Proton NMR relaxation studies on several nitroxyl-water systems

Proton spin-lattice and spin-spin relaxation times of several nitroxyl radicals in aqueous solutions have been measured between 10 kHz and 90 MHz. There are two regions where the relaxation times of the solvent water protons are frequency dependent. It is possible to extract from these experimental data structural and dynamical parameters such as the number of solvated water protons and/or their mean lifetime in the first hydration sphere around the paramagnetic center. These results were found to differ considerably from corresponding data reported in the literature which have been deduced mainly from chemical shift measurements. To whom correspondence should be addressed.     

Photoisomerization and relaxation dynamics of a structurally modified biomimetic photoswitch

Recent experimental and theoretical studies on N-alkylated indanylidene pyrroline Schiff bases (NAIP) show that these compounds exhibit biomimetic photoisomerization analogous to that in the chromophore of rhodopsin. The NAIP compounds studied previously isomerize rapidly and often evolve coherently on the ground-electronic surface after reaction. We present the results of transient electronic absorption spectroscopy on dMe-OMe-NAIP, a newly synthesized NAIP analogue that differs from other NAIP compounds in the substituents on its pyrrolinium ring. Following excitation with 400 nm light, dMe-OMe-NAIP relaxes from the electronic-excited state in less than 500 fs, which is slower than in other analogues, and does not show the prominent oscillations observed in other NAIP compounds. A reduction in the amount of twisting between the rings caused by removal of the methyl group is likely responsible for the slower isomerization. Measurements in solvents of varying viscosity and structure suggest that intramolecular processes dominate the relaxation of nascent photoproducts.     

NMR studies of lysozyme surface accessibility by using different paramagnetic relaxation probes

Paramagnetic probes, whose approach to proteins can be monitored by nuclear magnetic resonance (NMR) studies, have been found to be of primary relevance for investigating protein surfaces' accessibility. Here, a Gd(III) neutral complex which contains two metal ions, [Gd2(L7)(H2O)2], is suggested as a paramagnetic probe particularly suited for systematic NMR investigation of protein surface accessibility, due to an expected high relaxivity and to the lack of electric charge which could favor specific interactions. Hen egg white lysozyme has been used as a model system to verify the absence of preferential approaches of this paramagnetic probe to specific protein moieties by comparing paramagnetic perturbation profiles of 1H-13C HSQC signals obtained in the presence of TEMPOL and [Gd2(L7)(H2O)2]. From the similarity of the measured paramagnetic perturbation profiles induced by the two different probes, specific interactions of [Gd2(L7)(H2O)2] with the enzyme could be ruled out. The large size of the latter probe is suggested to be responsible for the strong paramagnetic perturbations observed for CalphaH groups which are located in convex surface-exposed regions. The combined use of the two probes reveals fine details of the dynamics controlling their approach toward the protein surface.     

Radiation-induced degradation of butyl rubber vulcanized by three different crosslinking systems

Butyl rubber (IIR) is an isobutylene/isoprene copolymer and is provided with good properties including low permeability to gases, good thermal stability and high resistance to oxygen and ozone action, among others. It is well known that the major effect of ionizing radiations on butyl rubber is chain scission accompanied with a significant reduction in molar mass. This work aimed to study the effects of gamma radiation on the properties of butyl rubbers vulcanized by three different curing systems, such as, the ones based on sulfur, sulfur donor and phenolic resin to identify which curing system is the most stable under irradiation. The butyl rubber vulcanized by three different systems was gamma irradiated with doses of 25 kGy, 50 kGy, 100 kGy, 150 kGy and 200 kGy. Irradiated and non-irradiated samples were characterized by the following techniques: tensile, elongation and hardness. It was observed that doses higher than 150 kGy practically destroy the assessed properties for all butyl compounds, irrespective of the vulcanization system used; however compounds cured with phenolic resin showed a decrease in properties proportional to the dose.     

Synthesis and crystal structures of two structurally related kryptoracemates

Kryptoracemates are racemic compounds (pairs of enantiomers) that crystallize in Sohnke space groups (space groups that contain neither inversion centres nor mirror or glide planes nor rotoinversion axes). Thus, the two symmetry‐independent molecules cannot be transformed into one another by any symmetry element present in the crystal structure. Usually, the conformation of the two enantiomers is rather similar if not identical. Sometimes, the two enantiomers are related by a pseudosymmetry element, which is often a pseudocentre of inversion, because inversion symmetry is thought to be favourable for crystal packing. We obtained crystals of two kryptoracemates of two very similar compounds differing in just one residue, namely rac‐N‐[(1S ,2R ,3S )‐2‐methyl‐3‐(5‐methylfuran‐2‐yl)‐1‐phenyl‐3‐(pivalamido)propyl]benzamide, C₂₇H₃₂N₂O₃, (I), and rac‐N‐[(1S ,2S ,3R )‐2‐methyl‐3‐(5‐methylfuran‐2‐yl)‐1‐phenyl‐3‐(propionamido)propyl]benzamide dichloromethane hemisolvate, C₂₅H₂₈N₂O₃·0.5CH₂Cl₂, (II). The crystals of both compounds contain both enantiomers of these chiral molecules. However, since the space groups [P 2₁2₁2₁ for (I) and P 1 for (II)] contain neither inversion centres nor mirror or glide planes nor rotoinversion axes, there are both enantiomers in the asymmetric unit, which is a rather uncommon phenomenon. In addition, it is remarkable that (II) contains two pairs of enantiomers in the asymmetric unit. In the crystal, molecules are connected by intermolecular N—H…O hydrogen bonds to form chains or layered structures.     

Grafting of montmorillonite with different functional silanes via two different reaction systems

Silane grafted montmorillonites were synthesized by using 3-aminopropyltriethoxysilane and trimethylchlorosilane via two different grafting reaction systems: (a) ethanol-water mixture and (b) vapor of silane. The resulting products were investigated using Fourier transform infrared (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA). XRD patterns demonstrate that silane was intercalated into the montmorillonite gallery, as indicated by the increase of the basal spacing. The product prepared by vapor deposition has a larger basal spacing than that obtained from solution, due to the different extent of silane hydrolysis in various grafting systems. TGA curves indicate that the methyl groups penetrate into the siloxane clay are the primary reason for the decrease of the dehydroxylation temperature of the grafted products. 3-Aminopropyltriethoxysilane in the grafted montmorillonite adopts a bilayer arrangement while trimethylchlorosilane adopts a monolayer arrangement within the clay gallery.     

Comparison of enthalpy relaxation between two different molecular masses of a bisphenol-A polycarbonate

Summary The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔH_a) and fictive temperature (T_f) as a function of aging temperature (T_a) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔH_a and T_f changes were only appreciable for (T_g -70 K)<T_a<T_g . While the influence of molecular mass was somewhat discernible, enthalpy measurements were not as sensitive as stress relaxation tests in differentiating molecular mass effects. In a second investigation, the kinetics of enthalpy relaxation upon isothermal aging at 130°C was evaluated using the peak shift method and found to be comparable to literature values. The plot of ΔH_a as a function of log (aging time) showed two distinct regions: a brief non-linear portion (less than 1 h aging) which is followed by a linear relationship as typically reported in the literature. In contrast to the linear region, the non-linear relaxation behaviour of the poorly aged state does not appear to be dependent on molecular mass.     

Magnetic field induced cross relaxation between two different spin transitions of triplet coumarin

At certain magnetic fields, when the energy separations between the τ_x — τ_y spin levels and between the τ_y — τ_z levels of triplet coumarin become equal, intensity changes are observed in the phosphorescence of coumarin doped in a single crystal of durence cooled to 1.6 K that can be attributed to cross relaxation (CR) between the τ_x — τ_y spin levels of one triplet coumarin molecule and the τ_y — τ_z spin levels of another triplet coumarin. Rate equations that describe the time behavior of the populations of the spin levels which are involved in the CR process are described and numerically solved. CR behavior is found to be strongly dependent on the total coumarin triplet concentration.     

Dilatometric studies of isotactic polypropylene with different morphology before and after thermal degradation in air

Summary Glass transition phenomenon in isotactic polypropylene has been studied in this paper basicly on dilatometric studies. New suggestions are made for explaining the occurrence of twoT _g values with their characteristic dependence on the degree of crystallinity of the samples having a different morphological structure. The effects produced by thermooxidation are discussed. T _g calledT _g1 is considered as the true glass transition temperature in the amorphous phase of IPP whereasTg2 is melting of a smectic phase in IPP,T _g2 >T _g1.Author thanks Prof. M. Kryszewski for useful discussion.     

Dynamic DSC purity analysis studies of two component doped systems

Up to the present time, the accuracy and precision of dynamic DSC purity analysis measurements have been tested by using simple binary systems as models. For the most part, pure organic substances have served as host and eutectic-forming low level dopant. As a first step in considering whether the DSC method can be applied to multicomponent impure substances, the simple ternary model system, phenacetin doped with benzamide and p-aminobenzoic acid, has been studied. Separately, the two dopants form well defined eutectics with the host melting at 370 K (benzamide dopant) and 385 K (p-aminobenzoic acid dopant). When the two dopants are combined, a third eutectic melting at 353 K is produced. Evidence is presented confirming the fact that in the range 100-98.5 mole % purity, the total DSC measured impurity is the sum of the two individual dopant levels, and is independent of their relative proportions.     

Dynamics of hyaluronan oligosaccharides revealed by 15N relaxation

Complex carbohydrates are considered to be flexible biomolecules, yet few experimental techniques are available to characterize their dynamics. In this study, we investigate the potential of 15N relaxation to probe the dynamics of hyaluronan oligosaccharides by adapting approaches previously applied to proteins. Unlike the 13C nucleus, 15N provides considerably enhanced spectral resolution, allowing position-specific information to be measured even in the middle of oligomers as large as decasaccharides. Moreover, isotopic incorporation maintains the 1H-15N group as an isolated spin-pair, allowing relaxation experiments to be performed and interpreted at low concentrations. A methodology is described for calculating the Lipari and Szabo model-free parameters at specific positions in hyaluronan oligomers and is used to produce a dynamic representation for the hexasaccharide. In this model, the glycosidic linkages and acetamido rotamer were determined to deviate by 18 degrees and 24 degrees from their mean positions, respectively. This approach allows the dynamic structural characterization of hyaluronan and other nitrogen-containing carbohydrates. The resultant models provide crucial insights into the physical properties and biology of these flexible molecules, which are at present poorly understood.     

Evaluation of the performance of microcrystalline cellulose in retarding degradation of two epoxy resin systems

Accelerated weathering studies are necessary to determine future risks arising from the loss of durability of materials under environmental conditions (e.g. ultraviolet irradiation from the sun, moisture from rainfall, temperature cycling). The influence of different accelerated weathering conditions such as UV light and moisture on the properties of two epoxy resin systems incorporating microcrystalline cellulose (MCC) was evaluated. This study aimed to assess changes in chemical properties (FTIR), mechanical properties (tensile tests), thermal properties (TGA and DSC) and morphology (SEM) before and after accelerated weathering. The samples exposed to different accelerated weathering times (1, 2, 3, 4, and 6?months) were based on the diglycidyl ether of bisphenol A, DGEBA, or hydrogenated diglycidyl ether of bisphenol A, HDGEBA, with amine crosslinker (2,2,4-trimethyl-1,6-hexanediamine, TMDA) and 2% MCC. Incorporation of MCC improved thermal stability, reduced surface oxidation, and gave better retention of mechanical properties after accelerated weathering. Both epoxy resins and epoxy composites exhibited a reduction in the tensile strength upon accelerated weathering with the composites showing less reduction in the tensile strength after 6 months. The glass transition temperatures (T_g) before and after accelerated weathering were also measured. DGEBA-TMDA/2%MCC and HDGEBA-TMDA/2% MCC composites reduced the decrease in the T_g after accelerated weathering, compared to that of DGEBA-TMDA and HDGEBA-TMDA samples. Degradation primarily decreased the mechanical properties of the composites, with some damaged specimens showing on the surfaces of DGEBA-TMDA/2% epoxy composites and HGEBA-TMDA/2%MCC composites. Fewer morphological changes with limited voids were seen on the DGEBA epoxy interface for HDGEBA compared to DGEBA composite samples. Incorporation of 2%MCC in DGEBA-TMDA and HDGEBA-TMDA increased resistance to thermal degradation after accelerated weathering.     

Physical aging and relaxation processes in epoxy systems

Relaxation transitions in epoxy oligomers of various chemical nature and three-dimensional polymer networks on their basis are considered. Additional factors affecting relaxation of the networks, namely, the conversion of reactive functional groups of monomers during curing and the crosslink density of the network being formed, are discussed. Special attention is given to the phenomenon of physical aging in glassy crosslinked epoxy systems, which is analyzed from the experimental and theoretical viewpoints.     

Spectroscopic studies on iron complexes of different anthracyclines in aprotic solvent systems

Iron complexes of daunorubicin, idarubicin, pirarubicin, and doxorubicin in anhydrous DMF were studied by UV/vis, CD, fluorescence, M?ssbauer, and EPR spectroscopy. Titration studies of the metal-free anthracyclines showed one (UV-detectable) deprotonation step requiring 2 equiv of base, compared to 1 equiv for quinizarine. Metal complexation was studied at three different metal/ligand ratios, and with increasing amounts of base. The results obtained from optical spectroscopy show the existence of two different complex species and give clear indications for the requirements of metal complexation. Complex species I, formed at a low iron-to-ligand ratio, is less dependent on base addition than complex species II formed with equimolar ferric ion. EPR and M?ssbauer experiments provide further insight into the structures of both complex species. Lack of spin density of the M?ssbauer samples in EPR indicates spin coupling between the metal centers. M?ssbauer spectra consist of single quadrupole doublets with values typical for high-spin ferric ion in an octahedral arrangement. The M?ssbauer spectroscopic features at 7 T exclude the presence of S = 0 dimers. Complex I represents a monomeric ferric iron complex whereas complex II is consistent with a more or less aggregrated oligomeric Fe-anthracycline system.     

Raman spectrocopic studies on the vibrational relaxation of CH3CN in different environments

Comparison of the frequencies and half-widths of v₁ in a variety of solvents shows that dispersion forces make a significant contribution to the effective intermolecular potential. Measurements of τ_v^?1 and [ω²] in the CH₃CN/CD₃OD/CCl₄ system demonstrates that long-range (hydrogen-bonding?) potentials significantly affect the relaxation rate.     

Solid state proton spin–lattice relaxation in four structurally related organic molecules

We report and interpret the temperature dependence of the proton spin–lattice relaxation rate at 8.50 and 22.5 MHz in four polycrystalline solids composed of structurally related molecules: 2-ethylanthracene, 2-t-butylanthracene, 2-ethylanthraquinone, and 2-t-butylanthraquinone. We have been unable to grow single crystals and therefore do not know the crystal structures. Hence, we use the NMR relaxometry data to make predictions about the solid state structures. As expected, we are able to conclude that the ethyl groups do not reorient in the solid state but that the t-butyl groups do. The anthraquinones have a “simpler” structure than the anthracenes. The best dynamical models suggest that there is a unique crystallographic site for the t-butyl groups in 2-t-butylanthraquinone and two sites, each with half the molecules, for the ethyl groups in 2-ethylanthraquinone. There are also two sites in 2-ethylanthracene, but with unequal weights, suggesting four sites in the unit cell with lower symmetry than the two anthraquinones. Finally, the observed relaxation rate data in 2-t-butylanthracene is very complex and its interpretation demonstrates the uniqueness problem that arises in interpreting relaxometry data without the knowledge of the crystal structure.     

NMR relaxation study of thermal degradation in cured PMR polyimides

We have studied cross-linking and thermal degradation of high-performance first-and second-generation PMR-15 polyimides, both thermoset and thermoplastic versions, by performing nonspectroscopic NMR solid echo T^*₂ relaxation measurements at temperatures up to 430°C using probes built for this purpose. We employ signal averaging and automated decomposition of the relaxation decays into two Gaussian components, the slower of which gradually appears above 300°C. Tracking the molecular mobility spectrum in terms of the relative intensity of the components and their relaxation times as temperature is cycled, we detect essentially no irreversible effects below the glass transition, measure permanent mobility reductions attributable to completion of cure, and find that exposure to temperatures above 380°C on the order of 1 h is required for substantial thermal degradation to occur. These results are closely supported by thermal and mechanical measurements on parallel specimens. Second-generation PMR resins appear to have higher microscopic rigidity and reduced viscous fraction at high temperatures. ©1995 John Wiley & Sons, Inc.     

Thermal degradation kinetics of structurally diverse poly(bispropargyl ethers-bismaleimide) blends

Structurally diverse bispropargyl ethers using resorcinol,quinol,4,4￠-dihydroxy biphenyl,bisphenol-A,4,4￠-dihydroxy diphenyl ketone,4,4￠-dihydroxy diphenylsulphone,trimethyl indane bisphenol and tetramethyl spirobiindane bisphenol were prepared by using phase transfer catalyst.Synthesized materials were separately blended with 4,4￠-bismaleimido diphenyl methane(BMIM)in mole ratios(0.5:0.5).The materials were thermally cured and the structural characterisation and the thermal properties of these cross-linked materials are investigated using Fourier-transform infrared(FTIR)spectrophotometer and thermogravimetric analyzer(TGA).Among the different materials investigated poly MRPE,poly MBPEBPA and poly MSPE show higher onset degradation temperature of 300°C indicating higher thermal stability.The degradation kinetics is investigated using Flynn-Wall-Ozawa(FWO),Vyazovkin(VYZ)and Friedman(FRD)methods.Amongst the various cured materials investigated,the activation energy(Ea-D)values obtained for poly MRPE and poly MKPE were observed to increase continuously froma=0.2 to 0.8 and the values range from 199 kJ/mol to 245 k J/mol and 153 k J/mol to 295 k J/mol respectively.The crosslinked materials resulting from these bispropargyl monomers definitely need more energy for bond cleavage due to the presence of more aromatic units.The volatile products obtained during the thermal degradation of the polymers were analyzed using thermogravimetric-Fourier transform infrared analyses(TG-FTIR).The phenols,substituted phenols,carbon monoxide,carbon dioxide and small amount of aniline were found to be the major products during thermal degradation of these cured blends.     

Carrier relaxation and lattice heating dynamics in silicon revealed by femtosecond electron diffraction

We report on the use of femtosecond electron diffraction to resolve the dynamics of electron-phonon relaxation in silicon. Nanofabricated free-standing membranes of polycrystalline silicon were excited below the damage threshold with 387 nm light at a fluence of 5.6 mJ/cm2 absorbed (corresponding to a carrier density of 2.2 x 10(21) cm(-3)). The diffraction pattern was captured over a range of delay times with a time resolution of 350 fs. All of the detected Bragg peaks exhibited intensity loss with a time constant of less than 2 ps. Beyond the initial decay, there was no further change in the diffracted intensity up to 700 ps. We find that the loss of intensity in the diffracted orders is accounted for by the Debye-Waller effect on a time scale indicative of a thermally driven process as opposed to an electronically driven one. Furthermore, the relaxation time constant is consistent with the excitation regime where the phonon emission rate is reduced due to carrier screening.     

Vibrational relaxation studies of systems with strong intermolecular interactions at low temperatures in the gas phase and in cryogenic solutions

Infrared fluorescence and double infrared resonance have been used to measure the V-T relaxation of CD₃F and CD₄ in mixtures with He, Ne, Ar, Kr, Xe and HCl in the gas phase and in the liquid Ar, Kr, Xe. It is shown that the temperature dependence of V-T probabilities in the interval 350–150°K can be attributed to an increasing influence of long range forces. In the mixture CD₃F + HCl, where H-bonded complexes CD₃F…HCl have been formed, the vibrational relaxation rate considerably increases with decreasing temperature. It is found that the probabilities of V-T energy transfer in cryogenic solutions are lower than those measured in the gas phae.