Cross-polarization NMR Research Articles

Efficiency of 1H→ 31P NMR cross-polarization in bone apatite and its mineral standards

Human bone mineral was studied using solid-state 31P NMR with cross-polarization (CP) from protons. The CP efficiency was determined for trabecular and cortical bone tissue from human adults and compared with synthetic mineral standards. The study shows the similarity between carbonatoapatite of type B and bone mineral as shown by their CP behaviour. The method can be used for the characterization of synthetic apatite-based implant materials.

Solution mechanisms of H 2O in depolymerized peralkaline melts

Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°–1300°C) at high pressure (0.8–2.0 GPa) have been examined in peralkaline melts in the system Na 2O-SiO 2-H 2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂X H2O, decreases with increasing total water content. At low water contents, the influence of H 2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q 4-species to form Q 3 and Q 2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q 3-species react with water to form Q 2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H- 29Si cross polarization NMR data.

Assessing the quantitative reliability of solid-state 13C NMR spectra of kerogens across a gradient of thermal maturity

Five type II kerogens, shown by elemental analysis and Rock-Eval pyrolysis to represent a gradient of thermal maturity, were further characterized using a range of solid-state 13C NMR spectroscopic techniques. 13C cross polarization (CP) NMR spectra of the kerogens confirmed the well-established pattern of increasing aromaticity with increasing thermal maturity. Spin counting showed that CP observability was around 50% for the immature kerogens, and only 14–25% for the mature kerogens. Spin counting also showed that the direct polarization (DP) observabilities were >80% for all but one of the kerogens. Despite the large differences in observability between the two techniques, aromaticities derived from corresponding CP and DP spectra differed by only 1–15%. The RESTORE technique showed that the low CP observability of the immature kerogens was due mostly to rapid T 1 ρH relaxation, whereas both rapid T 1 ρH relaxation and slow polarization transfer contributed to the low CP observability of the mature kerogens.

Does Solid-state 15N NMR Spectroscopy Detect all Soil Organic Nitrogen?

Virtually all of the N detected by 15N cross polarization (CP) NMR spectra of four HF-treated soil clay fractions is amide N. However, the intensity of this 15N CP NMR signal (per unit N) is 27–57% lower than detected for a wheat protein, gliadin. There are two possible explanations – either the amide N in the soil clay fractions produces proportionately less NMR signal than does the amide N in gliadin, or part of the N in the soil clay fractions produces little or no NMR signal. The cross polarization dynamics of the gliadin amide resonance and amide resonances detected for the soil clay fractions are very similar and thus should produce similar amounts of signal, ruling out the first possibility. Therefore up to half or even more of the organic N in these soil clay fractions must be in a form that is insensitive to NMR detection. For a model compound (caffeine), non-protonated heterocyclic N produced less than 20% of the signal of an equivalent amount of amide N in gliadin. Results from several 13C NMR techniques provide further evidence that much of the undetected N in the soil clay fractions may be heterocyclic.

Spectroscopic Identification of the Mixed Hydrogen and Carbon Dioxide Clathrate Hydrate

In this contribution, we first found the novel clathrate hydrate containing two gaseous guests of hydrogen and carbon dioxide by spectroscopic analysis. X-ray powder diffraction and NMR spectroscopy were used to identify structure and guest distribution of the mixed H2 + CO2 hydrate. X-ray diffraction result confirmed that the unit cell parameter was 11.8602 +/- 0.0010 A, and the formed hydrate was identified as structure I hydrate. 1H magic angle spinning (MAS) NMR and 13C cross-polarization (CP) NMR spectroscopy were used to examine the distribution of hydrogen and carbon dioxide molecules in the cages of structure I, respectively. These NMR spectra showed that carbon dioxide molecules occupied both small 512 cages and large 51262 cages, and hydrogen molecules only were occluded in small 512 cages of structure I. The new finding of the mixed hydrogen hydrate is expected to contribute toward the development of hydrogen production technology and, particularly, inclusion chemistry.

Measurement of NMR Cross-Polarization (CP) Rate Constants in the Slow CP Regime: Relevance to Structure Determinations of Zeolite−Sorbate and Other Complexes by CP Magic-Angle Spinning NMR

When analyzing I --> S variable contact time cross-polarization (CP) curves, the spin dynamics are usually assumed to be describable in the "fast CP regime" in which the growth of the S spin magnetization is governed by the rate of cross polarization while its decay is governed by the rate of I spin T1rho relaxation. However, in the investigation of the structures of zeolite-sorbate and other complexes by polarization transfer this will not necessarily be the case. We discuss the measurement of I --> S CP rate constants under the "slow CP regime" in which the rate of T1rho relaxation is fast compared to the rate of cross polarization, leading to a reversal of the usual assumptions such that the rate or growth is governed by the rate of I spin T1rho relaxation while the decay is governed by the rate of cross polarization (and the S spin T1rho relaxation). It is very important to recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will lead to erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolute values for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated to the scaling factor, the contribution from 29Si T1rho relaxation is ignored, and it is difficult to obtain reliable data at very long contact times. The use of a 29Si{1H} CP "drain" or "depolarization" experiment, which measures absolute values of the CP rate constants, is therefore proposed as being most appropriate for theses situations. To illustrate the importance of these observations, measurements of the 1H-29Si CP rate constants in the p-dichlorobenzene/ZSM-5 sorbate-zeolite complex by 29Si{1H} CP and CP drain magic-angle spinning (MAS) NMR experiments are presented and compared and used to determine the location of the guest sorbate molecules in the cavities of the host zeolite framework.

Investigation of the Role of Structural Domains Identified in Sedimentary Organic Matter in the Sorption of Hydrophobic Organic Compounds

The role of composition and structure of sedimentary organic matter (SOM) in the sorption of hydrophobic organic compounds (HOCs) was investigated by spiking 13C-labeled phenanthrene onto six estuarine sediments known to vary in SOM content and character. After equilibration and HF treatment, 13C NMR cross polarization and stable carbon isotope analyses indicated that the amount of desorption-resistant phenanthrene was related to aromatic carbon content. Application of the 13C NMR spectral editing technique proton spin relaxation editing (PSRE) demonstrated that all samples consisted of a rapidly relaxing and a slowly relaxing component, further evidence that SOM can be described as a structurally heterogeneous sorbent. Further, comparison of corresponding control and spiked PSRE subspectra revealed that, for each of the six sediments, desorption-resistant phenanthrene had become associated almost exclusively with the rapidly relaxing component. In only two of the sediments were there even small amounts of phenanthrene discernible in the slowly relaxing component, which is signficant as it was not always true that aromatic carbon was concentrated exclusively in the rapidly relaxing phase. The implication of these findings is that not all aromatic fractions have the same affinity for phenanthrene and that some fractions may indeed have little affinity at all. These results were interpreted as indicative that rapidly relaxing aromatic carbon associated with either sediment-associated charcoal or diagenetic organic matter plays a controlling role in the sorption of HOCs. However, the exact manner in which this rapidly relaxing aromatic phase relates to models presented elsewhere remains unclear.

Conformational heterogeneity of transmembrane residues after the Schiff base reprotonation of bacteriorhodopsin

bR, N-like and O-like intermediate states of [15N]methionine-labelled wild type and D85N/T170C bacteriorhodopsin were accumulated in native membranes by controlling the pH of the preparations. 15N cross polarization and magic angle sample spinning (CPMAS) NMR spectroscopy allowed resolution of seven out of nine resonances in the bR-state. It was possible to assign some of the observed resonances by using 13C/15N rotational echo double resonance (REDOR) NMR and Mn2+ quenching as well as D2O exchange, which helps to identify conformational changes after the bacteriorhodopsin Schiff base reprotonation. The significant differences in chemical shifts and linewidths detected for some of the resonances in N- and O-like samples indicate changes in conformation, structural heterogeneity or altered molecular dynamics in parts of the protein.

Efficient Recovery of CO2 from Flue Gas by Clathrate Hydrate Formation in Porous Silica Gels

Thermodynamic measurements and NMR spectroscopic analysis were used to show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixtures using water dispersed in the pores of silica gel. Kinetic studies with 1H NMR microimaging showed that the dispersed water in the silica gel pore system reacts readily with the gas, thus obviating the need for a stirred reactor and excess water. Hydrate phase equilibria for the ternary CO2-N2-water system in silica gel pores were measured, which show that the three-phase hydrate-water-rich liquid-vapor equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization NMR spectral analysis and direct measurement of the CO2 content in the hydrate phase suggested that the mixed hydrate is structure I at gas compositions of more than 10 mol % CO2, and that the CO2 molecules occupy mainly the more abundant 5(12)6(2) cages. This makes it possible to achieve concentrations of more than 96 mol % CO2 gas in the product after three cycles of hydrate formation and dissociation. 1H NMR microimaging showed that hydrate yields of better than 85%, based on the amount of water, could be obtained in 1 h when a steady state was reached, although approximately 90% of this yield was achieved after approximately 20 min of reaction time.

Dynamics of the quasiordered structure in the regioregulatedπ-conjugated polymer poly(4-methylthiazole-2,5-diyl)

The dynamics of regioregulated poly(4-methylthiazole-2,5-diyl) (HH-P4MeTz) was investigated by solid-state $^{1}\mathrm{H},\phantom{\rule{0.3em}{0ex}}^{2}\mathrm{D}$, and $^{13}\mathrm{C}$ NMR spectroscopies, and differential scanning calorimetry (DSC) measurements. DSC, $^{2}\mathrm{D}$ quadrupolar echo NMR, $^{13}\mathrm{C}$ cross-polarization and magic-angle spinning (CPMAS) NMR, and two-dimensional spin-echo CPMAS NMR spectroscopy suggest existence of a quasiordered phase in which backbone twists take place with weakened $\ensuremath{\pi}$ stackings. Two-dimensional exchange $^{2}\mathrm{D}$ NMR (2DEX) detected slow dynamics with a rate of an order of ${10}^{2}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$ for the $\mathrm{C}{D}_{3}$ group in ${d}_{3}$-HH-P4MeTz at 288 K. The frequency dependence of proton longitudinal relaxation rate at 288 K shows a ${\ensuremath{\omega}}^{\ensuremath{-}1∕2}$ dependence, which is due to the one-dimensional diffusionlike motion of backbone conformational modulation waves. The diffusion rate was estimated as $3\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$, which was approximately ${10}^{7}$ times larger than that estimated by 2DEX NMR measurements. These results suggest that there exists anomalous dispersion of modulation waves in HH-P4MeTz. The one-dimensional group velocity of the wave packet is responsible for the behavior of proton longitudinal relaxation time. On the other hand, the 2DEX NMR is sensitive to the phase velocity of the nutation of methyl groups that is associated with backbone twists. From proton ${T}_{1}$ and ${T}_{2}$ measurements, the activation energy was estimated as 2.9 and $3.4\phantom{\rule{0.3em}{0ex}}\mathrm{kcal}∕\mathrm{mol}$, respectively. These were in agreement with $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{kcal}∕\mathrm{mol}$ determined by a M\o{}ller-Plesset molecular orbital calculation. We also performed a chemical shielding calculation of the methyl carbon in order to understand chemical shift tensor behavior, leading to the fact that a quasiordered phase coexists with the crystalline phase.

Probing segmental order in lipid bilayers at variable hydration levels by amplitude- and phase-modulated cross-polarization NMR

The conformational response of dimyristoylphosphatidylcholine bilayers in the liquid crystalline phase to hydration is investigated by a novel magic-angle spinning cross-polarization NMR technique.

Quantum dynamics under coherent and incoherent effects of a spin bath in the Keldysh formalism: application to a spin swapping operation

We develop the Keldysh formalism for the polarization dynamics of an open spin system. We apply it to the swapping between two qubit states in a model describing an NMR cross-polarization experiment. The environment is a set of interacting spins. For fast fluctuations in the environment, the analytical solution shows effects missed by the secular approximation of the quantum master equation for the density matrix: a frequency decrease depending on the system-environment escape rate and the quantum quadratic short time behavior. Considering full memory of the bath correlations yields a progressive change of the swapping frequency.

Solid State NMR Characterisation of Encapsulated Molecules in Mesoporous Silica

Ibuprofen molecules have been encapsulated in mesoporous MCM-41 type-silica functionalised or not by amino groups. They have been characterised by 13C and 1H solid state NMR spectroscopy. The 13C MAS single pulse or cross polarization NMR spectra, as well as the 1H MAS NMR spectra demonstrate an extremely high mobility of the ibuprofen molecules when the matrix is not functionalised. On the contrary, when the silica matrix is functionalized by amino groups, the 13C NMR response shows less mobility suggesting the existence of interactions between the amino groups and the carboxylic groups. Benzoic acid as well as benzamide have also been encapsulated and their NMR responses compared to that of ibuprofen.

19F/ 29Si distance determination and heteronuclear spin counting under fast magic-angle spinning in fluoride-containing octadecasil

19F/ 29Si rotational-echo double-resonance (REDOR), θ-REDOR and Hartmann–Hahn cross-polarization (CP) NMR techniques have been applied under fast magic-angle spinning (MAS) to a powder sample of fluoride-containing octadecasil. The internuclear distance of the 19F– 29Si spin pairs formed by the silicons in the D4R units (T-1 site) and the fluoride anions deduced directly from the CP and REDOR measurements is in very good agreement with the X-ray crystal structure. Moreover, heteronuclear spin-counting θ-REDOR experiments unambiguously show that the 19F and the T-1 29Si nuclei form fairly isolated spin pairs, whereas multiple-spin behaviour is clearly evidenced at the T-2 site. To cite this article: P. Bertani et al., C. R. Chimie 7 (2004).

Spin-state-selective excitation in gradient-selected heteronuclear cross-polarization NMR experiments

Several heteronuclear coherence transfer mechanisms involved in proton-detected heteronuclear J-cross-polarization (HCP) NMR experiments have been theoretically derived and experimentally verified in isotropic solution. It is shown that in-phase and/or anti-phase heteronuclear coherence transfer can take place separately or simultaneously during the HCP process as a function of the relative phase between the HCP mixing sequence and the corresponding input magnetization. As the more important consequence, clean coherence-order and spin-state selective (S 3) excitation with maximum sensitivity can be achieved from gradient-enhanced HCP experiments by proper co-addition/subtraction of in-phase and anti-phase magnetizations, offering an attractive alternative to widely used HSQC-type experiments.

Structure and Guest Distribution of the Mixed Carbon Dioxide and Nitrogen Hydrates As Revealed by X-ray Diffraction and 13C NMR Spectroscopy

In this contribution, X-ray diffraction and 13C NMR spectroscopy were used to identify structure and guest distribution of the mixed N2 + CO2 hydrates. X-ray diffraction results of the mixed N2 + CO2 hydrates confirmed that the unit cell parameter was ∼11.8 A over the gas mixture composition range of 3−20 mol % CO2 and the formed hydrates were identified as structure I. When the composition of the gas mixture was reduced to 1 mol % CO2, the structure of the mixed hydrate was transformed to structure II showing the unit cell parameter of 17.26(2) A. 13C cross-polarization (CP) NMR spectroscopy was used to examine the distribution of carbon dioxide molecules in small 512 and large 51262 cages of structure I. From NMR spectra of the mixed N2 + CO2 hydrate formed from gas mixture of 20 and 10 mol % CO2, powder patterns having chemical shift anisotropy of −54.5 and −53.8 ppm were observed, respectively. There was no clear isotropic line indicating carbon dioxide molecules in the small 512 cages of structure I....

Ordering in nematic liquid crystals from NMR cross-polarization studies

The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented.

Resolving the different dynamics of the fluorine sublattices in the anionic conductor BaSnF(4) by using high-resolution MAS NMR techniques.

19F and (119)Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF(4), a member of the MSnF(4) family of fluorite-related anionic conductors containing double layers of Sn(2+) and M(2+) cations. Two fluorine sublattices were observed by (19)F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba(2+) double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba(2+) and Sn(2+) layers, and the few fluorine atoms that inhabit the Sn(2+)-Sn(2+) double layers. These ions are in rapid exchange with each other, and an extremely short correlation time tau(C) for the motion of these ions of <3 x 10(-)(5) s is obtained at -100 degrees C. T(1) measurements indicate that tau(C) approaches 10(-)(8) s at room temperature. (19)F-to-(119)Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional (19)F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (tau(C) approximately 10 ms at 250 degrees C). Low-temperature (19)F MAS and (19)F-to-(119)Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by -150 degrees C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF(2)-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6-7 orders of magnitude slower.

Investigation of Molecular Motions by Lee-Goldburg Cross-Polarization NMR Spectroscopy

We demonstrate the use of Lee-Goldburg cross-polarization (LG-CP) NMR under fast magic-angle spinning (MAS) to investigate the amplitude and geometry of segmental motions in biomolecular and polymeric solids. Motional geometry information was previously available only from 2 H NMR, which, however, has limited site resolution and requires site-specific isotopic labeling. Using a 2D LG-CP technique, we resolve the 1 3 C- 1 H or 1 5 N- 1 H dipolar couplings according to the 1 3 C or 1 5 N isotropic chemical shift. Applications to systems undergoing 180° phenylene ring flips show spectral line shapes reflecting the geometry of the motion. Using this LG-CP technique, we measured the 1 3 C- 1 H and 1 5 N- 1 H dipolar couplings in the water-soluble and membrane-bound states of the colicin la channel domain. The backbone motions of the membrane-bound colicin scale both the Ca-Ha and N-H couplings similarly, thus ruling out rotation of the α-helices around their axes as a specific mechanism of motion. We also show that the sensitivity of the LG-CP spectra can be enhanced by the addition of a phase-inverted 1 H- 1 3 C cross-polarization step, and the site resolution of the 1 5 N- 1 H LG-CP spectra can be enhanced by 1 3 C indirect detection.

Theoretical and experimental assessment of single- and multiple-quantum cross-polarization in solid state NMR

Continuous wave cross-polarization (CP) NMR experiments with magic angle spinning (MAS) are reviewed for the case of isolated spin pairs I-S, with spin quantum numbers I = ½ and S ½ (1/2, 3/2, …). For two spin-1/2 nuclei, the Hartmann-Hahn matching conditions are examined at various sample rotation rates νR, especially with regard to off-resonance behaviour. In addition to signal enhancement, the CPMAS experiment can be used for the selective determination of inter-nuclear distances between spin-1/2 nuclei. Theoretical examination of the CP transfers to single-quantum (1Q-CPMAS) and multiple-quantum (MQ-CPMAS) levels of quadrupolar nuclei is presented. Simple analytical formulae describing the Hartmann-Hahn matching under various experimental conditions are verified using computer simulations of the spin density matrix under MAS, and the experimental data. The strategies for the most efficient acquisition of 1Q-CPMAS and MQ-CPMAS spectra are extensively discussed.