Time-dependent pump-probe spectra of NeBr2

Time- and frequency-resolved pump-probe measurements on NeBr2 have been performed to better characterize its fragmentation dynamics on the B electronic state for vibrational levels in the energy region of the transition from direct vibrational predissociation to intramolecular vibrational relaxation dynamics. Above nu'=20 of the Br2 stretching mode, it was observed that the dependence of lifetime on the vibrational quantum number deviates from the energy-gap law by leveling off in the range of 10 psE transitions of the complex. These transitions are shifted 20 cm(-1) to lower energy from the free Br2 resonances, indicating an E state Ne-Br2 bond energy of 82 cm(-1). Measurements of NeBr2 vibrational predissociation via the delta nu=-2 channel were also performed for nu'=27, 28, and 29. The closing of the delta nu=-1 channel leads to an increase in the lifetimes of these vibrational levels. A new Nd:yttrium aluminum garnet pumped dual optical parametric oscillator/optical parametric amplifier system is described that allows us to conveniently record time-delayed pump-probe spectra with 2-cm(-1) spectral resolution and 15-ps time resolution.     

Titanosilicates as Supports for an Enzymatic Alcoholysis Reaction

Novel [Cr^III(amp)(bipy)(Cl)] (1) (H₂amp = N-(hydroxyphenyl)salicyldimine; bipy = 2,2-bipyridyl) and [Cr^III(app)(bipy)(Cl)]⁺ (2) (H₂app = N-(hydroxyphenyl)pyridine-2-carboxaldimine; bipy = 2,2-bipyridyl) complexes have been synthesized and characterized by physico-chemical methods. Complexes 1 and 2 have been employed as catalysts in the oxidation of both saturated and unsaturated hydrocarbons using tert-butylhydroperoxide (t-BuOOH). The significance of the results with respect to oxo-functionalization of C-H bonds both in unsaturated and saturated hydrocarbons is noted.     

Effects of trivalent phosphorus compounds on vinyl polymerizations—VII. A nonstationary state radical polymerization of methylmethacrylate in the presence of phenyldichlorophosphine

In the radical polymerization of methylmethacrylate in the presence of p-phenyldichlorophosphine, the initial rate and degree of polymerization increased with polymerization time. They first decreased with increase in the phosphine concentration but increased with further increase in the phosphine concentration. Termination was first order with respect to the initiator. The degree of polymerization was independent of the initiator concentration, but dependent on the conversion. The polymer contained no phosphorus units regardless of the phosphine concentration. The ESR spectra of system showed existence of a phosphorus radical. In order to explain these characteristics of the polymerization, it is proposed that there may be a nonstationary state in which the polymer radicals are regenerated during the polymerization from the phosphoranyl radicals.     

Some remarks on the homogenization of the dam problem

The homogenization of the dam problem is discussed in the general weak formulation by means of the technique of correctors. In the two special cases of horizontal layers and of the isotropic coefficients with separated variables the problem is solved. However, in general, the unsaturated region is not stable for the homogenization, as a counterexample by Alt shows.     

Assessing the non-specific hydroxyl radical scavenging properties of melanoidins in a Fenton-type reaction system

Soluble high molecular weight fraction (>10 kDa, named melanoidins) was isolated from Maillard reaction model systems, coffee, beer and sweet wine by ultrafiltration. Deoxyribose method was adjusted for measuring the hydroxyl radical scavenging properties of melanoidins. The presence of competitive melanoidins with deoxyribose for OH decrease the rate of deoxyribose degradation. Possible interferences to the deoxyribose method have been evaluated. Most of isolated melanoidins exhibited a variable and measurable non-site-specific hydroxyl scavenging activity in a Fenton-type reaction system. The iron reducing properties of melanoidins at the reaction conditions were evaluated with ferrozine. It has established a kinetic approach to assess the second rate constants of hydroxyl radical scavenging reactions of melanoidins. This approach may be a valuable tool for addressing the structure-activity relationships of melanoidins in a future. There is no correlation between browning (absorbance at 420 nm) and efficiency for scavenging hydroxyl radicals in solution.     

A density functional theory study on the electrocyclization of 1,2,4,6-heptatetraene analogues: converting a pericyclic to a pseudopericyclic reaction

A comprehensive B3LYP/6-31+G* study on the electrocyclization of 1,2,4,6-heptatetraene analogues was conducted. Starting from the cyclization of (2Z)-2,4,5-hexatrienal, a pericyclic disrotatory process favored by the assistance of a electron lone pair, we incorporated small modifications in its molecular structure to obtain a truly pseudopericyclic process. To this purpose electronegative atoms (fluorine and nitrogen) were added to give a more electrophilic character on the carbon atom which is attacked by the electron lone pair of the oxygen atom. The complete pathway for each reaction was determined, and changes in magnetic properties were monitored with a view to estimating the aromatization associated with each process. This information, together with the energetic and structural results, allowed us to classify the reactions as pseudopericyclic or pericyclic. Among all studied reactions only one was a truly pseudopericyclic process and another was a borderline case. The features of this unequivocally pseudopericyclic case were analyzed in depth.     

Analytical fractionation of aquatic humic substances and their metal species by means of multistage ultrafiltration

The molecular-size fractionation of aquatic humic substances (HS) and their metal species by means of a novel sequential-stage ultrafiltration (UF) device equipped with five appropriate ultramembranes (1, 5, 10, 50 and 100 kD) is described. First of all, the concentration dynamics of macromolecules, particulary HS, during five-stage UF and its subsequent washing step has been modelled. Based on these results, the fractionation of aquatic HS (from ground and bog water) by means of multistage UF has been optimized for an analytical scale (10 ml sample, 1 mg/ml HS, 10 ml washing solution, pH 6.0). The molecular size-distribution of selected aquatic HS (BOC 1/2 from the DFG-Versuchsfeld Bocholt, VM 5 from Venner Moor, Germany) studied by five-stage UF exhibited strong systematic influences of the procedure used for their isolation. The molecular-size distribution of HS obtained by on-line UF and gel permeation chromatography (GPC) showed a satisfactory agreement in the range 1–50 kD. Moreover, when interrupting multistage UF for > 48 h a slow transformation in the HS samples has been found as gradually additional HS fractions of < 1 kD have been formed. Besides unloaded HS molecules, the molecular-size distribution of freshly formed metal species of HS (1.0 mg metal/g HS of Al(III), Cd(II), Cu(II), Fe(III), Mn(II), Ni(II), Pb(II), Zn(II), each) has been characterized by multistage UF as a function of pH-value, degree of loading and complexation time. Metal determinations as carried out by flame AAS, showed that considerable metal fractions in HS especially are present in molecules > 50 kD, which seemed to be rather acid-inert. With complexation times of < 2 days a transient shift of the molecular size distribution of both HS and their metal species (e.g., Al(III), Fe(III) to higher values (> 10 kD) has been found.     

Global optimization of ionic Mg(n)F(2n) (n=1-30) clusters

The global optimization basin-hopping (BH) method has been used to locate the global minima (GM) of Mg(n)F(2n) (n=1-30) clusters using a Born-Mayer-type potential. Some of the GM were particularly difficult to find, requiring more than 1.5 x 10(4) BH steps. We have found that both the binding energy per MgF2 unit and the effective volume of the GM isomers increase almost linearly with n, and that cluster symmetry decreases with cluster size. The data derived from the BH runs reveal a growing density of local minima just above the GM as n increases. Despite this, the attraction basin around each GM is relatively large, since after all their atomic coordinates are randomly displaced by values as high as 2.0 bohrs, the perturbed structures, upon reoptimization, relax back to the GM in more than 50% of the cases (except for n=10 and 11). The relative stabilities derived from energy second differences suggest that n=8,10,13,15, and 20 are probably the magic numbers for these systems. Mass spectrum experiments would be very useful to clarify this issue.     

Studies on polymers and composites from lignin and fiber derived from sugar cane

Sugarcane fiber (i.e. bagasse) lignin has a larger fraction of aromatics unsubstitution in the ortho position than hardwood or softwood lignin and hence has the greater ability to be derivatized. Furthermore, organosolv lignin has a higher purity than sulfonated and kraft lignins. This work examines the purification of organosolv lignin derived from bagasse and the physico‐chemical properties of the lignin and lignin‐phenol formaldehyde (PF) resin coatings, and composites. The wetability tests have shown that lignin and lignin‐PF resin films are effective water barrier coatings, though the contact angles of lignin‐PF resin films were considerably less than the wax films. The overall mechanical properties (i.e. peak stress, peak strain and modulus) of the bagasse fiber composites were lower than the values obtained with the composites without the inclusion of bagasse fiber. Copyright © 2007 John Wiley & Sons, Ltd.