, N-γ-glutamyl-cysteinyl-glycine The Au:SG clusters appear to share the high stability and robustness of their hydrophobic n-alkyl analogues but are highly water soluble The most abundant cluster produced by these methods can be easily separated from its homologues by gel electrophoresis Its total molecular weight is ca 104 kDa, and the mass of its strongly bound inorganic core is 56 kDa, suggesting the composition Au28(SG)16 This composition is also consistent with the X-ray diffraction pattern of the crystalline molecular solid Distinct features in the optical absorption spectroscopy are inherently different from either larger clusters or smaller gold cluster compounds The compound is optically active, as evidenced by circular dichroism in the near-IR, visible, and near-UV regions The 13C N

Picosecond Dynamics of Silver Nanoclusters. Photoejection of Electrons and Fragmentation

Abstract: Silver colloids of particle diameter 40−60 nm have been synthesized using a chemical reduction method in aqueous medium. These nanoclusters are photoactive and exhibit transient bleaching in the 400−500 nm region followed by a strong absorption in the visible−near-infrared region when subjected to 355 nm laser-pulse excitation. The transient bleaching of the surface plasmon absorption band is a monophotonic process, while the absorption growth in the red region is a biphotonic process arising from the photoejection of electrons. The 40−60 nm clusters were observed to break up into smaller clusters (5−20 nm) with 355 nm laser-pulse excitation. The choice of excitation wavelength provides the size selectivity in the fragmentation of the clusters. For example, when the excitation wavelength was switched to 532 nm, only larger (or irregularly shaped) particles were found to break up.

Complex Nature of the UV and Visible Fluorescence of Colloidal ZnO Nanoparticles

Abstract: Time-resolved spectroscopy of colloidal ZnO nanoparticles has been carried out with a laser excitation at 248 nm. UV and visible fluorescence has been analyzed. Except for the known band gap 370-nm and impurity 510-nm emissions, we have found additional continua at ∼300, 430, and 545 nm. These continua were developed in solutions of different composition, with excess Zn2+ and OH- ions, and in function of time. All but 510-nm fluorescence bands exhibit short nanosecond or subnanosecond decays. The green fluorescence at 510 nm originating from 4-nm particles in Zn2+-rich solutions is a much longer-lived, 1.0 μs. The band natures are discussed. Cluster size distribution and growth kinetics have been recovered from spectral measurements.

Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals

Abstract: Relatively monodisperse, highly luminescent zinc blende ZnSe nanocrystals are synthesized in a hexadecylamine/trioctylphosphine coordination solvent. The controlled growth process affords tunable sample sizes. Pure band-edge fluorescence size-tunable between 2.8 and 3.4 eV is obtained at room temperature with quantum yields between 20% and 50% relative to Stilbene 420.

Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes

Abstract: We have investigated the storage capability of microporous carbon materials for gaseous hydrogen both theoretically and experimentally. In the grand canonical Monte Carlo calculation the hydrogen molecules are physisorbed by van der Waals interactions with the surface atoms of carbon slitpores and carbon nanotubes. At room temperature the optimum pore geometry is a slitpore consisting of two graphite platelets separated by a distance that corresponds approximately to two times the diameter of a hydrogen molecule. In this case for a storage pressure of 10 MPa a maximum adsorbed hydrogen density of 14 kg/m3 can be reached, which corresponds to a gravimetric storage capacity of 1.3 wt %. Only for low gas pressure a cylindrical geometry like that in carbon nanotubes can exceed the storage density of carbon slitpores owing to capillary forces.

Surface Structure and Morphology of Calcium Carbonate Polymorphs Calcite, Aragonite, and Vaterite: An Atomistic Approach

Abstract: Atomistic simulation techniques have been employed to investigate the effect of molecular adsorption of water on the low-index surfaces of calcite, aragonite, and vaterite. Calculated surface and hydration energies agree with experiment and previous calculations where available. Known experimental surface features are reproduced, i.e., 1 × 1 symmetry and structural features of the calcite {1014} surface and bulk termination of the {1011} and {1120} surfaces. Surface carbonate groups tend to rotate to lie flat in the surface. The morphologies of the hydrated crystals agree with experimentally found morphologies. The bulk lattice energies of the polymorphs reflect their thermodynamic stability.

Ultrafast Studies of Photoexcited Electron Dynamics in γ- and α-Fe2O3 Semiconductor Nanoparticles

Abstract: The ultrafast dynamics of photoexcited electrons in the semiconductor iron oxides, γ-Fe2O3 and α-Fe2O3, have been measured using femtosecond laser spectroscopy. Transmission electron microscopy shows the γ-Fe2O3 particles are spherical, with 1−2 nm average diameter, and the α-Fe2O3 are spindle-shaped, with average dimensions of 1 × 5 nm. Static electronic absorption measurements of the colloids suggest that they may be in the quantum confined regime. Steady-state emission measurements show that the <400 nm direct transitions are moderately emissive, but the indirect transitions in the visible do not produce measurable emission. The ultrafast transient absorption decay profiles measured for the γ-Fe2O3 and α-Fe2O3 samples synthesized in our lab as well as a commercial sample of γ-Fe2O3 are the same and are fit best with three exponentials with 0.36, 4.2, and 67 ps time constants. The decay profiles are independent of pump power, probe wavelength, and pH and were not affected by lattice doping with other me...

Modification of MCM-41 by Surface Silylation with Trimethylchlorosilane and Adsorption Study

Abstract: Siliceous MCM-41 samples were modified by silylation using trimethylchlorosilane (TMCS). The surface coverage of functional groups was studied systematically in this work. The role of surface silanol groups during modification was evaluated using techniques of FTIR and 29Si CP/MAS NMR. Adsorption of water and benzene on samples of various hydrophobicities was measured and compared. It was found that the maximum degree of surface attachments of trimethylsilyl (TMS) groups was about 85%, corresponding to the density of TMS groups of 1.9 per nm2. The degree of silylation is found to linearly increase with increasing pre-outgassing temperature prior to silylation. A few types of silanol groups exist on MCM-41 surfaces, among which both free and geminal ones are responsible for active silylation. Results of water adsorption show that aluminosilicate MCM-41 materials are more or less hydrophilic, giving a type IV isotherm, similar to that of nitrogen adsorption, whereas siliceous MCM-41 are hydrophobic, exhibit...

Room-temperature molten salts based on the quaternary ammonium ion

Abstract: The properties of a family of novel quaternary ammonium salts based on the bis(trifluoromethylsulfonyl)imide and triflate anions are reported. Binary phase diagrams for some of their mixtures and their electrochemical windows of stability are also reported. The highest conductivity observed in the pure salt systems at 25 °C was 7 × 10-4 S cm-1. An electrochemical window of stability of up to 5 V was measured on graphite electrodes. The effect of salt structure and solvent on conductivity of the salts is also discussed.

Assembly and Self-Organization of Silver Nanocrystal Superlattices: Ordered “Soft Spheres”

Abstract: Hydrophobic dodecanethiol-capped silver nanocrystals ranging from 50 to 80 A in diameter were synthesized using arrested growth methods. Size-selective precipitation was employed to isolate nanocrystals homogeneous in size and shape which exhibited close-packed structural order after drying on a carbon or mica substrate. Elemental analysis, H1 NMR and FTIR spectroscopies were used to characterize the compositional features of the adsorbed thiolate ligands on nanocrystals suspended in solution and condensed in nanocrystal films. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to probe the structure of both individual nanocrystals and superlattices, and to estimate the energetic interactions between these sterically stabilized particles. In terms of these measurements, the effects of the capping ligand coverage, particle faceting and shape, and interparticle attractions on superlattice formation are elucidated. In particular, the concept of nanocrystals as “soft sphe...

Measurement of Ultrafast Photoinduced Electron Transfer from Chemically Anchored Ru-Dye Molecules into Empty Electronic States in a Colloidal Anatase TiO2 Film

Abstract: Electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)bis(2,2‘-bipyridyl-4,4‘-dicarboxylate) into empty electronic states in a colloidal anatase TiO2 film was measured as a transient absorption signal of the injected hot electrons with a rise time <25 fs. Optical absorption of the anchored dye molecules led to the excited singlet state of the dye with a small admixture of charge transfer states. The electron transfer reaction reported here did not involve redistribution of vibrational excitation energy and was thus completely different from the well-known Marcus−Levich−Jortner−Gerischer type of electron transfer in the case of weak electronic interaction. It was also not a direct optical charge transfer transition from the donor to the acceptor level but rather an electron transfer reaction with an ultrashort but finite reaction time.

Application of the RESP Methodology in the Parametrization of Organic Solvents

Abstract: We present parametrizations for the nonaqueous solvents dimethyl sulfoxide, ethanol, CCl4, CHCl3, and CH2Cl2 that are compatible with the recent AMBER force field by Cornell et al. (J. Am. Chem. Soc. 1995, 117, 5179−5197). With the general procedure for generating new parameters and the RESP approach to obtain the atomic charges, we achieve flexible all-atom solvent models whose density, heat of vaporization, diffusion constant, and rotational correlation times areespecially for a generic force fieldin good agreement with available experimental data.

Wet-chemical synthesis of doped colloidal nanoparticles : yvo4:ln (ln = eu, sm, dy)

Abstract: Colloidal solutions and redispersible powders of nanocrystalline, lanthanide-doped YVO4 have been prepared via a hydrothermal method at 200 °C. High-resolution transmission electron micrographs of size-selected samples show highly crystalline particles ranging in size from about 10 to 30 nm. The particles exhibit the tetragonal zircon structure known for bulk material. Upon UV excitation of the vanadate host, the energy is transferred to the lanthanide ion and strong luminescence (f−f transitions) is observed. By analyzing line splitting and intensity pattern in the luminescence spectrum of the europium-doped sample, we are able to verify that the dopant ions enter the same lattice site as in bulk material despite the nanocrystalline nature of the sample and the low-temperature synthesis. For YVO4:Eu nanoparticles a luminescence quantum yield of 15% at room-temperature was observed.

Kinetically Controlled Growth and Shape Formation Mechanism of Platinum Nanoparticles

Abstract: Recently, we have been able to synthesize platinum colloidal nanoparticles of different shapes (Science, 1996, 272, 1924). In this report, we present transmission electron microscopic (TEM) results on the time-dependent shape distribution of platinum nanoparticles during the growth period and its dependence on the concentration of the capping polymer as well as the pH of the solution. The results suggest a shape-controlled growth mechanism in which the difference between the rate of the catalytic reduction process of Pt2+ on the {111} and {100} faces, the competition between the Pt2+ reduction and the capping process on the different nanoparticle surfaces, and the concentration-dependent buffer action of the polymer itself all control the final distribution of the different shapes observed.

Multistate Empirical Valence Bond Model for Proton Transport in Water

Abstract: A multistate empirical valence bond (MS-EVB) model for describing proton transport in aqueous systems is presented. In this approach the electrostatic interaction of the solvent water molecules with an exchange charge distribution is explicitly included in the off-diagonal elements of the EVB Hamiltonian. The MS-EVB model is parametrized to reproduce geometrical and energetic quantities of selected H3O+·(H2O)N clusters. The obtained geometries, formation energies, and energy barriers are in excellent agreement with results from high-level ab initio calculations. It is furthermore applied in a classical molecular dynamics simulation of condensed-phase water with an excess proton in order to estimate the proton-transfer rate.

Physisorption and Chemisorption of Alkanethiols and Alkyl Sulfides on Au(111)

Abstract: The energetics and kinetics of the adsorption of a variety of sulfur-containing hydrocarbons onto Au(111) have been explored using helium beam reflectivity and temperature-programmed desorption (TPD) techniques. Simple alkanethiols as well as dialkyl sulfides, dialkyl disulfides, and other sulfur-containing organics were found to adsorb with a low coverage physisorption enthalpy about 20% greater than the heat of vaporization in the bulk. In contrast to the dialkyl sulfides that only physisorb, alkanethiols and dialkyl disulfides also interact chemically with the gold with a chemisorption enthalpy of 126 kJ/mol that is independent of alkyl chain length. The presence of sterically hindering substituent groups on the carbon atom adjacent to the sulfur atom produces, however, a reduction in the chemisorption enthalpy of up to 15%. Temperature-programmed desorption of nonequilibrated, high exposure layers of alkanethiols with eight carbon atoms or longer displayed a second, higher energy, chemisorption peak a...