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

A simple procedure is provided for the quantitative analysis of Diels-Alder adduct prepared from commercial methylcyclopentadiene and maleic anhydride by NMR spectra without further separation of its components. The adduct is considered as a mixture of endo norbornene derivatives (C_n), (D_n), (E) and (F_n) which are obtained from 1-, 2- and 5-methylcyclopentadiene and cyclopentadiene, respectively. The quantitative analysis of the adduct can be made on a basis of the ratio of the signal intensities of the olefinic protons in the adduct. The result shows that the adduct prepared under mild conditions mainly consists of three norbornene derivatives (C_n), (D_n) and (F_n), with a negligibly small amount of (E). When the adduct prepared under mild conditions is heated, it contains the exo isomers (Cx), (Dx) and (Fx). The ratio of the endo and the exo norbornene derivatives, i.e. ((C_n) + (D_n) + (F_n)):((C_x) + (D_x) + (F_x)), may be estimated from the signal intensities of the 5- and 6-protons. As the result of the analyses of the adducts which are obtained by heating at different temperatures the adduct prepared under mild conditions, the mole fraction of 2-methyl isomers (D_n) and (D_x), which is found to be 58 % in the adduct prepared at ? 15°C, keeps on increasing with rising temperature and shows almost the same value, ～86%, when heated above 150°C. Therefore, the equilibrium between 1-methyl and 2-methyl isomers does not change above 150°C. On the other hand, the isomerization from endo to exo isomers is not found below 110°C, and then exo isomers keep on increasing above 140°C.     

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 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 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 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 spectra of codeine and isocodeine derivatives

The NMR spectra of a spiro oxirane ( 1 ) derivative of codeinone, codeine ( 2 ), isocodeine ( 3 ), 6-methylcodeine ( 4 ) and 6-methylisocodeine ( 5 ) were compared. NOE and double-resonance experiments were used to confirm the conformation of 1 , and the configuration about C-6 of 6-methylcodeine and 6-methylisocodeine. An interchange of the chemical shifts of the olefinic protons in 1 was noted, as compared with those in all of the other compounds. This interchange could be attributed to the bond anisotropies of the oxirane moiety.     

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 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 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.     

Proton magnetic resonance studies of 8-hydroxyquinoline derivatives

The PMR spectra of 8-hydroxyquinoline and eighteen derivatives have been obtained at 220 MHz. For several of the compounds, PMR spectra have not been reported previously. The use of 220-MHz frequency considerably facilitates the interpretation of spectra from such complex derivatives as 2-(2'-pyridyl) and 2-(2'-thienyl)-8-hydroxyquinoline.     

Nuclear magnetic resonance spectra of tropic acid and some derivatives

The NMR spectra of tropic acid, its methyl ester and acetyltropic acid methyl ester were measured at 100 and 220 MHz in various solvents. The spectra were analysed by means of an iterative computer procedure. The results indicate, for all the compounds studied, a predominance of the conformation where the phenyl and hydroxyl (or acetoxyl) groups are in anti.positions to each other. The solvent and concentration effects upon the vicinal coupling constants and hence upon the position of the conformational equilibria are rather weak.     

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 the microscopic protonation of L-cysteine

The protonation scheme of L-cysteine has been investigated by nuclear magnetic resonance. Chemical shift data as a function of pH are given for L-cysteine, L-cysteine methyl ester and S-methyl-L-cysteine. Use of derivatives of L-cysteine permits determination of the effect of protonation of the amino, sulfhydryl and carboxylic sites on the chemical shift of the -CH and -CH₂ protons. On the basis of these results, a set of simultaneous equations was written whose solution yielded the fraction of protonation of each site upon the addition of n equivalents of acid to the initially completely deprotonated molecule.     

Nuclear quadrupole resonance studies of structural and magnetic phase transitions

From the temperature dependence study of halogen NQR frequencies, various structural phase transitions have been reported for methylammonium hexahalometallates (IV) and tetramethylammonium hexachlorometallates (IV). These phase transitions and also those of the related compounds are collected and discussed by referring to the results of ¹H NMR and DTA measurements. We have studied several transition metal complexes, mainly copper (II) complexes, by NQR and found magnetic phase transitions from a paramagnetic to a magnetically ordered state, the existence of which has been confirmed by magnetic susceptibility measurements. These results are reviewed.