Nuclear magnetic resonance studies on dipeptides

Dipeptides of different amino acids have been studied by NMR in aqueous solutions at different pH values. The results obtained are discussed in terms of the different contributions to the chemical shift of the α-CH protons.     

Nuclear magnetic resonance studies of chloroaluminate melts

Nuclear magnetic resonance has been used to study the structures of fused (Na, K)Al₂Cl₇ at 170°C. Sodium, aluminum and chlorine magnetic resonance, and proton magnetic resonance on substitution with NH₄⁺, indicate the structure to be essentially ionic as M⁺ Al₂Cl₇⁻.     

Nuclear magnetic resonance studies of resorcinol-formaldehyde aerogels

In this article, we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, [resorcinol]/[catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized (129)Xe NMR in combination with solid-state (13)C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both coadsorption of chloroform and 2D EXSY (129)Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY (129)Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micropores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (V(g)/S) ratios for aerogels are measured for the first time without the use of geometric models. The V(g)/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in V(g)/S.     

Nuclear magnetic resonance studies of hydrazyl radicals

A study of the nuclear magnetic resonance spectra of six hydrazyl radicals in solution, including α,α′-diphenyl-β-picryl-hydrazyl (DPPH), has been carried out. From the values of the paramagnetic shifts the constants (α_H) of hyperfine coupling with the protons were found. For DPPH these have the following values: On the basis of the analysis of the hyperfine constants a conclusion is drawn concerning the substantial role of σ-electron spin density delocalization in aromatic rings. The geometric structure of radicals investigated is also discussed in the paper.     

Nuclear magnetic resonance studies of adsorbed polymer layers

Nuclear magnetic resonance spectroscopy (NMR) encompasses a large range of techniques which can probe both the structure and dynamics of materials in detail. Of particular interest to the study of polymer adsorption is the ability to distinguish adsorbed segments (trains) from free segments (loops and tails) in a quantitative way. This may he achieved by a variety of experimental approaches and the background to those will be discussed. A brief overview of the necessary NMR theory will be given but for a more detailed treatment, the reader is referred to the large number of texts available [1].     

Nuclear magnetic resonance studies of helical dipyrromethene-zinc complexes

[formula: see text] Analysis of helical chirality within dinuclear dipyrromethene-Zn(II) complexes has been achieved with the use of 1H NMR spectroscopy. The use of AgFOD and chiral lanthanide shift reagents gives fully resolved resonances attributable to two diastereomeric helical complexes.     

Nuclear magnetic resonance microscopy studies of membrane biofouling

There is a substantial need for novel measurement techniques that enable non-invasive spatially resolved observation of biofouling in nanofiltration (NF) and reverse osmosis (RO) membrane modules. Such measurements will enhance our understanding of the key design and operational parameters influencing biofilm fouling. In this study we demonstrate the first application of nuclear magnetic resonance microscopy (NMR) to a spiral wound reverse osmosis (RO) membrane module. The presented NMR protocols allow the extraction of the evolution with biofouling of (i) the spatial biofilm distribution in the membrane module, (ii) the spatially resolved velocity field and (iii) displacement propagators, which are distributions of molecular displacement of a passive tracer (in our case, water) in the membrane. From these measurements, the effective membrane surface area is quantified. Despite the opaque nature of membrane design, NMR microscopy is shown to be able to provide a non-invasive quantitative measurement of RO membrane biofouling and its impact on hydrodynamics and mass transport. Minimal biofilm growth is observed to have a substantial impact on flow field homogeneity.     

Nuclear magnetic resonance relaxation studies of polyacrylamide solution

 The cohesive interaction among polymer chains in a polyacrylamide (PAAm)–D₂O solution has been studied by NMR relaxation. The NMR relaxation times of PAAm in the good solvent D₂O were measured at different temperatures. The results show that the solution system has a high local viscosity and that its relaxation characteristic is soft-solid-like. The temperature dependence of the relaxation behavior of the solution is obviously different from that of ordinary polymer solutions. The difference lies in the relaxation behavior of the methylene protons in the main chain of PAAm, as shown by analyzing the relaxation process with single exponential and biexponential decays. As the temperature increases, the solvation is weakened, leading polymer chains to form curling coils, thus hindering the movement of the methylene protons among the main chains. It can be expected from the existence of 80% fast-relaxing protons that there are a zhigh number of entanglements among the polymer chains in PAAm solution. The information about entanglements among the polymer chains can be deduced from the biexponential dependence of the spin–spin relaxation on the concentration of the polymer solutions. Received: 14 April 1999/Accepted in revised form: 12 October 1999     

Nuclear magnetic triple resonance studies on formamide-14N

The magnitudes of the two spin-spin coupling constants J(N¹H …? ¹³C), and all relatives signs of the couplings J(¹H …? ¹H) and J(¹H …? ¹³C) of formamide-¹⁴N, were determined by triple resonance experiments of the types ¹H—{¹³C}—{¹⁴N} and ¹H—{¹H}—{¹⁴N}.     

Nuclear magnetic resonance studies of convection in the 1,4-cyclohexanedione-bromate-acid reaction

The manifestation and development of convection during pattern formation in the 1,4-cyclohexanedione-acid-bromate reaction was investigated using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) experiments. An apparatus was devised that enabled convection to be probed inside an NMR spectrometer and prevented hydrodynamic motion arising from extraneous sources, such as poor mixing or temperature gradients imposed by the experimental setup. PGSE experiments were performed concurrently with magnetic resonance imaging (MRI) experiments to show that convection arose spontaneously from inhomogeneities associated with the chemical patterns. Quantitative data on diffusion coefficients and hydrodynamic velocities are reported.     

Nuclear magnetic resonance studies of 5,6-dicarboxy-2-norbornene derivatives

A detailed study has been made on the ¹H nuclear magnetic resonance (NMR) spectra of 5,6-dicarboxy-2-norbornene derivatives, whose chemical shifts and geminal, vicinal and long-range spin-spin coupling constants were determined. Each signal of the spectra of these derivatives shows a slight low-field shifts compared with norbornene itself. In the case of five methyl esters studied in this work, the signal of the 7s-proton remains at approximately the same position in all the compounds, but that of the 7a-proton is more variable. For the chemical shifts of the 7a-proton of the five methyl esters, an additive rule seems to hold: that the magnitude of the low-field shift, from the 7a-signal of norbornene itself, is obtained by adding each of the effects of the carboxyl and the carbomethoxy groups substituted at the endo-5-(or 6-) or the exo-5-(or 6-) positions of the norbornene ring. In most of the derivatives considered in this work, the spin-spin coupling constants, except those between bridgehead and bridged methylene protons, are about 1 to 2 Hz smaller than those of many other norbornene derivatives already studied. This fact seems to suggest that norbornenes, which have two bulky adjacent substituents at 5- and 6-positions, may suffer distortions in the norbornene ring.