Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

Determination of impurities in uranium compounds by atomic absorption

A method is described for the determination of 14 elements (Al, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn, Ni, K, Na, Zn) in uranium and uranium compounds by atomic absorption spectroscopy. A sample is dissolved in 6–8 N nitric acid, from which the uranium is selectively removed by a single extraction with tributyl phos phate. The aqueous layer is evaporated to dryness and the residue is re-dissolved in 0.2 N hydrochloric acid. Any or all of the elements can then be determined in the solution by atomic absorption spectroscopy. The limits of error in the analyses are less than 10%. Thus, the method gives about the same precision as colorimetric procedures, and it is much more precise than emission spectroscopy.     

Effect of storage on quality parameters and phenolic content of Italian extra-virgin olive oils

Abstract

The quality of extra virgin olive oils is affected mainly by hydrolytic and oxidative reactions. The present paper investigated the changes of major and minor components and oxidation indices of three monovarietal extra virgin olive oils after 18?months of storage at room temperature and in dark glass bottles conditions. After storage, the basic quality parameters such as free acidity, peroxide values, extinction coefficients, fatty acids composition, chlorophyll and carotenoid content, did not exceed the upper limits set by European Community Regulations for extra-virgin olive oils. Given the importance of the phenolic fraction, UHPLC-HESI-MS metodology was used. A decrease in 3,4-DHPEA-EDA (oleacin) and p-HPEA-EDA (oleochantal) was detected whereas, an increase of tyrosol and hydroxytyrosol was measured as a consequence of degradation of ligstroside and oleuropein derivatives. Based on the results it is possible to observe the high nutritional value of the studied oils even after 18?months of conservation.     

Solvent extraction separations with bpha. applications to the microanalysis of niobium and zirconium in uranium

A detailed study of the benzoylphenylhydroxylamine (BPHA)-chloroform-hydrochloric acid solvent extraction system with 52 elements is described with emphasis placed on extraction of the easily hydrolyzed transition metals from strong hydrochloric acid. From this study, a separation procedure for hafnium, niobium, tantalum, titanium, vanadium, and zirconium from uranium was developed, and procedures are given for the microanalysis of niobium and zirconium in uranium. Niobium and zirconium are separated from uranium by extraction into BPHA-chloroform from 10-N HCl.The separated elements are then measured colorimetrically as the niobium-4-(2-pyridylazo)resorcinol and zirconium-arsenazo III complexes. The limit of detection is 1 μg/g U.     

A universal solvent extraction-titration method for the rapid and accurate determination of uranium in complex solutions

A rapid and accurate method has been developed for the determination of uranium in complex solutions produced in the recovery of uranium from nuclear fuels. These solutions usually contain 0.1–20 g U l^-1 and high concentrations of aluminum nitrate in addition to a variety of cations and anions associated with fuel-element constituents and dissolution media. The method involves the solvent extraction of uranium from an acid-deficient aluminum nitrate-tetrapropylammonium nitrate solution into 2% tributy1 phosphate in n-amy1 acetate. The uranium is then backextracted with a concentrated phosphoric acid solution, and titrated by the method of Davies and Gray. The uranium extraction efficiency for sample solutions weighing up to 50 g is 99.9% or better, and the limit of error per analysis with 95% confidence is ±0.6%. No prior sample preparation is necessary, no expensive equipment is required, and even unskilled personnel can do duplicate analyses in 1.5 h.     

Direct identification of intramolecular disulfide links in peptides using negative ion electrospray mass spectra of underivatised peptides. A joint experimental and theoretical study

[M--H]- parent anions of underivatised peptides containing an intramolecular disulfide bridge undergo characteristic loss of the elements of H2S2, a process diagnostic of the presence of the disulfide moeity. This facile process is initiated from a side-chain enolate anion. Theoretical calculations (at the HF/6-31G(d)//AM1 level of theory) indicate that the process is exothermic with a small barrier. When the disulfide link involves a C-terminal Cys, the negative ion spectrum shows an [(M--H)--(H2S2+CO2)] fragment anion which is usually the main peak of the spectrum. This process is also directed by an enolate anion: theoretical calculations suggest a stepwise sequence with loss of CO2 preceding loss of H2S2. Both [(M--H)--H2S2] and [(M--H)--(H2S2+CO2)] anions undergo backbone cleavage allowing identification of the amino acid sequence of the peptide.     

Does the Surface Matter? Hydrogen‐Bonded Chain Formation of an Oxalic Amide Derivative in a Two‐ and Three‐Dimensional Environment

We report on a multi‐technique investigation of the supramolecular organisation of N,N‐diphenyl oxalic amide under differently dimensioned environments, namely three‐dimensional (3D) in the bulk crystal, and in two dimensions on the Ag(111) surface as well as on the reconstructed Au(111) surface. With the help of X‐ray structure analysis and scanning tunneling microscopy (STM) we find that the molecules organize in hydrogen‐bonded chains with the bonding motif qualitatively changed by the surface confinement. In two dimensions, the chains exhibit enantiomorphic order even though they consist of a racemic mixture of chiral entities. By a combination of the STM data with near‐edge X‐ray absorption fine‐structure spectroscopy, we show that the conformation of the molecule adapts such that the local registry of the functional group with the substrate is optimized while avoiding steric hindrance of the phenyl groups. In the low coverage case, the length of the chains is limited by the Au(111) reconstruction lines restricting the molecules into fcc stacked areas. A kinetic Monte Carlo simulated annealing is used to explain the selective assembly in the fcc stacked regions.     

New chemistry of olefin complexes of platinum(II) unravelled by basic conditions: synthesis and properties of elusive cationic species

The evolution in basic medium ([RO-] = 1 M in methanol, R = H or Me) of five-coordinate platinum(II) compounds, [PtCl2(eta2-C2H4)(N-N)], 2a-c, (N-N = N,N,N',N'-tetramethyl-1,2-ethanediamine, a; 2,2'-bipyridyl, b; 1,10-phenanthroline, c) leads to the formation of [PtCl(eta1-CH2CH2-OCH3)(N-N)], 5a-c. The analogous compound 5d (N-N = 2,9-dimethyl-1,10-phenanthroline, d) can also be prepared, but not via transformation of the five-coordinate species 2d in basic medium where it is quite stable. 5d can instead be prepared by reaction of d with a strongly basic methanol solution of Zeise's anion [PtCl3(eta2-C2H4)](-), 1. In such a medium the di-anionic trans-[PtCl2(OR)(eta1-CH2CH2-OCH3)](2-) species (1") reacts with to form exclusively 5d. Hydrolysis of with acids bearing weakly coordinating anions leads to [PtCl(eta2-C2H4)(N-N)]+, 3a-c, as stable cations; upon the same treatment 5d does not generate 3d, but it reacts with HCl to give 2d in almost quantitative yield. Cationic complexes 3b, 3c, here reported for the first time, were reacted with some nucleophiles and their behaviour compared with that of the already known 3a. In 3b, 3c the metal centre competes with the coordinated ethene for binding to nucleophiles; therefore the acetylacetonate anion can either add to the olefin (affording compounds 6b, 6c ) or to the metal ion replacing the ethene ligand (yielding compounds 7b, 7c). Under similar conditions, 3a gives exclusively 6a. Secondary amines readily add to ethene in 3b, 3c, affording the addition products 8b, 8c, which undergo a ready cyclization to an azaplatinacyclobutane ring (9b, 9c). The remarkable ease of the four-membered ring formation has been related to the high electrophilic character of the metal core in 3b, 3c.