The effects of restricted diffusion in nuclear magnetic resonance microscopy

A stochastic computer simulation is used to investigate the effects of restricted diffusion in NMR microscopy. It is shown that diffusion contributes to a loss of interfacial resolution through two main mechanisms. The first applies to spatial regions bound by impermeable interfaces and involves diffusive averaging of the frequency differences set up by the applied field gradients. This effect can be made arbitrarily small by increasing the magnitude of the field gradient. The second mechanism involves diffusion through permeable membranes or interfaces defining the sample morphology. This effect can, in principle, be reduced by multiple echo imaging with short pulse spacings. The possibility of imaging diffusive flow through a permeable interface is discussed.     

Putrescine homologues control silica morphogenesis by electrostatic interactions and the hydrophobic effect

A systematic model study on the role(s) of putrescine homologues on silicification is presented and it is proposed that electrostatic forces between additive and silicic acid, and the hydrophobic behaviour of the additives are both important in silicification.     

On the 13C CP/MAS spectra of leucine.

The CP/MAS 13C NMR spectra of crystalline L-leucine and DL-leucine at 7 T are compared with previously reported spectra at lower field strengths. An increasing dominance of chemical shift effects over residual 14N-13C dipolar interactions is observed on the C alpha and C beta splittings with increasing field strength. A new structure is observed in the 25 ppm region of both samples. The spectra in this region were assigned by application of the depolarisation-repolarisation method. The assignment showed differences in the ordering of peaks between solid state and liquid state chemical shifts.     

Slow dynamics in glassy methyl alpha-l-rhamnopyranoside studied by 1D NMR exchange experiments

It is widely known that the ability of sugar glasses to preserve anhydrobiotic systems in nature is important but the process is not yet fully understood. Molecular motions in the glassy state are likely to be important in the process but until now have remained largely uncharacterized. Here we describe the use of 1D 13C NMR exchange experiments using CODEX (centreband only detection of exchange) methods to study the dynamics of the well characterised model glassy monosaccharide, methyl alpha-l-rhamnopyranoside. The glass was prepared by fast cooling of a melt inside an NMR rotor. Molecular motions in the range of seconds to milliseconds were observed in the glass, whereas identical experiments using the crystalline material displayed no observable motions in the time-scales covered by the experiment. At 13 to 14 K above Tg the nature of the motion in the glass changed probably due to the onset of larger scale reorientation. A bimodal distribution of jump angles combined with a broad distribution of correlation times was found to best represent the observed motions.     

Proton NMR relaxation studies of solid tyrosine derivatives and their mixtures with L-leucinamide.

Proton NMR relaxation time measurements were carried out on solid tyrosine derivatives: acetyl-L-tyrosine ethyl ester (Ac-Tyroet), N-carbobenzyloxy-L-tyrosine ethyl ester (CBZ-Tyroet), N-trifluoroacetyl-L-tyrosine ethyl ester (TFAc-Tyroet) and their mixtures with L-leucinamide. It was found that spin-lattice relaxation was driven mainly by methyl group rotation it low temperature for the pure solids and the mixtures. Benzene ring flipping motion and a third motion (possibly whole molecule tumbling) were found to be responsible for relaxing Ac-Tyroet and CBZ-Tyroet at high temperature. However, these motions were highly hindered in TFAc-Tyroet. Molecular tumbling motion was detected in the supercooled liquid mixtures of L-leuNH/CBZ-Tyroet and L-leuNH/Ac-Tyroet, while this motion is absent in the mixture L-leuNH/TFAc-Tyroet. The hindered motion in TFAc-Tyroet may be one of the factors affecting its ability to form a supercooled liquid with L-leucinamide.     

Exploring a Multicriteria Approach to Production Scheduling

A current problem in production scheduling is related to the multi-criteria nature of the task. No matter how effective a heuristic or algorithm is, there remains a need to use human judgement to find a balance between an organization's conflicting objectives, such as meeting due-dates and minimizing work-in-progress. The problem is that heuristics do not have convenient input parameters that relate to these organizational objectives. In this paper we introduce the concept of a ‘control mechanism’ that would allow the scheduler to direct the search for a schedule on the basis of preferences with respect to multiple performance criteria. The feasibility of such a system is dependent on there being a predictable relationship between the input parameters of the scheduling heuristic and the performance measures. We describe a prototype Visual Interactive implementation of the proposed system, present the results of some initial experimental work, which indicates that the method has some potential, and comment on potentially fruitful areas for further research.     

The chaotic-representation property for a class of normal martingales

Suppose $Z=(Z_t)_{t\ge0}$ is a normal martingale which satisfies the structure equation $$d[Z]_t = (\alpha(t)+\beta(t)Z_{t-}) dZ_t + dt$$ . By adapting and extending techniques due to Parthasarathy and to Kurtz, it is shown that, if α is locally bounded and β has values in the interval [-2,0], the process Z is unique in law, possesses the chaotic-representation property and is strongly Markovian (in an appropriate sense). If also β is bounded away from the endpoints 0 and 2 on every compact subinterval of [0,∞] then Z is shown to have locally bounded trajectories, a variation on a result of Russo and Vallois.     

Thermal desorption of H2 and CO on Pt/TiO2

Thermal desorption of D₂ from oxidized titanium surfaces, covered with various amounts of platinum, shows no detectable irreversible adsorption for up to three monolayers of Pt when the substrate temperature is 130 K. Twelve monolayers of Pt give a substrate that adsorbs significant amounts of both CO and H₂.