Home About us Contact
|
Home About us Contact | ||
|
|
||
Adsorbed Molecules (adsorbed + molecule)
Selected AbstractsUnderstanding Macroscopic Diffusion of Adsorbed Molecules in Crystalline Nanoporous Materials via Atomistic SimulationsCHEMINFORM, Issue 37 2006David S. Sholl Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Adsorption of water in mordenite,An ab initio studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2001Th. Demuth Abstract The adsorption of water in mordenite has been investigated using density functional theory using gradient corrections to the exchange,correlation functional. In the neutral complex water is strongly physisorbed through two different hydrogen bonds, the stronger between the acid site and the water oxygen atom, the weaker between a hydrogen atom of the adsorbed molecule and a framework oxygen atom. Strong polarizations and structural distortions of both the acid site and the molecule have been observed. To elucidate the question if a chemisorbed complex (creation of a hydroxonium ion) is possible, ab initio molecular dynamics have been performed, indicating that a protonation of water occurs even for the low coverage of one water molecule per unit cell. However, this ionic configuration cannot be stabilized by the electrostatic field of the zeolite framework and is therefore not a minimum of the potential energy surface. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 110,116, 2001 [source] Current,Voltage Characteristics of a Homologous Series of Polycyclic Aromatic HydrocarbonsCHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2007Thilo Böhme Dr. Abstract A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D2h symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic , system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I,V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current,voltage (I,V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO,LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction. [source] Mechanisms Controlling Crystal Habits of Gold and Silver ColloidsADVANCED FUNCTIONAL MATERIALS, Issue 7 2005C. Lofton Abstract Examples of gold and silver anisotropic colloids, such as prisms and rods, have appeared in the literature for many years. In most cases, the morphologies of these thermodynamically unfavorable particles have been explained by the particular reaction environment in which they were synthesized. The mechanisms used to explain the growth generally fall into two categories, one in which chemically adsorbed molecules regulate the growth of specific crystal faces kinetically, and the other where micelle-forming surfactants physically direct the shape of the particle. This paper raises questions about the growth of anisotropic metal colloids that the current mechanisms cannot adequately address, specifically, the formation of multiple shapes in a single homogeneous reaction and the appearance of similar structures in very different synthesis schemes. These observations suggest that any growth mechanism should primarily take into consideration nucleation and kinetics, and not only thermodynamics or physical constrictions. The authors suggest an alternative mechanism where the presence and orientation of twin planes in these face-centered cubic (fcc) metals direct the shape of the growing particles. This explanation follows that used for silver halide crystals, and has the advantage of explaining particle growth in many synthesis methods. In this mechanism, twin planes generate reentrant grooves, favorable sites for the attachment of adatoms. Shape and structural data are presented for gold and silver particles synthesized using several different techniques to support this new model. Triangular prisms are suggested to contain a single twin plane which directs that growth of the initial seed in two dimensions, but limits the final size of the prism. Hexagonal platelets are suggested to contain two parallel twin planes that allow the fast growing edges to regenerate one another, allowing large sizes and aspect ratios to form. Rods and wires were found to have a fivefold symmetry, which may only allow growth in one dimension. It is expected that a superior mechanistic understanding will permit shape-selective synthesis schemes to be developed. [source] Spontaneous segregation on a hybrid chiral surfaceJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2008Szabelski Abstract Segregation of enantiomers in two-dimensional adsorbed layers is a process that is usually controlled by anisotropic directional interactions between adsorbed molecules. In this contribution, we propose a simple theoretical model in which the chiral segregation occurs even though the lateral interactions are neglected. In particular, we consider a solid surface composed of two domains with different patterns of active sites being mirror images of each other. The domains of opposite handedness represent crystal facets of a composite chiral material which are adjoined to form a heterochiral adsorbing surface. To explore equilibrium properties of the system, we use Canonical Ensemble Monte Carlo method for a square lattice. The influence of factors such as energetic properties of the surface and density of the adsorbed layer on the extent of separation is examined. The obtained results indicate that effective two-dimensional separation on the hybrid chiral surface assumed in our model can be achieved only at sufficiently low adsorbate densities. The results also suggest that the segregation on the hybrid surface would be a promising method of enantiodiscrimination for those chiral molecules which do not exhibit strong lateral interactions. © 2008 Wiley Periodicals, Inc. J Comput Chem 2008 [source] Monolayers of sulfur-containing molecules at metal surfaces as studied using SERS: 3, 3,-thiodipropionic acid and 3-mercaptopropionic acid adsorbed on silver and copperJOURNAL OF RAMAN SPECTROSCOPY, Issue 6-7 2005Andrzej Kudelski Abstract The modification of metal surfaces with self-assembled thiol monolayers is the subject of intensive studies owing to both its fundamental interest in surface chemistry and its potential technological significance. In this work, the applicability of surface-enhanced Raman scattering (SERS) to determine the conformation and pKa values of ,-terminated thiol molecules was investigated. 3, 3,-Thiodipropionic acid (TDPA) and 3-mercaptopropionic acid (MPA) monolayers were spontaneously formed on silver and copper surfaces by adsorption from TDPA and MPA solutions, respectively. The structure of the monolayers formed was determined from SERS measurements. The SERS investigations showed that molecules forming TDPA monolayers prefer to adopt a gauche conformation of the,S,C,C chains, whereas in MPA monolayers formed from 1 mM MPA aqueous solution a significant part of the adsorbed molecules adopts a trans conformation. Formation on the metal surface of an MPA monolayer with high surface coverage of MPA makes dissociation of the carboxylic groups of MPA significantly more difficult. Analogous changes in the strength of similar adsorbed compounds have been reported by other groups. In contrast to those results, we found that the formation of monolayers from TDPA increases its acidic strength even at high surface coverage when monolayers are formed from relatively concentrated TDPA solutions. Probably the interaction of carboxylic groups with the metal surface is easier for TDPA than for MPA for geometric reasons. We consider that ionization of carboxylic groups increases the strength of this interaction. Copyright © 2005 John Wiley & Sons, Ltd. [source] Structures of monolayers formed from different HS,(CH2)2,X thiols on gold, silver and copper: comparitive studies by surface-enhanced Raman scatteringJOURNAL OF RAMAN SPECTROSCOPY, Issue 11 2003Andrzej Kudelski Abstract Molecules of 2-aminoethanethiol (cysteamine, CYS), 2-mercaptoethanol (MET), 3-mercaptopropionic acid (MPA), sodium 2-mercaptoethanesulphonate (MES) and 1-propanethiol (PTH) were spontaneously chemisorbed on electrochemically activated silver, copper and gold surfaces. The structure of monolayers formed was studied with surface-enhanced Raman scattering (SERS). In CYS and MPA monolayers the relative surface concentration of gauche conformers was higher than in MET, PTH and MES monolayers. This is probably due to double bonding to the metal surface by a fraction of adsorbed MPA and CYS molecules (via the sulphur moiety and the terminal carboxylic or amino group). In CYS, MET, MPA and PTH monolayers the surface concentration of trans conformers is significantly higher on Ag than on Au or Cu. The structures of MES monolayers on Ag, Au and Cu are similar, with very high surface concentrations of trans conformers. Statistically, the wavenumbers of ,(C,S) bands of both gauche and trans conformers are the highest on Au, slightly lower on Ag and the lowest for Cu. However the positions of ,(C,S) bands are also sensitive to the other parameters (e.g. C,C,S,metal torsion angle, the overall ordering of the monolayer). Therefore, the wavenumbers of ,(C,S) bands are not good indicators of differences in the interaction between chemisorbed thiols and the metal substrates. Desorption measurements suggest that part of the adsorbed molecules is bonded significantly weaker than the rest. Thiolate monolayers on Cu decomposed relatively easily, forming a layer of copper sulphide. The strength of adsorbed MPA acid is similar on all substrates. Copyright © 2003 John Wiley & Sons, Ltd. [source] Enrichment of peptides from plasma for peptidome analysis using multiwalled carbon nanotubesJOURNAL OF SEPARATION SCIENCE, JSS, Issue 6 2007Xin Li Abstract Human plasma contains a complex matrix of proteolytically derived peptides (plasma peptidome) that may provide a correlate of biological events occurring in the entire organism. Analyzing these peptides from a small amount of serum/plasma is difficult due to the complexity of the sample and the low levels of these peptides. Here, we describe a novel peptidome analysis approach using multiwalled carbon nanotubes (MWCNTs) as an alternative adsorbent to capture endogenous peptides from human plasma. Harvested peptides were analyzed by using liquid chromatography-mass spectrometry as a means of detecting and assessing the adsorbed molecules. The improved sensitivity and resolution obtained by using liquid chromatography-mass spectrometry allowed detection of 2521 peptide features (m/z 300,1800 range) in about 50 ,L of plasma. 374 unique peptides were identified with high confidence by two-dimensional liquid chromatography system coupled to a nano-spray ionization linear ion trap-mass spectrometer. High recovery of BSA digest peptides enriched with MWCNTs, in both standard buffer and high abundance protein solution, was observed. Comparative studies showed that MWCNTs were superior to C18 and C8 for the capture of the smaller peptides. This approach could hold promise of routine plasma peptidome analysis. [source] Theoretical Simulation of Vibrational Sum-Frequency Generation Spectra from Density Functional Theory: Application to p -Nitrothiophenol and 2,4-DinitroanilineCHEMPHYSCHEM, Issue 12 2009Julien Guthmuller Dr. Abstract The molecular orientation of adsorbed molecules forming self-assembled monolayers can be determined by combining vibrational sum-frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p -nitrothiophenol and 2,4-dinitroaniline. It is found that a suitable choice of basis set as well as of exchange-correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsorbed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6-311++G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p -nitrothiophenol. Moreover, the investigated XC functionals reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree,Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors. [source] | ||||||||||