Search

A compressed exponential function (CEF) is shown to be represented by a distribution of Gaussian functions. The properties and characteristics of the distribution are detailed and discussed, and are illustrated with reference to the change in the NMR spectroscopy proton Hahn echo relaxation response that takes place in a composite material (prepared by melt compounding a mixture of graphite nanoparticles and pyromellitic anhydride modified polypropylene carbonate (PPC)) during aging at 90 °C. The results show that both the width and the average value of the spin-spin relaxation rate distribution decrease during aging, suggesting that the molecular motion becomes less constrained and less heterogeneous.

The molecular weight distribution characteristics derived from an inverse Laplace transformation (ILT) of a stretched exponential PGSTE NMR response function (SEF) is discussed with reference to the scaling law between diffusivity (D) and molecular mass (M) (D = KM−α). Two general equations are presented, enabling the polydispersity index (PDI) and the average molecular weights (Mw, Mn, and Mmed) to be determined from α and the two characteristic parameters β and D0 of the (normalized) SEF (= exp [(−D0t)β]). A detailed discussion regarding the derivation of the two equations is presented.

Assuming the signal response in a pulsed gradient stimulated echo (PGSTE) experiment (on a polymer) to be described by a simple stretched-exponential function (SEF) and knowing the scaling law between diffusivity and molecular weight, the molecular weight distribution (MWD) characteristics (kurtosis, skewness, moment, and width) are derived and compared to the corresponding distribution characteristics obtained by a log-normal function fit (to the same MWD). Also, the challenge involved in obtaining a reliable weight average molecular weight from an SEF response function is discussed.

A log/normal MWD is characterized by two parameters, its mean molecular weight M0 and width s. It is demonstrated how these parameters can be obtained by model fitting a stretched exponential function (SEF), as characterized by two parameters bandDS, to the PGSTE response curve. Based on simulations, two general empirical equations relating b and DS to M0 and s are found. The model enables the MWD characteristics to be determined if the scaling law between diffusivity and molecular weight is known. The sensitivity and relative error of s and M0 are discussed and the applicability of the model is illustrated by analyzing experimental NMR response curve of some PEO samples. The key advantages of this technique are its simplicity, numerical robustness, and reliability.

The iron(III) spin-crossover compounds [Fe(Hthsa)(thsa)]⋅H2O (1), [Fe(Hth5Clsa)(th5Clsa)2]⋅H2O (2), and [Fe(Hth5Brsa)(th5Brsa)2]⋅H2O (3) (H2thsa=salicylaldehyde thiosemicarbazone, H2th5Clsa=5-chlorosalicylaldehyde thiosemicarbazone, and H2th5Brsa=5-bromosalicylaldehyde thiosemicarbazone) have been synthesized and their spin-transition properties investigated by magnetic susceptibility, Mossbauer spectroscopy, and differential scanning calorimetry measurements. The three compounds exhibit an abrupt spin transition with a thermal hysteresis effect. The more polarizable the substituent on the salicylaldehyde moiety, the more complete is the transition at room temperature with an increased degree of cooperativity. The molecular structures of 1 and 2 in the high-spin state are revealed. The occurrence of the light-induced excited-spin-state trapping phenomenon appears to be dependent on the substituent incorporated into the 5-position of the salicylaldehyde subunit. Whereas the compounds with an electron-withdrawing group (-Br or ‒Cl) exhibit light-induced trapped excited high-spin states with great longevity of metastability, the halogen-free compound does not, even though strong intermolecular interactions (such as hydrogen-bonding networks and π stacking) operate in the system. For compound 2, the surface level of photoconversion is less than 35 %. In contrast, compound 3 displays full photoexcitation.

Canadian econometric studies dealing with the livestock-feed sector have tended to concentrate on individual commodities, or on a particular component of the market equilibrium process. In this article an integrated model of the sector is presented which depicts the nature of commodity interactions both at the consumer level and the producer level. The feed sector was conceptualized primarily in terms of derived demand. The model contained a total of 82 equations, of which 64 were related to the livestock subsystem. The estimated model was subsequently used for predicting the impact of changes in selected instrumental variables on the livestock-feed sector. Les dernieres etudes econometriques canadiennes traitant du pâturage se sont concentrees sur les produits separes on sur un compose particulier repondant au besoin commercial. Cet article presents un modele integre du secteur montrant la nature des actions reciproques entrc les produits au niveau du consommateur et du producteur. Le secteur pâturage est explique en fonction de la demande. Le modele contenait 82 equations, 64 desquelles se rapportaient aľelevage; par consequent le modele est employe pour predire ľinfluence dans les pâturages de changements operes sur les composants choisis.

A dipolar filter pulse sequence combined with cross-polarization-MAS is applied to characterize the phase distribution, morphology, and spin diffusion within a high-density polyethylene sample. A new method to obtain quantitative C NMR by combining cross-polarization-MAS and spin diffusion NMR is presented. The derived crystallinity is consistent with the corresponding crystallinity obtained by 1 H NMR.

The self-diffusion coefficient of MAO confined within a silica membrane is not experimentally accessible by traditional PFG NMR measurements, probably due to a susceptibility effect resulting in rather broad, unobservable resonance line. A more indirect approach was therefore applied to probe the diffusivity of MAO within a silica membrane. MAO confined in a bulk toluene solution on one side of the membrane was free to diffuse through the membrane (saturated with toluene) and into a corresponding deuterated toluene solution on the other side of the membrane where is proton NMR signal intensity was monitored as a function of time. By introducing a relevant diffusion model, the diffusivities of both toluene and MAO were derived by model-fitting. By refining the diffusion model, the formation of methane within the silica was explained, and its diffusivity estimated. The results suggest a significant reduction in diffusivities of both MAO and toluene within the silica as compared to the corresponding diffusivities in bulk solution.

In this study, antimicrobial polymers are synthesized by the organocatalytic ring-opening polymerization of an eight-membered heterocyclic carbonate monomer that is subsequently quaternized with methyl iodide. These polymers demonstrate activity against clinically relevant Gram-positive Staphylococcus epidermidis and Staphylococcus aureus, Gram-negative Escherichia coli and Pseudomonas aeruginosa, and fungus Candida albicans with fast killing kinetics. Importantly, the polymer efficiently inhibits biofilm growth and lyses existing biofilm, leading to a reduction in biomass and cell viability. In addition, the macromolecular antimicrobial is less likely to induce resistance as it acts via a membrane-lytic mechanism. The polymer is not cytotoxic toward mammalian cells with LD50 of 99.0 ± 11.6 mg kg-1 in mice through i.v. injection. In an S. aureus blood stream infection mouse model, the polymer removes bacteria from the blood more rapidly than the antibiotic Augmentin. At the effective dose, the polymer treatment does not damage liver and kidney tissues or functions. In addition, blood electrolyte balance remains unchanged after the treatment. The low cost of starting materials, ease of synthesis, nontoxicity, broad spectrum activity with fast killing kinetics, and in vivo antimicrobial activity make these macromolecular antimicrobials ideal candidates for prevention of sepsis and treatment of infections.

A critical evaluation of model-fitted and observed 1H NMR spin-lattice relaxation time data (T1) of methylaluminoxane (MAO) dissolved in toluene suggests that the relaxation time of MAO is predominantly controlled by rotational motions. Within the Al concentration (CAl) range (10–3M T > 20°C) studied, the diffusion of MAO contributes with less than 20% (at the maximum concentration investigated) to the overall relaxation rate. For concentrations smaller than 0.5 M, the diffusion has no significant impact on the spin-lattice relaxation time (

The crosslinking of poly(vinyl alcohol) (PVA) with glutaraldehyde at 80 °C was characterized by viscosity and pulse field gradient (PFG) nuclear magnetic resonance (NMR) techniques. NMR signified an initial dormant period of approximately 6 hr, in which the self-diffusion coefficient of PVA was found to be constant and independent of time. During the next 7 hr (the “primary” gel period), this induction period was succeeded by a fast decay of the self-diffusion coefficient of rate (9.13 ± 0.45) × 10−5 sec−1 followed by a slower decay rate of (3.22 ± 0.30) × 10−5 sec−1 (the “secondary” gel period). The viscosity of the solution showed the same time behavior, i.e. an initial dormant period (∼ 6 hrs), followed by a fast increase of the viscosity for the next 7 –8 hr. During the “secondary” gel regime, the viscosity became too large to be reliably determined. However, within the time regime where both techniques produced reliable data, they gave identical information regarding the kinetics of the gel process, suggesting that PFG NMR enables in situ monitoring of gelation within porous materials (for instance sandstone). Copyright © 2000 John Wiley & Sons, Ltd.

A novel synthetic approach to the construction of the zaragozic acids, which was used for the asymmetric synthesis of zaragozic acid A/squalestatin S1 (1), is described. Fragment 5, representing the tricarboxylic acid core portion, is assembled in three key steps: 1) Stille coupling to establish the carbon framework; 2) enantioselective dihydroxylation to introduce the absolute stereochemistry; and 3) diastereoselective dihydroxylation to complete the required carbon-oxygen connectivity. The convergency of this synthesis is demonstrated by the dithiane addition of a variety of C 1 side chains (e.g., 78) to advanced intermediate 5. A multi-event acid-catalyzed rearrangement yielded the zaragozic acid core 86, which was converted to an intermediate obtained from degradation of zaragozic acid A. A second-generation synthesis of the core of the zaragozic acids is also described. When aldehyde 90 was used instead of 5, both the yield and diastereoselectivity of the dithiane addition reaction were improved, although the degree of convergency was slightly lower.

A solid state electrochemical reactor is described in which reactants can be oxidised at high temperatures over an anode/catalyst using co-fed oxygen gas as well as electrochemically supplied oxygen. The setup permits injection of isotopic pulses in the reactant streams. The composition and isotopic distribution in the products are recorded with a quadrupole mass spectrometer. The use of the system is exemplified by oxidation of methane over a Au + SrFeO3−δ//YSZ anode at 800–850°C. Pulses of 18O2 in the stream of co-fed O2 were used to study the reactivity and products of gaseous oxygen as distinguished from the electrochemically supplied oxygen. The results indicate that the anode used supports oxygen pumping, but is only moderately active for methane oxidation. The products are mainly CO and CO2. The content of 18O in the products is low, indicating that methane oxidation takes place by 16O-rich lattice oxygen. In comparison, a reference Au//YSZ electrode was found to be a slower anode for oxygen pumping, but a better catalyst for the reaction between CH4 and gaseous O2, seemingly involving adsorbed oxygen.

Natural products continue to provide ever-increasing challenges to synthetic chemists with their complex and sensitive structures. With proper selection, these challenges can be turned into wonderful opportunities to invent and discover new synthetic reactions, to design new strategies and to fashion novel molecular architectures. Developments in these areas often profoundly influence and shape the science of chemical synthesis, biology and medicine. In this article, a number of recent total syntheses of challenging natural substances from the authors's laboratories are highlighted.