Alexander Goldberg

DFT study of hydrogen adsorption on Al13 clusters

In this work, adsorption of hydrogen on Al13 clusters has been investigated theoretically using the density functional theory (DFT) approach. We have performed geometry optimization of atomic and molecular hydrogen in the proximity of Al13 and calculated the binding energy and electronic properties of the stable Al13+Hn assemblies. We have also calculated the energy barrier for the hydrogen atom transition between different adsorption sites on Al13 cluster as well as the activation energy for the dissociation and adsorption of molecular hydrogen. We found that the hydrogen atom adsorbs on the surface of Al13 cluster, without an energy barrier onto atop, bridge and hollow sites. A small barrier for H atom transition from one adsorption site to another together with the minor energy difference between the most stable isomers points towards high mobility of the hydrogen atom on the surface. The calculated dissociation–adsorption barrier for the hydrogen molecule of ∼14u2009kcal/mol and a desorption barrier of ∼19u2009kcal/mol together with a high theoretical storage capacity of Al13 clusters suggest further investigations of Al nanostructures for application in hydrogen storage.

Taurolidine Antiadhesive Properties on Interaction with E. coli; Its Transformation in Biological Environment and Interaction with Bacteria Cell Wall

The taurine amino-acid derivative, taurolidine, bis-(1,1-dioxoperhydro-1,2,4-thiabiazinyl–4)methane, shows broad antibacterial action against gram-positive and gram-negative bacteria, mycobacteria and some clinically relevant fungi. It inhibits, in vitro, the adherence of Escherichia coli and Staphylococcus aureus to human epithelial and fibroblast cells. Taurolidine is unstable in aqueous solution and breaks down into derivatives which are thought to be responsible for the biological activity. To understand the taurolidine antibacterial mechanism of action, we provide the experimental single crystal X-ray diffraction results together with theoretical methods to characterize the hydrolysis/decomposition reactions of taurolidine. The crystal structure features two independent molecules linked through intermolecular H-bonds with one of them somewhat positively charged. Taurolidine in a biological environment exists in equilibrium with taurultam derivatives and this is described theoretically as a 2-step process without an energy barrier: formation of cationic taurolidine followed by a nucleophilic attack of O(hydroxyl) on the exocyclic C(methylene). A concerted mechanism describes the further hydrolysis of the taurolidine derivative methylol-taurultam. The interaction of methylol-taurultam with the diaminopimelic NH2 group in the E. coli bacteria cell wall (peptidoglycan) has a negative ΔG value (−38.2 kcal/mol) but a high energy barrier (45.8 kcal/mol) suggesting no reactivity. On the contrary, taurolidine docking into E. coli fimbriae protein, responsible for bacteria adhesion to the bladder epithelium, shows it has higher affinity than mannose (the natural substrate), whereas methylol-taurultam and taurultam are less tightly bound. Since taurolidine is readily available because it is administered in high doses after peritonitis surgery, it may successfully compete with mannose explaining its effectiveness against bacterial infections at laparoscopic lesions.

New Insight into Differences in Cycling Behaviors of a Lithium-ion Battery Cell Between the Ethylene Carbonate- and Propylene Carbonate-Based Electrolytes

Density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations have been performed to gain insight into the difference in cycling behaviors between the ethylene carbonate (EC)-based and the propylene carbonate (PC)-based electrolytes in lithium-ion battery cells. DFT calculations for the ternary graphite intercalation compounds (Li+(S)iCn: S=EC or PC), in which the solvated lithium ion Li+(S)i (i=1~3) was inserted into a graphite cell, suggested that Li+(EC)iCn was more stable than Li+(PC)iCn in general. In addition, MD simulations were carried out to examine the solvation structures at a high salt concentration: 2.45 mol kg-1. The results showed that the solvation structure was significantly interrupted by the counter anions, having a smaller solvation number than that at a lower salt concentration (0.83 mol kg-1). The results from both DFT calculations and MD simulations are consistent with the recent experimental observations.

Crystal and molecular structure of manganese(II) lapacholate, a novel polymeric species undergoing temperature-reversible metal to ligand electron transfer.

Lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphtoquinone) (HLap, C(15)H(14)O(3)) reacts with Mn(2+) producing a novel polymeric complex with formula: [Mn(Lap)(2)](n). Two ligands chelate the metal through their ortho oxygen (O1, O2) moiety while two para oxygens, from other Lap ligands, complete the octahedral coordination sphere. Thus far, all reported Lap metal complexes are mononuclear, lack the metal-trans-quinonic (para) oxygen binding and have Lap as a bidentate ligand. Synthesis, X-ray diffraction, IR, and UV-visible spectroscopic properties, thermogravimetric analysis, and differential thermal analysis of this complex are reported along with a density functional theory study describing electron transfer from the Mn to the Lap ligand at low temperature. X-ray structure determinations at 125, 197, and 300 K describe the progressive trend of a Mn contribution to the Mn-O1 bond length as a function of T. The Mn-O1 bond distance increases with temperature and may be therefore associated with a semiquinonate action at low T by the carbonyl O1 donor (and corresponding to Mn(III)). It transforms to a more classical coordinative bond at room T and stabilizes a Mn(II) species; this is a reversible phenomenon involving Mn(II)-Mn(III) oxidation states.

Modelling of poly(4-vinyl pyridine) and poly(4-vinyl pyridine)/pyridine composites: structural and optical properties

The interactions of the polymer poly(4-vinyl pyridine) moieties with free pyridine molecules in concentrated solution develop protonated and hydrogen-bonded species on the polymer backbone and turn the viscous solution to gel. Direct irradiation at proton transfer centre on the protonated polymer moiety promotes an amorphous-to-crystalline transition. The polymer crystals exhibit completely different optical properties when compared to the amorphous material. The proposed mechanism of the photoinduced crystallisation is the following: direct excitation to the proton transfer centre generates in abundance protonated polymer moieties, which have rigid quinone structure. Rigid quinone conformations stimulate the crystallisation of the polymer chains; in their turn, increasing polymer ordering stabilises the photoinduced protonated species. Photoinduced phase transition is reversible, meaning, that crystalline phase is metastable. To clarify the mechanism of the phase transition, in the present issue, using molecular modelling, we investigate the conformational behaviour of the polymer species depending on the state of protonation, interaction with adjacent solvent molecules and polymer side-chain units. The Density Functional Theory (DFT) calculations show the protonated pyridine moiety as a quinone structure that is clearly stable, thus emphasising the ability of such structure to play a key role as a ‘working’ species.