Ab-initio molecular dynamics based on non-local density functional procedure with Gaussian basis; study of structural and temperature behaviour of metallic clusters

An ab-initio molecular dynamics procedure without precalculation of the Born-Oppenheimer energy surface based on an iterative non-local density functional method employing Gaussian atomic basis has been developed. Analytical gradients are calculated and used for the propagation of nuclei. Sufficiently long trajectories can be calculated at an acceptable computational cost, allowing for analysis of dynamical behaviour of small metallic clusters. This is illustrated on an example of the Li₈ cluster. Temperature behaviour of different type of isomers has been investigated. Calculated power spectra allow to identify the presence of more than one isomeric forms along the given trajectories.     

Determination of chloride by stripping chronopotentiometry with silver-film electrode

The experimental parameters of cathodic stripping chronopotentiometry of chloride at a silver-film electrode are investigated and optimized. The chloride preconcentration is achieved in the form of silver chloride by a controlled potential oxidation of the working electrode under vigorous stirring. Cathodic stripping of the deposit is obtained by the constant current, under the condition of diffusive mass transfer. Deaeration of the solution is not necessary. A detection limit of 35 μ dm⁻³ (10⁻⁶ mol dm⁻³) is obtained at a deposition time of 180 s, with a reproducibility of 6.7 % (expressed as relative standard deviation, RSD).     

Oxygen adsorption on hydrated gold cluster anions: experiment and theory

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.     

1,8-bis(dimethylethyleneguanidino)naphthalene: tailoring the basicity of bisguanidine "proton sponges" by experiment and theory

1,8-bis(dimethylethyleneguanidino)naphthalene (DMEGN), the second example of a peralkyl guanidine "proton sponge" based on the 1,8-naphthalene backbone, was prepared and fully characterized. The crystal structure analysis of monoprotonated DMEGN reveals an unsymmetrical intramolecular hydrogen bridge. A decrease in the basicity with respect to the noncyclic parent 1,8-bis(tetramethylguanidino)naphthalene was found. Nevertheless, a new proton sponge provides a new crossbar in the ladder of highly basic neutral organic compounds. A detailed theoretical study of DMEGN and related cyclic guanidines explains this surprising experimental result. Homodesmotic reactions reveal that the intramolecular hydrogen bond contributes effectively 10 kcal/mol to proton affinity of DMEGN.     

Detection of DNA probes using Diels Alder cycloaddition and SERRS

A number of methods for detecting specific DNA sequences have been used to provide data for use in diagnosis of disease states and examination of gene expression. This study shows how the use of labelled oligonucleotides created by Diels Alder cycloaddition can be used as surface enhanced resonance Raman scattering (SERRS) active probes that provide easily identifiable signals at low concentrations in a mixture. The probes were produced by first tagging the oligonucleotides with a furan residue at the 5'-terminus to act as the diene. Three specifically designed benzotriazole azo maleimide dyes were then used as dienophiles to undergo cycloaddition with the furan modified oligonucleotide to generate SERRS active probes. These probes gave excellent SERRS signals from a silver-PVA film. Surface mapping of the silver PVA film indicated that the distribution of the dyes was uniform and that the number of moles of probe detected at any one time was approximately in the attomole region.     

Bis(PheOH) maleic acid amide-fumaric acid amide photoizomerization induces microsphere-to-gel fiber morphological transition: the photoinduced gelation system

The photoinduced gelation system based on 1 (non-gelling) to 2 (gelling) molecular photoisomerization in water results by microspheres (1) to gel fibers (2) transformation at the supramolecular level.     

Theoretical scrutinization of nine benzoic acid dimers: Stability and energy decomposition analysis

Aromatic carboxylic acids are able to form diverse dimers and multimers due to their hydrogen bond donor and acceptor cites, as well as the aromatic rings. In this work, we examine nine benzoic acid dimers stabilized by hydrogen bonding and stacking interactions. Interacting quantum atoms methodology revealed that dominant attractive interactions in all of them, including hydrogen bonded systems, are due to exchange-correlation. Coulomb interactions are significant only in the most stable dimer with a double hydrogen bond, although the corresponding energy term is almost two times lower compared to the nonclassical one. Since interacting quantum atoms approach treats monomers binding by considering electronic energy only, in order to examine dissociation kinetics we performed density functional theory-based molecular dynamics simulations of selected stacked dimers: in 40% of the studied systems at 300 K thermal energy was sufficient to overpower barrier for dissociation within 1 ps, which resulted in the separation of the monomers, whereas 20% of them remained in the stacked position even after 5 ps. These results highlight the importance of noncovalent interactions, particularly weak stacking interactions, on the structure and dynamics of carboxylic acids and their derivatives.     

Gels with exceptional thermal stability formed by bis(amino acid) oxalamide gelators and solvents of low polarity

Some bis (amino acid) oxalamide gelators form common thermo-reversible gels with various organic solvents but also gels of exceptional thermal stability with some solvents of medium and low polarity; the latter gels can be heated up to 50 degrees C higher temperatures than the bp of the solvent without apparent gel-to-sol transition.