Prediction of sound velocity and compressibility via molecular simulation at fixed entropy

A molecular-level simulation route is proposed to compute the isentropic thermodynamic properties in a fluid system by Monte Carlo simulation at fixed entropy. The method involves computation of the pressure response of a system to an infinitesimal change in system density by introduction of a single molecule, while retaining the system volume as well as the absolute molar entropy. The probability for accepting a change in temperature during the Monte Carlo moves was weighted against the argument proposed by Smith et al. [W.R. Smith, M. Lísal, I. Nezbeda, Chem. Phys. Lett. 426 (2006) 436–440]. Application to fluid argon has confirmed superior accuracy for the technique within the gas state to yield results within 1.2% of the measured values for the range of thermodynamic conditions investigated.     

The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution

The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the [110] facets only in the NR, which have higher surface energy.     

Direct Synthesis of PbO Nanoparticles From a Lead(II) Nano Coordination Polymer Precursor: Synthesis,Crystal Structure,and DFT Calculations of [Pb2(dmp)2(μ‐N3)2(μ‐ClO4)2]n with the First Pb2‐(μ‐ClO4)2 Unit

Nanostructures of a new coordination polymer of divalent lead with the ligand 2, 9‐dimethyl‐1, 10‐phenanthroline (dmp) containing the first Pb₂‐(μ‐ClO₄)₂ motif, [Pb₂(dmp)₂(μ‐N₃)₂(μ‐ClO₄)₂]_n ( 1 ), was synthesized by a sonochemical method that produces the coordination polymers at nano size. The new nanostructure was characterized by scanning electron microscopy, X‐ray powder diffraction, IR, ¹H NMR and ¹³C NMR spectroscopy, and elemental analysis. Compound 1 was structurally characterized by single‐crystal X‐ray diffraction and the single‐crystal X‐ray data shows that the coordination number of Pb^II ions is six, (PbN₄O₂), with two N‐donor atoms from aza‐aromatic base ligands and four O‐donors from two perchlorate anions and two N‐donors from two azide anions. It has a “stereo‐chemically active” electron lone pair, and the coordination sphere is hemidirected. The supramolecular features in these complexes are guided and controlled by weak directional intermolecular interactions. The chains interact with each other through π–π stacking interactions creating a 3D framework. The structure of the title complex was optimized by density functional theory calculations. Calculated structural parameters and IR spectra for the title complex are in agreement with the crystal structure. The PbO nanoparticles were obtained by thermolysis of 1 at 180 °C with oleic acid as a surfactant. The average diameter of the nanoparticles was estimated by the Scherrer equation to be 23 nm. The morphology and size of the prepared PbO samples were further observed using SEM.     

Surface modification and wear test of carbon steel by plasma electrolytic nitrocarburizing

Plasma electrolytic nitrocarburizing (PEN/C) was applied to the surface of carbon steel under the boiling condition of saturated urea electrolyte. In addition to the general effect of the bath temperature, different applied voltages and processing times were also considered in this new process. Optical and scanning electron microscopy, X‐ray diffraction, microhardness and pin‐on‐disc wear tests were used to characterize the PEN/C‐treated surfaces. A mixture of θ‐(Fe₃C) and ε‐(Fe_2–3N) was found in the compound layers. At certain conditions, dense surface layers with minimum porosity were observed at the top of the samples. The boiling condition resulted in special character of the compound layers on the surface. The layers consisted of some irregularities grown inward the samples andaffected the characteristics of the surface layers. The microhardness of the PEN/C‐treated layers increased up to 1280 HV0.1, which was 3 to 4 times higher than that for untreated material and higher than that obtained by other investigators (750 HV0.1). PEN/C decreased the wear loss of carbon steel significantly due to the change of the adhesive wear of untreated material to the abrasive mode of treated surfaces. The major advantage of this technique was a higher growth rate of the nitrocarburized layers and a more significant improvement in the tribological performance of the treated samples if compared to similarly oriented surface treatments. Copyright © 2011 John Wiley & Sons, Ltd.     

Bulk versus interfacial aqueous solvation of dicarboxylate dianions

Solvation of dicarboxylate dianions of varying length of the aliphatic chain in water clusters and in extended aqueous slabs was investigated using photoelectron spectroscopy and molecular dynamics simulations. Photoelectron spectra of hydrated succinate, adipate, and tetradecandioic dianions with up to 20 water molecules were obtained. Even-odd effects were observed as a result of the alternate solvation mode of the two negative charges with increasing solvent numbers. The competition between hydrophilic interactions of the charged carboxylate groups and hydrophobic interactions of the aliphatic chain leads to conformation changes in large water clusters containing dicarboxylates bigger than adipate. It also leads to a transition from bulk aqueous solvation of small dicarboxylates to solvation at the water/vapor interface of the larger ones. Whereas oxalate and adipate solvate in the inner parts of the aqueous slab, suberate and longer dicarboxylate dianions have a strong propensity to the surface. This transition also has consequences for the folding of the flexible aliphatic chain and for the structure of aqueous solvation shells around the dianions.     

A novel method for the one-pot conversion of carboxylic acids to N,N-dimethylamides

A simple, efficient, and new method has been developed for the preparation of N,N-dimethylamides from carboxylic acids. As described below, treatment of a variety of aromatic carboxylic acids with N,N-dimethylsulfamoyl imidazole or N,N-dimethylsulfamoyl chloride in the presence of a mixture of methanesulfonic acid/phosphorus pentoxide (2:1, v/w) proceeded effectively to afford the corresponding N,N-dimethylamides in moderate to good yields. This method is easy, rapid, and good yielding for the synthesis of N,N-dimethylamides from carboxylic acids.     

Host–Guest Assembly of a Molecular Reporter with Chiral Cyanohydrins for Assignment of Absolute Stereochemistry

The absolute stereochemistry of cyanohydrins, derived from ketones and aldehydes, is obtained routinely, in a microscale and derivatization‐free manner, upon their complexation with Zn‐MAPOL, a zincated porphyrin host with a binding pocket comprised of a biphenol core. The host–guest complex leads to observable exciton‐coupled circular dichroism (ECCD), the sign of which is easily correlated to the absolute stereochemistry of the bound cyanohydrin. A working model, based on the ECCD signal of cyanohydrins with known configuration, is proposed.