High-resolution solid-state 13C NMR study of ethylcellulose films

Ethylcellulose films cast from concentrated solutions of chloroform, benzene, and carbon tetrachloride were subjected to the NMR relaxation measurements including ¹H spin-lattice relaxation time (T_1H), rotating-frame ¹H spin-lattice relaxation time (T_1ρH), and ¹³C spin-lattice relaxation time (T_1C). The values of T_1ρH for carbons in the glucose units of ethyl-cellulose were of the same order of magnitude as those reported for the crystalline and noncrystalline regions of ramie cellulose. The values of T_1C for unsubstituted C2, C3 carbons were smaller than those for the corresponding carbons in the noncrystalline region of native celluloses. The T_1C values for unsubstituted C2, C3, and substituted C6 carbons showed a small but definite dependence on the solvent from which the films were cast. © 1993 John Wiley & Sons, Inc.     

High-resolution solid-state MAS 73Ge NMR spectra of some organogermanes

High-resolution solid-state magic angle spinning (73)Ge NMR spectra of some organogermanium compounds were measured. Most tetrasubstituted germanes with identical substituents exhibited signals except for one case. Tetrasubstituted germanes with two kinds of different but somewhat similar substituents exhibited broad peaks. Trisubstituted germanes failed to show signals, indicating the importance of symmetry around germanium.     

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy

Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.     

13C, 25Mg and 11B solid-state NMR study of a fire retarded ethylene-vinyl acetate copolymer

Flame retardance of ethylene-vinyl acetate copolymer (EVA) can be achieved using magnesium hydroxide (Mg(OH)₂) incorporated in the polymeric matrix. The adduct of small amount of zinc borate as synergistic agent in the formulation increases the fire-proofing properties. Multinuclei solid-state NMR appears as a powerful means to characterise materials before and after combustion. We show that endothermic dehydration, water vapour evolved and formation of a glassy coating provide the flame retardancy of interest to the polymeric matrix.     

13C shielding scale for MAS NMR spectroscopy

We have performed the direct measurements of ¹³C magnetic shielding for pure liquid TMS, solution of 1% TMS in CDCl₃ and solid fullerene. The measurements were carried out in spherical ampoules exploring the relation between the resonance frequencies, shielding constants and magnetic moments of ¹³C and ³He nuclei. Next the ¹³C shielding constants of glycine, hexamethylbenzene and adamantane were established on the basis of appropriate chemical shifts measured in the solid state. All the new results are free from susceptibility effects and can be recommended as the reference standards of ¹³C shielding scale in the magic angle spinning NMR experiments. Copyright © 2011 John Wiley & Sons, Ltd.     

Crystalline order in polyethylene: A 13C NMR CP/MAS solid-state T1 study

The carbon-13 spin-lattice relaxation times T₁ of the crystalline components of four solid ethylene-octene copolymers have been studied as a function of thermal history, branching number, and branching distribution. Slowly cooled samples (1 deg/min from melt to room temperature) exhibited similar or longer T₁s with respect to the same sample quench cooled (from the melt into 20°C water). The greater the degree of branching and the more homogeneous the branching distribution, the shorter were the observed crystal lattice T₁s. Differences of up to a factor of 3 in T₁ were observed for the same sample undergoing the two thermal treatments. Different degrees of branching homogeneity (for the same total number of branches) resulted in differences approaching a factor of 7 for samples with the same thermal history. These variations were attributed to the differing effects of side-chain disruption of the crystal lattice.     

High-resolution 13C and 43Ca solid-state NMR and computational studies of the ethylene glycol solvate of atorvastatin calcium

We report ⁴³Ca and ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopic studies of the ethylene glycol solvate of atorvastatin calcium. The ¹³C and ⁴³Ca chemical shift and ⁴³Ca quadrupolar coupling tensor parameters are reported. The results are interpreted in terms of the reported X-ray diffraction crystal structure of the solvate and are compared with the NMR parameters of atorvastatin calcium trihydrate, the active pharmaceutical ingredient in Lipitor®. Hartree–Fock and density functional theory calculations of the NMR parameters based on a cluster model derived from the optimized X-ray diffraction crystal structure of the ethylene glycol solvate of atorvastatin calcium are in reasonable agreement with the experimental ⁴³Ca and ¹³C NMR measurables.     

Industrial applications of high-resolution solid-state 13C NMR techniques

High-resolution solid-state ¹³C NMR is a versatile, non-destructive technique that is useful for the characterization of intractable solids. The NMR experiment provides a unique combination of structural and dynamic information that can be related to the chemical and physical properties of solid materials.     

A 13C solid-state NMR analysis of steroid compounds

(13)C CP/MAS solid-state NMR spectroscopy has been utilized to analyze six steroid compounds, namely testosterone (Tes), hydrocortisone (Cor), trans-dehydroandrosterone (Adr), prednisolone (Prd), prednisone (Pre) and estradiol (Est). Among them, Tes displays a doublet pattern for all residues, whereas Prd, Pre and Est, exhibit exclusively singlets. For Cor and Adr, the (13)C spectra contain both doublet and singlet patterns. The (13)C doublet signal, with splittings of 0.2-1.5 ppm, are ascribed to local differences in the ring conformations associated with polymorphism. We have assigned all of the (13)C resonances to the different residues in these steroid compounds on the basis of solution NMR data. The C-7, C-8, C-10, C-15 and C-16 residues of Tes, Cor and Adr consistently give rise to singlets or doublets with splittings of less than 0.5 ppm, indicating similar local conformations. Accompanying hydration and dehydration processes, a reversible phase transformation between delta- and alpha-crystal forms has been observed in Tes, corresponding to singlet and doublet (13)C patterns, respectively. To further characterize the ring conformations in the alpha-form, we have successfully extracted chemical shift tensor elements for the (13)C doublets. It is demonstrated that (13)C solid-state NMR spectroscopy provides a reliable and sensitive means of characterizing polymorphism in steroids.     

Reaction of o-phenylenediamine with diacetyl monoxime: characterisation of the product by solid-state 13C and 15N MAS NMR

2,3-dimethylquinoxaline (DMQ) and dimethylglyoxime (DMGH2) form a 1:1 hydrogen-bonded complex in the solid state, which is completely dissociated in methanol solution. There are small differences in solid-state 13C shifts between the separated components DMQ and DMGH2 and the complex. The changes in 15N solid-state chemical shifts are more significant: the hydrogen bond imparting a low frequency shift of ca 19 ppm. The effect of direct protonation on the DMQ solid-state 15N shifts was measured, and the experimental 15N data correlated with those from GIAO molecular orbital (MO) calculations.     

Supramolecular interactions probed by 13C-13C solid-state NMR spectroscopy

We present a robust solid-state NMR approach for the accurate determination of molecular interfaces in insoluble and noncrystalline protein-protein complexes. The method relies on the measurement of intermolecular (13)C-(13)C distances in mixtures of [1-(13)C]glucose- and [2-(13)C]glucose-labeled proteins. We have applied this method to Parkinson's disease-associated α-synuclein fibrils and found that they are stacked in a parallel in-register arrangement. Additionally, intermolecular distance restraints for the structure determination of the fibrils at atomic resolution were measured.     

11B and13C NMR studies on silica modified by borate esters

¹¹B and¹³C NMR has been applied along with programmed desorption and boron analysis to the surface of aerosil specimens modified by trimethoxyborane and dimethoxybutyl-borane. It is concluded that methanol is coordinated to the boron atoms in the dimethoxyborosilyl groups.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 118–121, January–February, 1988.     

High-resolution solid-state NMR measurements of oligomeric polysilanes

High-resolution solid-state ¹³C and ²⁹Si NMR spectra have been obtained for (Ph₃Si)₂, (Ph₂Si)₄ and (Ph₂Si)₅ under conditions of high-power decoupling, cross-polarization and magic-angle rotation. The spectra reveal crystallographic inequivalences for (Ph₂Si)₅ but not for the other two compounds. There are significant ²⁹Si chemical shift variations.     

The use of variable temperature 13C solid-state MAS NMR and GIPAW DFT calculations to explore the dynamics of diethylcarbamazine citrate

Experimental ¹³C solid-state magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) as well as Density-Functional Theory (DFT) gauge-including projector augmented wave (GIPAW) calculations were used to probe disorder and local mobility in diethylcarbamazine citrate, (DEC)⁺(citrate)⁻. This compound has been used as the first option drug for the treatment of filariasis, a disease endemic in tropical countries and caused by adult worms of Wuchereria bancrofti, which is transmitted by mosquitoes. We firstly present 2D ¹³C─¹H dipolar-coupling-mediated heteronuclear correlation spectra recorded at moderate spinning frequency, to explore the intermolecular interaction between DEC and citrate molecules. Secondly, we investigate the dynamic behavior of (DEC)⁺(citrate)⁻ by varying the temperature and correlating the experimental MAS NMR results with DFT GIPAW calculations that consider two (DEC)⁺ conformers (in a 70:30 ratio) for crystal structures determined at 293 and 235 K. Solid-state NMR provides insights on slow exchange dynamics revealing conformational changes involving particularly the DEC ethyl groups.     

7Li and 13C solid-state NMR spectra of lithium cuprates

7Li and 13C solid-state MAS NMR spectra of three lithium cuprates with known X-ray structures--lithium([12]crown-4)2 dimethyl and diphenyl cuprate (1,2) and lithium(thf)4-[tris(trimethylsilyl) methyl]2 cuprate (3)--have been measured and analysed with respect to the quadrupolar coupling constants of lithium-7, chi(7Li), and the asymmetry parameters of the quadrupolar interactions, eta(7Li), as well as the 6, 7Li and 13C chemical shifts. The chi(7Li) values of 23, 30, and 18 kHz for 1, 2 and 3, respectively, are in line with the high symmetry around the lithium nucleus in the solvent-separated structures and may be used as reference data for this structural motif. Calculations based on charges derived from ab initio 6-31 G* HF computations using the point charge model (PCM) and the program GAMESS support the experimental findings.