1H-Observe 1H?2H Dipolar Recoupling by REDOR and Rotary Resonance Recoupling

The measurement of heteronuclear dipolar couplings by solid-state NMR is an important tool for determining structures in solids. ¹H-observe ¹H ²H REDOR and rotary resonance recoupling experiments with fast magic-angle spinning are presented. These dipolar recoupling experiments show promise for measuring heteronuclear dipolar couplings with ¹H nuclei. Potential limitations of the experiments include short transverse relaxation times of the ¹H magnetization and Bloch Siegert effects.     

Techniques for the Measurement of Small Anisotropies in Magic Angle Spinning NMR Spectra

Two novel methods for enhancing the effect of small anisotropies on magic angle spinning (MAS) NMR spectra are described. The first method is a simple two-dimensional (2D) spin echo experiment, which generates rotational sidebands at a fraction of the spinning frequency in the ω₁ dimension, while maintaining the effective spinning speed in ω₂. It is shown that the rotational sidebands in the 2D spectrum are much larger than in a normal one-dimensional MAS spectrum obtained at the same spinning speed, and the relative intensities of the two-dimensional sideband array can be analyzed to yield the anisotropy and asymmetry parameter of the coupling tensor. The second method is designed to measure homonuclear dipolar couplings. We show that the effect of a dipolar coupling between members of an isolated spin pair can be enhanced by adjusting the spinning speed such that nω_r = ω_Δ^iso, where n is an integer, ω_r is the spinning speed, and ω_Δ^iso is the difference between isotropic shifts. When this condition, termed rotational resonance, is satisfied, a broadening or a splitting of the normally sharp resonance lines is observed. In addition, rapid exchange of Zeeman order between the dipolar coupled spins is seen. This method holds promise for the measurement of through space dipolar couplings and thus internuclear distances in polycrystalline solids.     

C NMR study of C-enriched single-wall carbon nanotubes synthesized by catalytic decomposition of methane

¹³C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% ¹³C enrichments and natural abundance (1.1% ¹³C) prepared by catalytic decomposition of CH₄. The ¹³C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the ¹³C enrichment. The ¹³C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the ¹³C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between ¹³C spins as well as between ¹³C and ¹H spins at defects or edges. ¹H NMR spectra confirm the presence of H-containing species. The ¹³C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.     

1H and 13C n.m.r. spectroscopic methods for the analysis of fossil fuel materials: Some novel approaches

A number of¹H and¹³C n.m.r. methods, apart from conventional procedures which rely on chemical shift classifications alone, can be used to identify chemical or structural characteristics of fossil fuel materials. The information available via multiplet selection in ¹³C n.m.r., two-dimensional J spectroscopy, homonuclear ¹H-¹H correlation spectroscopy, and heteronuclear ¹³C-¹H correlation procedures, is discussed and illustrated.     

Characterization of epoxidized natural rubber by 2D NMR spectroscopy

Assignment of signals in aliphatic region of ¹H NMR spectrum for epoxidized natural rubber was carried out through NMR spectroscopy. The epoxidized natural rubber was prepared by epoxidation of purified natural rubber with peracetic acid in latex stage followed by degradation with propanal and ammonium persulfate. The resulting liquid epoxidized natural rubber was characterized through 1D- and 2D-NMR spectroscopy. The unknown signals in the aliphatic region of the ¹H NMR spectrum were assigned through ¹³C NMR and two-dimensional heteronuclear shift correlation (HETCOR) measurement. The assignments were proved by two-dimensional inverse detected heteronuclear long-range shift correlation (HMBC) and two-dimensional homonuclear shift correlation (COSY) measurements, and they were supported with epoxidized squalene as a model compound through NMR spectroscopy.     

Measurement of interatomic connectivities in molecular sieves using MQMAS-based methods

The multiple-quantum magic angle spinning (MQMAS) NMR method can be extended to allow for investigation of the interatomic connectivities between quadrupolar and spin- nuclei. Several new experiments will be reviewed that combine MQMAS with cross polarization (CP) or heteronuclear recoupling via rotational echo double resonance (REDOR) in order to probe the dipolar interactions between quadrupolar and spin- nuclei under high-resolution conditions. The theoretical and experimental aspects of the spin dynamics involved in these new methods will be discussed. Examples of experimental results will be presented, involving the ¹H–²⁷Al , and ¹⁹F–²⁷Al spin pairs in fluorinated and hydrated aluminophosphates.     

Two-dimensional n.m.r. analysis of aromatic fractions from a coal liquefaction solvent

Two-dimensional n.m.r. experiments correlating heteronuclear ¹H-¹³C and homonuclear ¹H-¹H spin-spin coupling have enabled various aliphatic groups to be identified unambiguously in complex mono- and diaromatic fractions from a coal liquefaction solvent. These groups include those that are generally difficult to identify by g.c.-m.s. such as five membered rings and CH₃ substituted in cyclic structures.     

Mesomorphic behavior of polydiethylphosphazenes as described by13C,14N, and2H NMR

The existence of mesomorphism in polydiethylphosphazene was recently established by MAS NMR and X-ray diffraction characterization. In the present work the mechanism of motion of the ethyl side groups in the high-temperature polymorph tabove 45 C) is identified and compared to the arrangement of side groups in the low-temperature polymorph. For this purpose a few NMR active nuclei (¹³C,¹⁴N, and²H) were exploited to define the side-chain motions occurring at transition. Experiments performed at varying temperatures close to the onset of solid transition suggest the presence of jumps between two conformations in the pretransition state. Rotor-synchronized triple-resonance NMR of the high-temperature phase determined the average distances between the carbons and the nitrogens in the polymorphs. The theoretical prediction of the dipolar interaction between the nuclei supports the hypothesis that ethyl groups can undergo a complete rotation about the P CH₂ bond by jumping across a conformational barrier. The mechanism of motion of the ethyl groups must be cooperative and the collapse of the rigid shell around the main chain is described at the transition.Presented at the 1st Italian Workshop on Cyclo- and Poly (phosphazene) Materials. February 15 16, 1996, at the CNR Research Area in Padova, Italy.     

Mesomorphic behavior of polydiethylphosphazenes as described by13C,14N,and2H NMR

The existence of mesomorphism in polydiethylphosphazene was recently established by MAS NMR and X-ray diffraction characterization. In the present work the mechanism of motion of the ethyl side groups in the high-temperature polymorph tabove 45 C) is identified and compared to the arrangement of side groups in the low-temperature polymorph. For this purpose a few NMR active nuclei (¹³C,¹⁴N, and²H) were exploited to define the side-chain motions occurring at transition. Experiments performed at varying temperatures close to the onset of solid transition suggest the presence of jumps between two conformations in the pretransition state. Rotor-synchronized triple-resonance NMR of the high-temperature phase determined the average distances between the carbons and the nitrogens in the polymorphs. The theoretical prediction of the dipolar interaction between the nuclei supports the hypothesis that ethyl groups can undergo a complete rotation about the P CH₂ bond by jumping across a conformational barrier. The mechanism of motion of the ethyl groups must be cooperative and the collapse of the rigid shell around the main chain is described at the transition.     

Backbone and side-chain 13C and 15N signal assignments of the alpha-spectrin SH3 domain by magic angle spinning solid-state NMR at 17.6 Tesla

The backbone and side-chain 13C and 15N signals of a solid 62-residue (u-13C,15N)-labelled protein containing the alpha-spectrin SH3 domain were assigned by two-dimensional (2D) magic angle spinning (MAS) 15N-13C and 13C-13C dipolar correlation spectroscopy at 17.6 T. The side-chain signal sets of the individual amino acids were identified by 2D 13C-13C proton-driven spin diffusion and dipolar recoupling experiments. Correlations to the respective backbone nitrogen signals were established by 2D NCACX (CX=any carbon atom) experiments, which contain a proton-nitrogen and a nitrogen-carbon cross-polarisation step followed by a carbon-carbon homonuclear transfer unit. Interresidue correlations leading to sequence-specific assignments were obtained from 2D NCOCX experiments. The assignment is nearly complete for the SH3 domain residues 7-61, while the signals of the N- and C-terminal residues 1-6 and 62, respectively, outside the domain boundaries are not detected in our MAS spectra. The resolution observed in these spectra raises expectations that receptor-bound protein ligands and slightly larger proteins (up to 20 kDa) can be readily assigned in the near future by using three-dimensional versions of the applied or analogous techniques.     

Structural Characterization of Frustrated Lewis Pairs and Their Reaction Products Using Modern Solid-State NMR Spectroscopy Techniques

Frustrated Lewis pair (FLP) chemistry has provided a new strategy for small-molecule binding and/or catalytic activation. The most prominent FLPs are based on intramolecular phosphane borane adducts, the catalytic properties of which can be tailored over wide ranges of reactivity and selectivity. Advanced solid-state NMR spectroscopic techniques, together with DFT calculations, can provide new structural insights in these systems. This review illustrates the utility of ³¹P and ¹¹B NMR chemical shifts, ¹¹B electric field gradient tensors, and ³¹P ¹¹B indirect and direct dipole dipole interactions for characterizing intramolecular borane phosphane FLPs. We demonstrate the potential of this method to 1) quantify the extent of boron phosphorus bonding interactions (and hence, the “degree of frustration”); 2) reveal specific structural details (i.e., boron phosphorus distances and other local geometric aspects) related to their catalytic activities; and (3) characterize products of FLP reactions with regard to molecular structure, stereochemistry, and aggregation properties in terms of internuclear distances, bonding connectivities, and orientational parameters.     

Solid‐state NMR characterization of cyclomaltoheptaose (β‐cyclodextrin) polymers using high‐resolution magic angle spinning with gradients

Solid‐state nuclear magnetic resonance (NMR) techniques were used to characterize cyclomaltoheptaose (β‐cyclodextrin, β‐CD) polymers. These insoluble materials have been investigated by cross‐polarization magic angle spinning with dipolar decoupling (CP/MAS), magic angle spinning without dipolar decoupling (MAS), and high‐resolution magic angle spinning with gradients (HRMAS). These NMR spectra allow the assignment of the principal ¹H and ¹³C signals. The presence of two distinct components (cross‐linked β‐CD and polymerized epichlorohydrin) in the materials was clearly demonstrated. These polymers were used as sorbents and the resulting NMR spectra are presented and discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1288–1295, 2000     

Hydrogen bond interaction and dynamics in PMMA/PVPh polymer blends as revealed by advanced solid-state NMR

The hydrogen bond interactions and dynamics of the poly (methyl methacrylate) (PMMA) and poly (4-vinyl phenol) (PVPh) polymer blends were studied by a variety of advanced solid-state NMR techniques. The possible hydrogen bond interactions between the carbonyl group of PMMA and hydroxyl group of PVPh were successfully elucidated by two-dimensional ¹H–¹H spin-exchange ¹³C–¹H heteronuclear chemical-shift correlation (HETCOR) NMR experiment. The ¹³C 2D separation of undistorted powder patterns by effortless recoupling (SUPER) experiments combined with quantum chemical calculations for the theoretical predictions of chemical-shift anisotropy (CSA) parameters were utilized to investigate the intermolecular hydrogen-bonding interactions and molecular conformation of the blends. ¹³C–¹H PISEMA (polarization inversion and spin-exchange at magic angle) experiments at different temperatures were used to reveal the heterogeneous dynamics resulting from the cooperative motion associated with the hydrogen bond interaction. PISEMA analysis shows that the motion of aromatic group in PVPh is affected by the rotating motions of methyl in PMMA.     

1H-NMR-, 13C-NMR- und IR-Untersuchungen zur Tautomerie von 15N-markiertem 3-Methyl-1-phenyl-Δ2-pyrazolin-5-on

¹H-N. M. R., ¹³C N. M. R., and I. R.-Investigations Concerning Tautomerism of ¹⁵N-Labeled 3-Methyl-1-phenyl-Δ²-pyrazolin-5-one The behaviour of the ¹⁵N-mono and dilabeled compound in dependence on temperature and solvent polarity is described. Chemical shifts, ⁿJ(¹⁵N ¹H), and ⁿJ(¹⁵N ¹³C) coupling constants (n = 1 – 3) have been determined. In the cases of 2, 2, 2-trifluoroethanole and hexafluoroisopropanole the Δ⁴-pyrazolin-3-one is stabilized and detected by ³J(¹⁵N 2 H 4) = 3 Hz, ¹J(¹³C 3 ¹⁵N 2) = 11, 0 Hz and δ(¹³C 5). In HMPT the 5-hydroxypyrazole form predominates and is characterized by ³J(¹⁵N 1 H 4) = 5, 1 Hz and δ (¹³C 5). Coupling constants ³J(¹⁵N 2 H 4) and J(¹³C 3 ¹⁵N 2) could not be observed in this solvent. In accordance with ¹H- and ¹³C-n. m. r.-parameters i. r. measurements in dependence on temperature lead to the conclusion that in DMSO and THF for instance all three tautomers are present and variation of temperature does not change the equilibrium between the tautomers. A good relationship between the content of the CH₂, OH and NH forms and the Donor Numbers (DN) and Acceptor Numbers (AN) by Gutmann has been found.     

A Morphological Study of Poly[Bis(Trifluoroethoxy)Phosphazene] Using Solid-State NMR: Introducing Domain Selective 1H and 19F Decoupled 13C MAS NMR

Using solid-state NMR methods the morphological behavior of poly[bis(trifluoroethoxy)phosphazene] was studied, employing four nuclei of interest – ¹H, ¹⁹F, ³¹P and ¹³C. Measurements on all four nuclei support that at ambient temperature the crystalline and amorphous phases coexist. Variable temperature studies showed that above T(1) = 90° only a single highly mobile phase exists, which is presumed to be the 2D mesophase. All four nuclei showed that when heat cycling the polymer, repeatedly above T(1), an increase in crystallinity occurs with each cycle. For the first time ¹³C MAS NMR spectra, using high power ¹⁹F and ¹H decoupling, were obtained, which exhibited the same behaviour domain. Filtered ¹³C{¹H,¹⁹F} MAS spectra containing signal from the crystalline domain using the discrimination induced by variable amplitude minipulses (DIVAM) sequence were measured. Heat treated and solvent cast material showed differences in these ¹³C spectra, that were consistent with a decrease in backbone conformations upon heating, suggesting an increase in the extended chain form corresponding to the γ form. Analogous sensitivity to variations in crystal phase composition has not been seen previously using ¹H, ¹⁹F and ³¹P methods, emphasizing the importance of ¹³C MAS methods to morphological studies of phosphazenes. This paper is dedicated to Professor Harry R. Allcock     

Formation of Si–H groups during the functionalization of mesoporous silica with Grignard reagents

A Grignard reaction is carried out upon halogenated SBA-15 type mesoporous silica, with different alkyl groups and reaction conditions, in order to modify its surface with these groups. The resulting solids are analyzed by infrared, thermal analysis, ¹⁹F MAS NMR, ¹H MAS NMR and elemental analysis. Parallel to the Grignard reaction an unusual reaction producing hydrosilane end groups (Si–H) is observed. A possible mechanism for the production of these groups from the fluorinated material is proposed, which involves a transition state of a six members ring formed with two Grignard molecules and a silicon bonded to at least two fluorine atoms.     

Study of microbial polyhydroxyalkanoates using two-dimensional Fourier-transform infrared correlation spectroscopy

The premelting behavior of bacterially synthesized polyester poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), abbreviated as P(HB-co-HHx), was investigated by two-dimensional Fourier-transform infrared (2D FTIR) correlation spectroscopy. The temperature-dependent dynamic spectra were measured over a temperature range of 25–300°C. We focused our study on the thermally induced intensity fluctuations of bands for CO (1700–1760 cm⁻¹), C H (2910–3010 cm⁻¹) and C O C groups (1220–1310 cm⁻¹) stretching vibrations. Changes of crystalline conformation due to the thermal perturbation could be detected by the intensity and location variations of those characteristic bands responding to the variations of dipole moments. 2D correlation analysis indicated that the appearance of fully amorphous component did not happen simultaneously with the disappearance of crystalline component, suggesting that there was an intermediate state between ordered crystalline and amorphous states in P(HB-co-HHx). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 934–940, 2001     

<Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR characterization and stereochemical assignments of bile acids in aqueous media

The unconjugated bile acids cholic acid, deoxycholic acid, and chenodeoxycholic acid; their glycine and taurine conjugated glycocholic acid, glycodeoxycholic acid, glycochenodeoxycholic acid, taurocholic acid, taurodeoxycholic acid, and taurochenodeoxycholic acid; and a taurine conjugated ursodeoxycholic acid, tauroursodeoxycholic acid, were characterized through ¹H and ¹³C NMR in aqueous media under the physiological pH region (7.4±0.1). Assignments of ¹H and ¹³C signals of all the bile acids were made using a combination of several one- and two-dimensional, homonuclear (¹H−¹H) and heteronuclear (¹H−¹³C) correlations as well as spectral editing NMR methods. Stereochemical assignment of the five-membered ring of the bile acids is reported here for the first time. The complete characterization of various bile acids in aqueous media presented here may have implications in the study of the pathophysiology of biliary diseases through human biliary fluids using NMR spectroscopy.