Photoelectron spectroscopy on thin films of extended zinc porphyrazines

Photoemission measurements were performed on a series of stepwise benzoannelated zinc porphyrazine molecules in thin films. The electronic structure of tert-butyl-substituted zinc tetraazaporphyrin, phthalocyanine, and naphthalocyanine is investigated using mainly EUV synchrotron radiation. A detailed analysis of the zinc satellites in the spectra of the valence region is performed in an attempt to infer the effect of ligand size extension on the metal-ligand interactions. No differences in the character of the bond between zinc and ligand were detected as a function of ligand size. The results are compared with those for the respective metal-free and copper-containing molecules.     

Intact size-selected Au(n) clusters on a TiO2(110)-(1 x 1) surface at room temperature

Experiments in which mass-selected gold clusters were deposited on a surface have found that the catalytic properties depend strongly on cluster size. However, these experiments have not established definitively that the clusters maintain their size after deposition. We report here work in which we deposit low kinetic energy, mass-selected Aun+ (n = 1-8) clusters on a rutile TiO2(1 x 1) surface and use ultrahigh vacuum scanning tunneling microscopy (UHV-STM) to determine their size and shape.     

Cyclisations par l'intermédiaire de fonctions phénylthioet phénoxy-carbamyle. I. Diphényl-1,2-alcoyl-5-dioxo-3,6-hexahydro-triazines-1,2,4

The N-(p-nitrophenoxy-carbonyl) and N-(phenylthio-carbonyl) derivatives of α-amino-acyl-hydrazides cyclize in presence of diazomethane, yielding hexahydro-1, 2, 4-triazines with simultaneous formation of p-nitro-anisole or thio-anisole respectively. The molecules with a phenoxycarbonyl function such as N-(phenoxycarbonyl)-glycyl-(N, N′-diphenylhydrazide) give the corresponding ring products and anisole only in solvents with high dielectric constant (e.g. nitromethane). The phenylthio-carbonyl derivatives give the same ring products in presence of lead acetate by the intervention of HO^?. The p-nitrophenoxycarbonyl derivatives give rise to the same cyclisations in the presence of pyridine by the intermediate of carbamyle-pyridinium ion acting as electrophile. These different types of intramolecular reactions are illustrated by the formation of 1, 2-diphenyl-5-alcoyl-3, 6-dioxo-1, 2, 4-hexahydro-triazines.     

The use of polarized light scattering for the study of the dynamics of the spinodal decomposition

Due to the fact that in a system undergoing spinodal decomposition the long wavelength fluctuations are enhanced, we expect that multiple events can be observed when the passage of light through the sample is measured. We study here the intensity of double scattering for both polarized and depolarized measurements, as a function of time and angle of detection. The time dependence is nonexponential and the dependence on the scattering wave vector is less pronounced than in the case of single scattering. For systems with spherical molecules it is possible to separate double and single scattering contributions if both polarized and depolarized measurements are made. This increases the amount and quality of information about the dynamics of the spinodal decomposition that can be obtained from light scattering experiments.     

Theoretical study of some heterocyclic amines with applications to the chemistry of 9-amino-1,2,3,4-tetrahydroacridine

9-Amino-1,2,3,4-tetrahydroacridine (THA ), a potent cholinesterase inhibitor, was recently used in the treatment of Alzheimer's disease. On attempting to prepare a dihydropyridine ? pyridinium salt-based redox chemical delivery system (CDS ) to enhance brain delivery of THA , several of the practical synthetic challenges were examined by using a theoretical MO approach. The structures, reactivities and stability of THA , derivatives of THA and a model compound, 4-aminopyridine, a simple dibasic heterocyclic amine, were studied in the framework of the AM -1 approximation. The study included the possible protonated forms of THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine, consistent with experimental results. Electron delocalization is responsible for the remarkable stability of these molecules and for the observed lack of reactivity of the amino group, both in the basic and protonated forms. The site of N-alkylation of the 9-nicotinamide derivative of THA (an intermediate in the synthesis of THA -CDS ) is controlled by electronic, thermodynamic, and steric factors.     

Use of weighted least-squares splines for calibration in analytical chemistry

Weighted least-squares spline functions are discussed and applied for calibration processes in analytical chemistry. Different weighting techniques are also considered, and for the evaluation of the results some quality coefficients are proposed. Depending upon the structure of the data, some weighting procedures may improve the results dramatically. Considering the results obtained in the case of TLC densitometry, it seems that nonlinear weighting procedures based upon the distance to the function are the best ones, with a plus for the y-distance type. It is difficult to give general rules regarding the optimal parameters of the weighted calibration splines-function order (m), number (N) and distribution of knots, and weighting technique. These depend upon the structure of the data. However, higher order splines are not recommended since the result might become extremely unstable. The example used to illustrate the performances of the procedures discussed here involved only a single independent variable. The method is general and extends practically to any number of variables, thus resulting in a multivariate approach.     

Colour formation in poly(ethylene terephthalate) during melt processing

The discolouration, that occurs in virgin poly(ethylene terephthalate) - PET during melt processing, was studied using various bulk and surface analytical techniques. Proton nuclear magnetic resonance (¹H NMR) was used to study the bulk chemical changes occurring in the polymer during thermo-oxidative degradation. Chemical derivatisation with trifluoroacetic anhydride (TFAA) was used to label the hydroxyl groups introduced on the polymer surface by thermal oxidation.From the surface analysis studies using photoacoustic Fourier transform infrared spectroscopy (PA/FT-IR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS) it was evident that colour formation starts initially with the hydroxylation of the terephthalic ring. Further, the formation of additional carbonyl functionalities and conjugated chromophoric systems complete the colour formation process.     

A Basis for Orbital Symmetry Rules

The influence of molecular symmetry on reaction rates is examined with an approach in which reactions are viewed as electronic transitions between states of reacants and products (described, in turn by quasiadiabatic potential surface). The moleculer Hamiltonian is used to derive selection rules for these transitions. The complete Hamilatonian has no useful symmetery. Neglect of non-Born-Oppenheimer and spin-orbit terms (and of other angular momentum coupling terms) leads to an apporixmate Hamiltonian and to selection rules which from the basis of the Woodward-Hoffmann rules. This apporch provides an alternative to the adiabatic potantial surfaces, reaction coordinates, and transition state theory used in more familiar discussions of the Woodward-Hoffmann rules. Further, it provides a particulary clear method for discussing violations of these symmetry rules, and for differentiating concerted and nonconcerted reactions.     

Structure of hydrated Na-Nafion polymer membranes

We use molecular dynamics simulations to investigate the structure of the hydrated Na-Nafion membranes. The membrane is "prepared" by starting with the Nafion chains placed on a cylinder having the water inside it. Minimizing the energy of the system leads to a filamentary hydrophilic domain whose structure depends on the degree of hydration. At 5 wt % water the system does not have enough water molecules to solvate all the ions that could be formed by the dissociation of the -SO3Na groups. As a result, the -SO3Na groups aggregate with the water to form very small droplets that do not join into a continuous phase. The size of the droplets is between 5 and 8 A. As the amount of water present in the membrane is increased, the membrane swells, and SO3Na has an increasing tendency to dissociate into ions. Furthermore, a transition to a percolating hydrophilic network is observed. In the percolating structure, the water forms irregular curvilinear channels branching in all directions. The typical dimension of the cross section of these channels is about 10-20 A. Calculated neutron scattering from the simulated system is in qualitative agreement with experiment. In all simulations, the pendant sulfonated perfluorovinyl side chains of the Nafion hug the walls of the hydrophilic channel, while the sulfonate groups point toward the center of the hydrophilic phase. The expulsion of the side chains from the hydrophilic domain is favored because it allows better interaction between the water molecules. We have also examined the probability of finding water molecules around the Na+ and the -SO3(-) ions as well as the probability of finding other water molecules next to a given water molecule. These probabilities are much broader than those found in bulk water or for one ion in bulk water (calculated with the potentials used in the present simulation). This is due to the highly inhomogeneous nature of the material contained in the small hydrophilic pores.