An investigation of the hydrogen-bonding structure in bilirubin by 1H double-quantum magic-angle spinning solid-state NMR spectroscopy

A solid-state NMR method for characterization of pharmaceutical eutectics

Pharmaceutical eutectics are extremely useful for designing formulations, and currently, there are no techniques other than differential scanning calorimetry (DSC) that can confirm their formation. In this study, we demonstrate that 1H fast magic angle spinning (MAS) solid-state NMR (SSNMR) experiments can confirm the formation of eutectics by detecting their intermolecular hydrogen bonding interactions.

Importance of Nuclear Quantum Effects for Molecular Cocrystals with Short Hydrogen Bonds

Many efforts have been recently devoted to the design and investigation of multicomponent pharmaceutical solids, such as salts and cocrystals. The experimental distinction between these solid forms is often challenging. Here, we show that the transformation of a salt into a cocrystal with a short hydrogen bond does not occur as a sharp phase transition but rather a smooth shift of the positional probability of the hydrogen atoms. A combination of solid-state NMR spectroscopy, X-ray diffraction, and diffuse reflectance measurements with density functional theory calculations that include nuclear quantum effects (NQEs) provides evidence of temperature-induced hydrogen atom shift in cocrystals with short hydrogen bonds. We demonstrate that for the predictions of the salt/cocrystal solid forms with short H-bonds, the computations have to include NQEs (particularly hydrogen nuclei delocalization) and temperature effects.

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process.

Though solution-processed conjugated polymers with inverted temperature activated transport have been reported, the origin of this behaviour is unclear. Here, the authors realize temperature-independent electron transport above 280 K in a donor-acceptor copolymer through microstructural engineering.

Elucidation of correlated disorder in zeolite IM-18

Classical crystallography is based on the translational periodicity of crystals and the analysis of discrete Bragg reflections. However, it is inadequate for determining disordered structures, of which the diffuse scattering is vital to evaluate the disorder level. The correlated disorder of IM-18 presents as zigzag chains arranged in translational periodicity and the double four-ring units randomly distributed along two dimensions. Supercell models regulated by multiple probabilities were systematically built to simulate the single-crystal and powder X-ray diffraction patterns in order to ascertain the specific disorder configuration in the single-crystal or polycrystalline samples of IM-18. The presence of defects in the polycrystalline sample was proved by combining ²⁹Si magic angle spinning (MAS) NMR and ¹H-¹H double quantum MAS NMR spectra, and was quantitatively explored by the simulation method. The method could also elucidate other disordered structures in polycrystalline or single-crystal samples, despite the presence of defects or multidimensional disorder.

Detailed mechanisms of 1H spin-lattice relaxation in ammonium dihydrogen phosphate confirmed by magic angle spinning

Mechanisms of the ¹H spin-lattice relaxation in NH₄H₂PO₄ were studied in detail by use of the effect of magic angle spinning on the relaxation. The acid and the ammonium protons have different relaxation times at the spinning rates higher than 10 kHz due to suppression of spin diffusion between the two kinds of protons. The intrinsic relaxation times not affected by the spin diffusion and the spin-diffusion assisted relaxation times were evaluated separately, taking into consideration temperature dependence. Both mechanisms contribute to the ¹H relaxation of the acid protons comparatively. The spin-diffusion assisted relaxation mechanism was suppressed to the level lower than the experimental errors at the spinning rate of 30 kHz.

Single-crystal X-ray diffraction and NMR crystallography of a 1:1 cocrystal of dithianon and pyrimethanil

A single-crystal X-ray diffraction structure of a 1:1 cocrystal of two fungicides, namely dithianon (DI) and pyrimethanil (PM), is reported [systematic name: 5,10-dioxo-5H,10H-naphtho[2,3-b][1,4]dithiine-2,3-dicarbonitrile-4,6-dimethyl-N-phenylpyrimidin-2-amine (1/1), C14H4N2O2S2·C12H13N2]. Following an NMR crystallography approach, experimental solid-state magic angle spinning (MAS) NMR spectra are presented together with GIPAW (gauge-including projector augmented wave) calculations of NMR chemical shieldings. Specifically, experimental 1H and 13C chemical shifts are determined from two-dimensional 1H-13C MAS NMR correlation spectra recorded with short and longer contact times so as to probe one-bond C-H connectivities and longer-range C...H proximities, whereas H...H proximities are identified in a 1H double-quantum (DQ) MAS NMR spectrum. The performing of separate GIPAW calculations for the full periodic crystal structure and for isolated molecules allows the determination of the change in chemical shift upon going from an isolated molecule to the full crystal structure. For the 1H NMR chemical shifts, changes of 3.6 and 2.0 ppm correspond to intermolecular N-H...O and C-H...O hydrogen bonding, while changes of -2.7 and -1.5 ppm are due to ring current effects associated with C-H...π interactions. Even though there is a close intermolecular S...O distance of 3.10 Å, it is of note that the molecule-to-crystal chemical shifts for the involved sulfur or oxygen nuclei are small.

An NMR crystallography study of the hemihydrate of 2', 3'-O-isopropylidineguanosine

An NMR crystallography study of the hemihydrate of 2', 3'-O-isopropylidineguanosine (Gace) is presented, together with powder X-ray diffraction and thermogravimetric analysis. (1)H double-quantum and (14)N-(1)H HMQC spectra recorded at 850MHz and 75kHz MAS (using a JEOL 1mm probe) are presented together with a (1)H-(13)C refocused INEPT spectrum recorded at 500MHz and 12.5kHz MAS using eDUMBO-122(1)H homonuclear decoupling. NMR chemical shieldings are calculated using the GIPAW (gauge-including projector augmented wave) method; good two-dimensional agreement between calculation and experiment is observed for (13)C and (1)H chemical shifts for directly bonded CH and CH3 peaks. There are two Gace molecules in the asymmetric unit cell: differences in specific (1)H chemical shifts are rationalised in terms of the strength of CH-π and intermolecular hydrogen bonding interactions.

Predominant role of water in native collagen assembly inside the bone matrix

Bone is one of the most intriguing biomaterials found in nature consisting of bundles of collagen helixes, hydroxyapatite, and water, forming an exceptionally tough, yet lightweight material. We present here an experimental tool to map water-dependent subtle changes in triple helical assembly of collagen protein in its absolute native environment. Collagen being the most abundant animal protein has been subject of several structural studies in last few decades, mostly on an extracted, overexpressed, and synthesized form of collagen protein. Our method is based on a (1)H detected solid-state nuclear magnetic resonance (ssNMR) experiment performed on native collagen protein inside intact bone matrix. Recent development in (1)H homonuclear decoupling sequences has made it possible to observe specific atomic resolution in a large complex system. The method consists of observing a natural-abundance two-dimensional (2D) (1)H/(13)C heteronuclear correlation (HETCOR) and(1)H double quantum-single quantum (DQ-SQ) correlation ssNMR experiment. The 2D NMR experiment maps three-dimensional assembly of native collagen protein and shows that extracted form of collagen protein is significantly different from protein in the native state. The method also captures native collagen subtle changes (of the order of ∼1.0 Å) due to dehydration and H/D exchange, giving an experimental tool to map small changes. The method has the potential to be of wide applicability to other collagen containing biomaterials.

Indirectly detected heteronuclear correlation solid-state NMR spectroscopy of naturally abundant 15N nuclei

Two-dimensional indirectly detected through-space and through-bond (1)H{(15)N} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are evaluated. Remarkable efficiency of polarization transfer can be achieved at a MAS rate of 40 kHz by both cross-polarization and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D (1)H{(15)N} HETCOR spectrum of natural abundance surface species is also reported.

Dynamic nuclear polarization enhanced NMR spectroscopy for pharmaceutical formulations

Dynamic nuclear polarization (DNP) enhanced solid-state NMR spectroscopy at 9.4 T is demonstrated for the detailed atomic-level characterization of commercial pharmaceutical formulations. To enable DNP experiments without major modifications of the formulations, the gently ground tablets are impregnated with solutions of biradical polarizing agents. The organic liquid used for impregnation (here 1,1,2,2-tetrachloroethane) is chosen so that the active pharmaceutical ingredient (API) is minimally perturbed. DNP enhancements (ε) of between 40 and 90 at 105 K were obtained for the microparticulate API within four different commercial formulations of the over-the-counter antihistamine drug cetirizine dihydrochloride. The different formulations contain between 4.8 and 8.7 wt % API. DNP enables the rapid acquisition with natural isotopic abundances of one- and two-dimensional (13)C and (15)N solid-state NMR spectra of the formulations while preserving the microstructure of the API particles. Here this allowed immediate identification of the amorphous form of the API in the tablet. API-excipient interactions were observed in high-sensitivity (1)H-(15)N correlation spectra, revealing direct contacts between povidone and the API. The API domain sizes within the formulations were determined by measuring the variation of ε as a function of the polarization time and numerically modeling nuclear spin diffusion. Here we measure an API particle radius of 0.3 μm with a single particle model, while modeling with a Weibull distribution of particle sizes suggests most particles possess radii of around 0.07 μm.

Exploiting the Synergy of Powder X-ray Diffraction and Solid-State NMR Spectroscopy in Structure Determination of Organic Molecular Solids

We report a strategy for structure determination of organic materials in which complete solid-state nuclear magnetic resonance (NMR) spectral data is utilized within the context of structure determination from powder X-ray diffraction (XRD) data. Following determination of the crystal structure from powder XRD data, first-principles density functional theory-based techniques within the GIPAW approach are exploited to calculate the solid-state NMR data for the structure, followed by careful scrutiny of the agreement with experimental solid-state NMR data. The successful application of this approach is demonstrated by structure determination of the 1:1 cocrystal of indomethacin and nicotinamide. The ¹H and ¹³C chemical shifts calculated for the crystal structure determined from the powder XRD data are in excellent agreement with those measured experimentally, notably including the two-dimensional correlation of ¹H and ¹³C chemical shifts for directly bonded ¹³C-¹H moieties. The key feature of this combined approach is that the quality of the structure determined is assessed both against experimental powder XRD data and against experimental solid-state NMR data, thus providing a very robust validation of the veracity of the structure.

Applications of high-resolution 1H solid-state NMR

This article reviews the large increase in applications of high-resolution (1)H magic-angle spinning (MAS) solid-state NMR, in particular two-dimensional heteronuclear and homonuclear (double-quantum and spin-diffusion NOESY-like exchange) experiments, in the last five years. These applications benefit from faster MAS frequencies (up to 80 kHz), higher magnetic fields (up to 1 GHz) and pulse sequence developments (e.g., homonuclear decoupling sequences applicable under moderate and fast MAS). (1)H solid-state NMR techniques are shown to provide unique structural insight for a diverse range of systems including pharmaceuticals, self-assembled supramolecular structures and silica-based inorganic-organic materials, such as microporous and mesoporous materials and heterogeneous organometallic catalysts, for which single-crystal diffraction structures cannot be obtained. The power of NMR crystallography approaches that combine experiment with first-principles calculations of NMR parameters (notably using the GIPAW approach) are demonstrated, e.g., to yield quantitative insight into hydrogen-bonding and aromatic CH-π interactions, as well as to generate trial three-dimensional packing arrangements. It is shown how temperature-dependent changes in the (1)H chemical shift, linewidth and DQ-filtered signal intensity can be analysed to determine the thermodynamics and kinetics of molecular level processes, such as the making and breaking of hydrogen bonds, with particular application to proton-conducting materials. Other applications to polymers and biopolymers, inorganic compounds and bioinorganic systems, paramagnetic compounds and proteins are presented. The potential of new technological advances such as DNP methods and new microcoil designs is described.

Identifying guanosine self assembly at natural isotopic abundance by high-resolution 1H and 13C solid-state NMR spectroscopy

By means of the (1)H chemical shifts and the proton-proton proximities as identified in (1)H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)(2) and dG(C3)(2) derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by (15)N refocused INADEQUATE spectra in a previous study of (15)N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc.2005, 127, 16018). The NH (1)H chemical shift is observed to be higher (13-15 ppm) for ribbon-like self-assembly as compared to 10-11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH(2)(1)H chemical shifts is also inverted, with the NH(2) proton closest in space to the NH proton having a higher or lower (1)H chemical shift than that of the other NH(2) proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)(2) derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, (14)N-(1)H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH(2) chemical shifts for the dG(C3)(2) derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.

Interplay between structure and dynamics in chitosan films investigated with solid-state NMR, dynamic mechanical analysis, and X-ray diffraction

Modern solid-state NMR techniques, combined with X-ray diffraction, revealed the molecular origin of the difference in mechanical properties of self-associated chitosan films. Films cast from acidic aqueous solutions were compared before and after neutralization, and the role of the counterion (acetate vs Cl(-)) was investigated. There is a competition between local structure and long-range order. Hydrogen bonding gives good mechanical strength to neutralized films, which lack long-range organization. The long-range structure is better defined in films cast from acidic solutions in which strong electrostatic interactions cause rotational distortion around the chitosan chains. Plasticization by acetate counterions enhances long-range molecular organization and film flexibility. In contrast, Cl(-) counterions act as a defect and impair the long-range organization by immobilizing hydration water. Molecular motion and proton exchange are restricted, resulting in brittle films despite the high moisture content.

Multinuclear solid state NMR investigation of two polymorphic forms of ciprofloxacin-saccharinate

Two polymorphic forms of a novel pharmaceutical compound, ciprofloxacin-saccharinate (CIP-SAC), are analyzed using one dimensional (1D) and two dimensional (2D) (1)H nuclear magnetic resonance (NMR) at fast magic angle spinning (MAS). Additionally (15)N spectroscopy and (1)H-(13)C correlation experiments were performed to complement our conclusions. The 1D (1)H NMR spectra of CIP and complexes reveal valuable information about the ionic bonding between ciprofloxacin and saccharine. Additionally, these spectra allow us to perform a clear characterization of each solid form, giving the number of molecules per unit cell in one of the polymorphs. From 2D (1)H-(1)H spectra obtained through double quantum correlations we can arrive at important conclusions about the hydrogen bonding, conformation, and intra and inter-molecular interactions present in these compounds. Comparing and contrasting the (1)H-(1)H correlation data obtained for both polymorphic forms and taking into account the single crystal structure data existing for the solid form CIP-SAC (II) was possible to extract some conclusions on the polymorph CIP-SAC (I) where no single crystal information is available. (1)H MAS NMR is shown to be an important tool in the field of polymorphism and for the characterization of multicomponent pharmaceutical compounds.

NMR crystallography of campho[2,3-c]pyrazole (Z' = 6): combining high-resolution 1H-13C solid-state MAS NMR spectroscopy and GIPAW chemical-shift calculations

(1)H-(13)C two-dimensional magic-angle spinning (MAS) solid-state NMR correlation spectra, recorded with the MAS-J-HMQC experiment, are presented for campho[2,3-c]pyrazole. For each (13)C moiety, there are six resonances associated with the six distinct molecules in the asymmetric unit cell (Z' = 6). The one-bond C-H correlations observed in the 2D (1)H-(13)C MAS-J-HMQC spectra allow the experimental determination of the (1)H and (13)C chemical shifts associated with the separate CH, CH(2), and CH(3) groups. (1)H and (13)C chemical shifts calculated by using the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach are presented. Calculations for the whole unit cell (12 × 29 = 348 atoms, with geometry optimization of all atoms) allow the assignment of the experimental (1)H and (13)C chemical shifts to the six distinct molecules. The calculated chemical shifts for the full crystal structure are compared with those for isolated molecules as extracted from the geometry-optimized crystal structure. In this way, the effect of intermolecular interactions on the observed chemical shifts is quantified. In particular, the calculations are sufficiently precise to differentiate the small (<1 ppm) differences between the (1)H chemical shifts of the six resonances associated with each distinct CH or CH(2) moiety.

Complete (1)H resonance assignment of beta-maltose from (1)H-(1)H DQ-SQ CRAMPS and (1)H (DQ-DUMBO)-(13)C SQ refocused INEPT 2D solid-state NMR spectra and first principles GIPAW calculations

A disaccharide is a challenging case for high-resolution (1)H solid-state NMR because of the 24 distinct protons (14 aliphatic and 10 OH) having (1)H chemical shifts that all fall within a narrow range of approximately 3 to 7 ppm. High-resolution (1)H (500 MHz) double-quantum (DQ) combined rotation and multiple pulse sequence (CRAMPS) solid-state NMR spectra of beta-maltose monohydrate are presented. (1)H-(1)H DQ-SQ CRAMPS spectra are presented together with (1)H (DQ)-(13)C correlation spectra obtained with a new pulse sequence that correlates a high-resolution (1)H DQ dimension with a (13)C single quantum (SQ) dimension using the refocused INEPT pulse-sequence element to transfer magnetization via one-bond (13)C-(1)H J couplings. Compared to the observation of only a single broad peak in a (1)H DQ spectrum recorded at 30 kHz magic-angle spinning (MAS), the use of DUMBO (1)H homonuclear decoupling in the (1)H DQ CRAMPS experiment allows the resolution of distinct DQ correlation peaks which, in combination with first-principles chemical shift calculations based on the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach, enables the assignment of the (1)H resonances to the 24 distinct protons. We believe this to be the first experimental solid-state NMR determination of the hydroxyl OH (1)H chemical shifts for a simple sugar. Variable-temperature (1)H-(1)H DQ CRAMPS spectra reveal small increases in the (1)H chemical shifts of the OH resonances upon decreasing the temperature from 348 K to 248 K.

Determining relative proton-proton proximities from the build-up of two-dimensional correlation peaks in 1H double-quantum MAS NMR: insight from multi-spin density-matrix simulations

The build-up of intensity-as a function of the number, n(rcpl), of POST-C7 elements used for the excitation and reconversion of double-quantum (DQ) coherence (DQC)-is analysed for the fifteen distinct DQ correlation peaks that are observed experimentally for the eight separate (1)H resonances in a (1)H (500 MHz) DQ CRAMPS solid-state (12.5 kHz MAS) NMR spectrum of the dipeptide beta-AspAla (S. P. Brown, A. Lesage, B. Elena, and L. Emsley, J. Am. Chem. Soc., 2004, 126, 13230). The simulation in SPINEVOLUTION (M. Veshtort and R. G. Griffin, J. Magn. Reson., 2006, 178, 248) of t(1) ((1)H DQ evolution) FIDs for clusters of eight dipolar-coupled protons gives separate simulated (1)H DQ build-up curves for the CH(2)(a), CH(2)(b), CH(Asp), CH(Ala), NH and OH (1)H single-quantum (SQ) (1)H resonances. An analysis of both the simulated and experimental (1)H DQ build-up leads to the following general observations: (i) considering the build-up of (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the DQC corresponding to the shortest H-H distance; (ii) for the maximum intensity (1)H DQ peak at a particular SQ frequency, the recoupling time for the observed maximum intensity depends on the corresponding H-H distance, e.g., maximum intensity for the CH(2)(a)-CH(2)(b) (H-H distance = 1.55 A) and OH-CH(Asp) (H-H distance = 2.49 A) DQ peaks is observed at n(rcpl) = 2 and 3, respectively; (iii) for DQ peaks involving a CH(2) proton at a non-CH(2) SQ frequency, there is much reduced intensity and a maximum intensity at a short recoupling time; (iv) for the other lower intensity (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the same (or close to the same) recoupling time, but the relative intensity of the DQ peaks is a reliable indicator of the relative H-H distance-the ratio of the maximum intensities for the peaks at the CH(Ala) SQ frequency due to the two DQCs with the NH and OH protons are found to be approximately in the ratio of the squares of the corresponding dipolar coupling constants. While the simulated (1)H DQ build-up curves reproduce most of the features of the experimental curves, maximum intensity is often observed at a longer recoupling time in simulations. In this respect, simulations for two to eight spins show a trend towards a faster decay for an increasing number of considered spins. Finally, simulations show that increasing either the Larmor frequency (to 1 GHz) or the MAS frequency (to 125 kHz) does not lead to changes in the marked differences between the (1)H DQ build-up curves at the CH(Asp) SQ frequency for DQCs to the CH(2)(a) and OH protons that correspond to similar H-H distances (2.39 A and 2.49 A, respectively).

Solid-state NMR studies of weak interactions in supramolecular systems

The field of application of solid-state NMR to the study of supramolecular systems is growing rapidly, with many research groups involved in the development of techniques for the study of crystalline and amorphous phases. This Feature Article aims to provide an overview of the recent contributions of our research group to this field, paying particular attention to the study of the weak interactions such as hydrogen bonds in supramolecular systems through solid-state NMR investigations. The structure and dynamic behaviour of selected host-guest systems will be also discussed.

Origins of linewidth in 1H magic-angle spinning NMR

A detailed study of the factors determining the linewidth (and hence resolution) in 1H solid-state magic-angle spinning NMR is described. Although it has been known from the early days of magic-angle spinning (MAS) that resolution of spectra from abundant nuclear spins, such as 1H, increases approximately linearly with increasing sample rotation rate, the difficulty of describing the dynamics of extended networks of coupled spins has made it difficult to predict a priori the resolution expected for a given sample. Using recently developed, highly efficient methods of numerical simulation, together with experimental measurements on a variety of test systems, we propose a comprehensive picture of 1H resolution under MAS. The "homogeneous" component of the linewidth is shown to depend primarily on the ratio between an effective local coupling strength and the spin rate, modified by geometrical factors which loosely correspond to the "dimensionality" of the coupling network. The remaining "inhomogeneous" component of the natural linewidth is confirmed to have the same properties as in dilute-spin NMR. Variations in the NMR frequency due to chemical shift effects are shown to have minimal impact on 1H resolution. The implications of these results for solid-state NMR experiments involving 1H and other abundant-spin nuclei are discussed.

Probing proton-proton proximities in the solid state: high-resolution two-dimensional 1H-1H double-quantum CRAMPS NMR spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.

Molecular understanding of the formation of surface zirconium hydrides upon thermal treatment under hydrogen of [([triple bond]SiO)Zr(CH2tBu)3] by using advanced solid-state NMR techniques

The reaction of [([triple bond]SiO)Zr(CH(2)tBu)(3)] with H(2) at 150 degrees C leads to the hydrogenolysis of the zirconium-carbon bonds to form a very reactive hydride intermediate(s), which further reacts with the surrounding siloxane ligands present at the surface of this support to form mainly two different zirconium hydrides: [([triple bond]SiO)(3)Zr-H] (1a, 70-80%) and [([triple bond]SiO)(2)ZrH(2)] (1b, 20-30%) along with silicon hydrides, [([triple bond]SiO)(3)SiH] and [([triple bond]SiO)(2)SiH(2)]. Their structural identities were identified by (1)H DQ solid-state NMR spectroscopy as well as reactivity studies. These two species react with CO(2) and N(2)O to give, respectively, the corresponding formate [([triple bond]SiO)(4-x)Zr(O-C(=O)H)(x)] (2) and hydroxide complexes [([triple bond]SiO)(4-x)Zr(OH)(x)] (x = 1 or 2 for 3a and 3b, respectively) as major surface complexes.

Distribution effects on 1H double-quantum MAS NMR spectra

The effect of a distribution in the (1)H-(1)H dipolar coupling on (1)H double-quantum (DQ) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spinning sideband patterns is considered. In disordered or amorphous materials a distribution in the magnitude of the (1)H-(1)H dipolar coupling is a realistic possibility. Simulations of the (1)H DQ MAS NMR spinning sideband spectra were performed with the two-spin approximation. These simulations reveal that a dipolar coupling distribution can greatly affect the DQ spectral shape and behavior of the DQ build-up. The spectral line shapes are quantified by measurement of the relative intensities of the DQ sidebands. These variations in the (1)H DQ NMR spectra are evaluated as a function of the width of the dipolar coupling distribution. As an example, the experimental DQ spinning sideband spectrum for a hydrated polyoxoniobate containing 15 H(2)O molecules per hexaniobate cluster, are better simulated with a distribution of dipolar couplings opposed to a single coupling constant.

Experimental aspects of proton NMR spectroscopy in solids using phase-modulated homonuclear dipolar decoupling

In this paper we demonstrate experimentally that the continuously phase-modulated homonuclear decoupling sequence DUMBO-1 is suitable for high-resolution proton NMR spectroscopy of rigid solids. Over a wide range of experimental conditions, we show on the model sample L-alanine as well as on small peptides that proton linewidths of less than 0.5 ppm can be obtained under DUMBO-1 decoupling. In particular the DUMBO-1 sequence yields well resolved proton spectra both at slow and fast MAS. The DUMBO-1 decoupling scheme can in principle be inserted in any multi-nuclear or multi-dimensional solid-state NMR experiment which requires a high-resolution 1H dimension. An example is provided with the 13C-1H MAS-J-HMQC experiment.

REDOR-based heteronuclear dipolar correlation experiments in multi-spin systems: rotor-encoding, directing, and multiple distance and angle determination

We review a variety of recently developed 1H-X heteronuclear recoupling techniques, which rely only on the homonuclear decoupling efficiency of very-fast magic-angle spinning. All these techniques, which are based on the simple rotational-echo, double-resonance (REDOR) approach for heteronuclear recoupling, are presented in a common context. Advantages and possibilities with respect to the complementary application of conventionally X and 1H-inversely detected variants are discussed in relation to the separability and analysis of multiple couplings. We present an improved and more sensitive approach to the determination of 1H-X dipolar couplings by spinning-sideband analysis, termed REREDOR, which is applicable to XHn groups in rigid and mobile systems and bears some similarity to more elaborate separated local-field methods. The estimation of medium-range 1H-X distances by analyzing signal intensities in two-dimensional REDOR correlation spectra in a model-free way is also discussed. More specifically, we demonstrate the possibility of combined distance and angle determination in H-X-H or X-H-X three-spin systems by asymmetric recoupling schemes and spinning-sideband analysis. Finally, an 1H-X correlation experiment is introduced which accomplishes high sensitivity by inverse (1H) detection and is therefore applicable to samples with 15N in natural abundance.

A study of a moleculartweezer host-guest system by a combination of quantum-chemical calculations and solid-state NMR experiments

A study of a host-guest system consisting of a naphthalene-spaced tweezer with a 1,4 dicyanobenzene guest molecule is presented. The complex is investigated using a combination of quantum-chemical calculations and solid-state NMR experiments. The advantages of such an approach are illustrated. The focus is on the calculation of (1) 1H NMR and (2) 13C NMR chemical shifts for model fragments of the solid-state structure, (3) the analysis of host-guest interactions important for molecular recognition, and (4) the investigation of the process of a guest molecule rotation. For modeling the solid-state structure, up to three host-guest units are considered and the convergence with respect to the size of the solid-state fragment is investigated.

High-sensitivity multinuclear NMR spectroscopy of a smectite clay and of clay-intercalated polymer

The nuclear magnetic resonance (NMR) properties of a smectite clay low in paramagnetic ions, and NMR experiments to detect organic material near the silicate surfaces with high sensitivity, have been explored by 1H, 29Si, and 13C NMR. In oven-dried hectorite clay, 1H NMR reveals a sharp signal at 0.35 ppm that narrows significantly with spinning speed. It is assigned to the "inner" OH protons of the silicate layers. In fluorohectorite, where the OH groups are replaced by fluorines, no such 1H peak is observed. The assignment is further confirmed by the efficient cross-polarization observed in two-dimensional (2D) 1H-29Si HETCOR spectra, and by 29Si-detected REDOR experiments with 1H-dephasing in the 29Si dipolar field, which yield a 1H-29Si distance of 2.9 + 0.4 A. In these 1H-29Si experiments, the sensitivity of the 29Si signal is enhanced at least fivefold by refocusing the decay resulting from the inhomogeneous broadening of the single 29Si peak, stretching the 29Si signal out over 80 ms. The small 1H linewidth of this signal at spinning frequencies exceeding 4 kHz is attributed to the large proton-proton distances in the clay. The upfield isotropic chemical shift of the OH groups is explained by their inaccessibility to hydrogen-bonding partners, as a result of their location in hexagonal "cavities" of the clay structure. The well-resolved, easily selectable OH-proton signal and the high-sensitivity 29Si detection open excellent perspectives for NMR studies of composites of clays with organic molecules. Two-dimensional 1H-29Si and 1H-1H chemical-shift correlation experiments enable efficient detection of the 1H spectrum of organic segments near the clay surface. Combined with 1H spin diffusion, the organic segments at up to several nanometers from the clay surfaces can be probed. A 2D 1H-13C correlation experiment yields the 13C spectrum of the organic species near the clay surfaces. A mobility gradient of intercalated poly(ethylene oxide), PEO, segments is proven in 1H-3Si WISE experiments with spin diffusion.

2D(13)C-(13)C MAS NMR correlation spectroscopy with mixing by true (1)H spin diffusion reveals long-range intermolecular distance restraints in ultra high magnetic field

An improved 2D (13)C-(13)C CP(3) MAS NMR correlation experiment with mixing by true (1)H spin diffusion is presented. With CP(3), correlations can be detected over a much longer range than with direct (1)H-(13)C or (13)C-(13)C dipolar recoupling. The experiment employs a (1)H spin diffusion mixing period tau(m) sandwiched between two cross-polarization periods. An optimized CP(3) sequence for measuring polarization transfer on a length scale between 0.3 and 1.0 nm using short mixing times of 0.1 ms < tau(m) < 1 ms is presented. For such a short tau(m), cross talk from residual transverse magnetization of the donating nuclear species after a CP can be suppressed by extended phase cycling. The utility of the experiment for genuine structure determination is demonstrated using a self-aggregated Chl a/H(2)O sample. The number of intramolecular cross-peaks increases for longer mixing times and this obscures the intermolecular transfer events. Hence, the experiment will be useful for short mixing times only. For a short tau(m) = 0.1 ms, intermolecular correlations are detected between the ends of phytyl tails and ring carbons of neighboring Chl a molecules in the aggregate. In this way the model for the structure, with stacks of Chl a that are arranged back to back with interdigitating phytyl chains stretched between two bilayers, is validated.

The direct detection of a hydrogen bond in the solid state by NMR through the observation of a hydrogen-bond mediated (15)N [bond] (15)N J coupling

A new method for detecting hydrogen bonds in the solid state is presented. Using two-dimensional NMR correlation experiments, it is shown that a hydrogen-bond mediated J coupling can be observed in a powder under magic-angle spinning conditions, even though the J coupling is 2 orders of magnitude smaller than the dominant anisotropic interactions encountered in solid-state NMR. Specifically, the observation of a pair of peaks in a two-dimensional 15N-15N solid-state INADEQUATE experiment due to two nitrogens that have no covalent connectivity is attributed to the presence of a J coupling across a linking hydrogen bond.

Rotor-encoded heteronuclear MQ MAS NMR spectroscopy of half-integer quadrupolar and spin I=1/2 nuclei

A new two-dimensional heteronuclear multiple-quantum magic-angle spinning (MQ MAS) experiment is presented which combines high resolution for the half-integer quadrupolar nucleus with information about the dipolar coupling between the quadrupolar nucleus and a spin I=1/2 nucleus. Homonuclear MQ coherence is initially created for the half-integer quadrupolar nucleus by a single pulse as in a standard MQ MAS experiment. REDOR recoupling of the heteronuclear dipolar coupling then allows the creation of a heteronuclear multiple-quantum coherence comprising multiple- and single-quantum coherence of the quadrupolar and spin I=1/2 nucleus, respectively, which evolves during t1. Provided that the t1 increment is not rotor synchronized, rotor-encoded spinning-sideband patterns are observed in the indirect dimension. Simulated spectra for an isolated IS spin pair show that these patterns depend on the recoupling time, the magnitude of the dipolar coupling, the quadrupolar parameters, as well as the relative orientation of the quadrupolar and dipolar principal axes systems. Spectra are presented for Na2HPO4, with the heteronuclear 23Na-1HMQ MAS experiments beginning with the excitation of 23Na (spin I=3/2) three-quantum coherence. Coherence counting experiments demonstrate that four- and two-quantum coherences evolve during t1. The heteronuclear spinning-sideband patterns observed for the three-spin H-Na-H system associated with the Na(2) site are analyzed. For an IS2 system, simulated spectra show that, considering the free parameters, the spinning-sideband patterns are particularly sensitive to only, first, the angle between the two IS internuclear vectors and, second, the two heteronuclear dipolar couplings. It is demonstrated that the proton localization around the Na(2) site according to the literature crystal structure of Na2HPO4 is erroneous. Instead, the experimental data is consistent with two alternative different structural arrangements, whereby either there is a deviation of 10 degrees from linearity for the case of two identical Na-H distances, or there is a linear arrangement, but the two Na-H distances are different. Furthermore, the question of the origin of spinning-sidebands in the (homonuclear) MQ MAS experiment is revisited. It is shown that the asymmetric experimental MQ sideband pattern observed for the low-C(Q) Na(2) site in Na(2)HPO4 can only be explained by considering the 23Na chemical shift anisotropy.