Through-space Correlation Research Articles

Effects of surface area and porosity on behavior of IL molecules in meso and macroporous polymeric networks

In this work, ionogels fabricated by incorporating room temperature ionic liquids (ILs) in meso- and macroporous syndiotactic polystyrene (sPS) gels, are studied by using differential scanning calorimetry (DSC), polarized optical microscopy (POM), rheology, and nuclear magnetic resonance (NMR) spectroscopy. sPS gel network is prepared via thermo-reversible gelation in tetrahydrofuran and the pores are subsequently filled with the ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI). Using DSC and POM it is found that PYR14TFSI molecules in sPS gels show higher melting temperatures than that for the neat PYR14TFSI. 1H static NMR signals provide overall mobility of ILs in the sPS gels. Surprisingly, the ionogel prepared with 0.04 g/mL sPS solution apparently shows the broadest 1H NMR signals among the gels prepared with 0.04, 0.08, and 0.12 g/mL sPS concentration. This is attributed to a quasi-bulk state of ILs. Moreover, two-dimensional 1H NOESY spectra indicate that PYR14TFSI molecules in sPS show strong non-bonded through-space correlation peaks and their intensities increase proportionally with polymer concentration in the gel, while such correlation peaks are not observed for the neat PYR14TFSI molecules. The observed concentration dependence of constrained PYR14TFSI dynamics is interpreted in terms of the confined PYR14TFSI at the surface or closer to the surface of sPS scaffold. From these experimental results, it is suggested that PYR14TFSI melting-temperature are more related to quasi-bulk component than the constrained PYR14TFSI in the sPS gels.

An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study of the Halogen Bond with Explicit Analysis of Electron Correlation

Energy profiles of seven halogen-bonded complexes were analysed with the topological energy partitioning called Interacting Quantum Atoms (IQA) at MP4(SDQ)/6–31 + G(2d,2p) level of theory. Explicit interatomic electron correlation energies are included in the analysis. Four complexes combine X2 (X = Cl or F) with HCN or NH3, while the remaining three combine ClF with HCN, NH3 or N2. Each complex was systematically deformed by translating the constituent molecules along its central axis linking X and N, and reoptimising its remaining geometry. The Relative Energy Gradient (REG) method (Theor. Chem. Acc. 2017, 136, 86) then computes which IQA energies most correlate with the total energy during the process of complex formation and further compression beyond the respective equilibrium geometries. It turns out that the covalent energy (i.e., exchange) of the halogen bond, X…N, itself drives the complex formation. When the complexes are compressed from their equilibrium to shorter X…N distance then the intra-atomic energy of N is in charge. When the REG analysis is restricted to electron correlation then the interatomic correlation energy between X and N again drives the complex formation, and the complex compression is best described by the destabilisation of the through-space correlation energy between N and the “outer” halogen.

1H-detected MAS solid-state NMR experiments enable the simultaneous mapping of rigid and dynamic domains of membrane proteins

Magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy is emerging as a unique method for the atomic resolution structure determination of native membrane proteins in lipid bilayers. Although 13C-detected ssNMR experiments continue to play a major role, recent technological developments have made it possible to carry out 1H-detected experiments, boosting both sensitivity and resolution. Here, we describe a new set of 1H-detected hybrid pulse sequences that combine through-bond and through-space correlation elements into single experiments, enabling the simultaneous detection of rigid and dynamic domains of membrane proteins. As proof-of-principle, we applied these new pulse sequences to the membrane protein phospholamban (PLN) reconstituted in lipid bilayers under moderate MAS conditions. The cross-polarization (CP) based elements enabled the detection of the relatively immobile residues of PLN in the transmembrane domain using through-space correlations; whereas the most dynamic region, which is in equilibrium between folded and unfolded states, was mapped by through-bond INEPT-based elements. These new 1H-detected experiments will enable one to detect not only the most populated (ground) states of biomacromolecules, but also sparsely populated high-energy (excited) states for a complete characterization of protein free energy landscapes.

71Ga-77Se connectivities and proximities in gallium selenide crystal and glass probed by solid-state NMR

We introduce two-dimensional (2D) 71Ga-77Se through-bond and through-space correlation experiments. Such correlations are achieved using (i) the J-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (J-RINEPT) method with 71Ga excitation and 77Se Carr-Purcell-Meiboon-Gill (CPMG) detection, as well as (ii) the J- or dipolar-mediated Hetero-nuclear Multiple-Quantum Correlation (J- or D-HMQC) schemes with 71Ga excitation and quadrupolar CPMG (QCPMG) detection. These methods are applied to the crystalline β-Ga2Se3 and the 0.2Ga2Se3-0.8GeSe2 glass. Such glass leads to a homogeneous and reproducible glass–ceramic, which is a good alternative to single-crystalline Ge and polycrystalline ZnSe materials for making lenses transparent in the IR range for thermal imaging applications. We show that 2D 71Ga-77Se correlation experiments allow resolving the 77Se signals of molecular units, which are not resolved in the 1D 77Se CPMG spectrum. Additionally, the build-up curves of the J-RINEPT and the J-HMQC experiments allow the estimate of the 71Ga-77Se J-couplings via one and three-bonds in the three-dimensional network of β-Ga2Se3. Furthermore, these build-up curves show that the one-bond 1J71Ga-77Se couplings in the 0.2Ga2Se3-0.8GeSe2 glass are similar to those measured for β-Ga2Se3. We also report 2D 71Ga Satellite Transition Magic-Angle Spinning (STMAS) spectrum of β-Ga2Se3 using QCPMG detection at high magnetic field and high Magic-Angle Spinning frequency using large radio frequency field. Such spectrum allows separating the signal of β-Ga2Se3 and that of an impurity.

Minimizing the t1-noise when using an indirect 1H high-resolution detection of unlabeled samples

The most utilized through-space correlation 1H-{X} methods with proton indirect detection use two consecutive transfers, 1H → X and then X → 1H, with the evolution time t1 in the middle. When the X isotope is not 100% naturally abundant (NA), only the signal of the protons close to these isotopes is modulated by the 1H-X dipolar interactions. This signal is theoretically disentangled with phase-cycling from the un-modulated one. However, this separation is never perfect and it may lead to t1-noise in case of isotopes with very small NA, such as 13C or even worse 15N. One way to reduce this t1-noise is to minimize, ‘purge’, during t1 the un-modulated 1H magnetization before trying to suppress it with phase-cycling.We analyze experimentally several sequences following the HORROR condition, which allow purging the 1H transverse magnetization. The comparison is made at three spinning speeds, including very fast ones for 1H resolution: 27.75, 55.5 and 111 kHz. We show (i) that the efficiency of this purging process increases with the spinning speed, and (ii) that the best recoupling sequences are the two simplest ones: XY and S1 = SR212.We then compare the S/N that can be achieved with the two most used 1H-{X} 2D methods, called D-HMQC and CP-CP. The only difference in between these two methods is that the transfers are done with either two π/2-pulses on X channel (D-HMQC), or two Cross-Polarization (CP) transfers (CP-CP).The first method, D-HMQC, is very robust and should be preferred when indirectly detecting nuclei with high NA. The second method, CP-CP, (i) requires experimental precautions to limit the t1-noise, and (ii) is difficult to use with quadrupolar nuclei because the two CP transfers are then not efficient nor robust. However, CP-CP is presently the best method to indirectly detect isotopes with small NA, such as 13C and 15N.

Stabilization of Atactic-Polyacrylonitrile under Nitrogen and Air As Studied by Solid-State NMR

Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13C-labeled atactic-polyacrylonitrile (a-PAN) heat-treated at various temperatures (Ts) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13C–13C INADEQUATE and 1H–13C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different Ts values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through thi...

Investigation of zinc alkali pyrophosphate glasses. Part II: Local and medium range orders analysed by 1D/2D NMR

The structure of the (66-x)ZnO-xNa2O-33.4P2O5 composition line, selected for the development of low-Tg and stable glasses, has been investigated by 1D/2D NMR spectroscopy. If standard 1D 31P MAS-NMR experiments give access to the Qn speciation and show the presence of Q0, Q1 and Q2 sites within the glass structure, application of the homonuclear through-space correlation technique (31P DQ-SQ) allows for a more accurate description of the phosphate units. Clear distinction between the Q1 sites involved in dimmers or in longer chains has been derived from 2D NMR correlation maps and leads to the re-assignment of Q1 into Q1,1 and Q1,2 species. 23Na and 23Na(31P) REDOR MAS-NMR experiments have been used to analyse the Na+ ions distribution and its interaction with the phosphate network. 67Zn static NMR experiments, performed at very high field, were carried out and suggest a constant Zn2+ coordination state all along the composition line. The results have been used to discuss the impact of the Zn2+/Na+ ratio on the extent of disorder within the glass network expressed in terms of Qn dismutation equilibrium constant and phosphate chain length distribution.

Real-time homonuclear broadband and band-selective decoupled pure-shift ROESY.

Unambiguous spectral assignments in (1)H solution-state NMR are central, for accurate structural elucidation of complex molecules, which is often hampered by signal overlap, primarily because of scalar coupling multiplets, even at typical high magnetic fields. The recent advances in homodecoupling methods have shown powerful means of achieving high resolution pure-shift (1)H spectra in 1D and also in 2D J-correlated experiments, by effectively collapsing the multiplet structures. The present work extends these decoupling strategies to through-space correlation experiments as well and describes two new pure-shift ROESY pulse schemes with homodecoupling during acquisition, viz., homodecoupled broadband (HOBB)-ROESY and homodecoupled band-selective (HOBS)-ROESY. Furthermore, the ROESY blocks suppress the undesired interferences of TOCSY cross peaks and other offsets. Despite the reduced signal sensitivity and prolonged experimental times, the HOBB-ROESY is particularly useful for molecules that exhibit an extensive scalar coupling network spread over the entire (1)H chemical shift range, such as natural/synthetic organic molecules. On the other hand, the HOBS-ROESY is useful for molecules that exhibit well-separated chemical shift regions such as peptides (NH, Hα and side-chain protons). The HOBS-ROESY sensitivities are comparable with the conventional ROESY, thereby saves the experimental time significantly. The power of these pure-shift ROESY sequences is demonstrated for two different organic molecules, wherein complex conventional ROE cross peaks are greatly simplified with high resolution and sensitivity. The enhanced resolution allows deriving possibly more numbers of ROEs with better accuracy, thereby facilitating superior means of structural characterization of medium-size molecules.

PH-triggered formation of nanoribbons from yeast-derived glycolipid biosurfactants.

In the present paper, we show that the saturated form of acidic sophorolipids, a family of industrially scaled bolaform microbial glycolipids, unexpectedly forms chiral nanofibers only at pH below 7.5. In particular, we illustrate that this phenomenon derives from a subtle cooperative effect of molecular chirality, hydrogen bonding, van der Waals forces and steric hindrance. The pH-responsive behaviour was shown by Dynamic Light Scattering (DLS), pH-titration and Field Emission Scanning Electron Microscopy (FE-SEM) while the nanoscale chirality was evidenced by Circular Dichroism (CD) and cryo Transmission Electron Microscopy (cryo-TEM). The packing of sophorolipids within the ribbons was studied using Small Angle Neutron Scattering (SANS), Wide Angle X-ray Scattering (WAXS) and 2D (1)H-(1)H through-space correlations via Nuclear Magnetic Resonance under very fast (67 kHz) Magic Angle Spinning (MAS-NMR).

Synthesis, characterization and dynamic NMR studies of a novel chalcone based N-substituted morpholine derivative

The synthesis of a novel chalcone based N-substituted morpholine derivative namely, (E)-1-(biphenyl-4-yl)-3-(4-(5-morpholinopentyloxy) phenyl) prop-2-en-1-one (BMPP), using a two step protocol is reported. The compound is characterized by FTIR, GC–MS and FTNMR spectroscopy techniques. Advanced 2D NMR techniques such as gradient enhanced COSY, HSQC, HMBC and NOESY were employed to establish through-bond and through-space correlations. Dynamic NMR measurements were carried out to obtain the energy barrier to ring inversion of the morpholine moiety.

High-resolution through-space correlations between spin-1/2 and half-integer quadrupolar nuclei using the MQ-D-R-INEPT NMR experiment

The multiple-quantum dipolar refocused-INEPT (MQ-D-R-INEPT) NMR experiment is presented. This new pulse sequence allows the generation of two-dimensional through-space NMR correlation spectra between spin-½ and quadrupolar nuclei under high-resolution. Compared to the existing sequences, the MQ-D-R-INEPT sequence benefits from increased robustness, due to the reduced sensitivity to offsets and radio-frequency field inhomogeneity of the R-INEPT recoupling scheme as compared to the classical cross-polarization transfer involving a quadrupolar nucleus, and applicability to quadrupolar nuclei with any half-integer spin value. Here, the heteronuclear dipolar couplings have been reintroduced using the rotary resonance recoupling (R(3)) sequence, chosen because it is γ-encoded, and thus robust to spinning speed instabilities and to radio-frequency inhomogeneities in the case of nuclei subjected to large chemical shift anisotropy (e.g., (31)P). In the course of the article, a new definition of γ-encoding is introduced, more general and practical than the previous ones, as it does not depend on the implementation of the recoupling sequence, and is independent of the considered frame. The efficiency of the MQ-D-R-INEPT experiment is demonstrated for the observation of (31)P-(27)Al proximities. It is first tested on AlPO4-VPI-5, and then applied to a complex aluminophosphate AlPO4-(Al5P7)-DAE of incompletely established structure. For this latter sample, the high-resolution provided by the MQMAS filter allows the resolution of a large number of (31)P-(27)Al correlations, which represent an essential step towards the determination of its structural model.

Probing Molecular Motion by Double-Quantum (13C,13C) Solid-State NMR Spectroscopy: Application to Ubiquitin

We demonstrate the use of two-dimensional ((13)C,(13)C) double-quantum spectroscopy to detect molecular dynamics by solid-state NMR. Data collected on tyrosine-ethylester (TEE) are in line with previously determined ((1)H,(13)C) order parameters. Application of these experiments to microcrystalline ubiquitin reveals the presence of dynamics on millisecond or faster time scales and differences in local mobility depending on microcrystal preparation. In addition, solid-state NMR-based structure calculation indicates conformational variability of loop regions between different solid-phase ubiquitin preparations. Our data relate preparation-dependent changes observed in NMR spectral parameters such as chemical shifts and through-space correlations to differences in ubiquitin dynamics and conformation and suggest a prominent role of molecular mobility in microcrystalline ubiquitin.

Connectivity and Proximity between Quadrupolar Nuclides in Oxide Glasses: Insights from through-Bond and through-Space Correlations in Solid-State NMR

The connectivity and proximity among framework cations and anions in covalent oxide glasses yields unique information whereby their various transport and thermodynamic properties can be predicted. Recent developments and advances in the reconstruction of anisotropic spin interactions among quadrupolar nuclides (spin > (1)/(2)) in solid-state NMR shed light on a new opportunity to explore local connectivity and proximity in amorphous solids. Here, we report the 2D through-bond (J-coupling) and through-space (dipolar coupling) correlation NMR spectra for oxide glasses where previously unknown structural details about the connectivity and proximity among quadrupolar nuclides ((27)Al, (17)O) are determined. Nonbridging oxygen peaks in Ca-aluminosilicate glasses with distinct connectivity, such as Ca-O-Al and Al-O-(Al, Si) are well distinguished in {(17)O}(27)Al solid HMQC NMR spectra. Both peaks shift to a lower frequency in direct and indirect dimensions upon the addition of Si to the Ca-aluminate glasses. The 2D (27)Al double quantum magic angle spinning NMR spectra for Mg-aluminoborate glasses indicate the preferential proximity between ([4])Al and ([5])Al leading to the formation of correlations peaks such as ([4])Al-([4])Al, ([4])Al-([5])Al, and ([5])Al-O-([5])Al. A fraction of the ([6])Al-([6])Al correlation peak is also noticeable while that of ([4,5])Al-([6])Al is missing. These results suggest that ([6])Al is likely to be isolated from the ([4])Al and ([5])Al species, forming ([6])Al clusters. The experimental realization of through-bond and through-space correlations among quadrupolar nuclides in amorphous materials suggests a significant deviation from the random distribution among framework cations and a spatial heterogeneity due to possible clustering of framework cations in the model oxide glasses.

Characterizing Slight Structural Disorder in Solids by Combined Solid-State NMR and First Principles Calculations

A general approach for structural interpretation of local disorder in partially ordered solids is proposed, combining high-resolution two-dimensional (2D) nuclear magnetic resonance (NMR) and first principles calculations. We show that small chemical shift variations of the order of a ppm can be interpreted in detailed structural terms with advanced density functional theory methods. Focusing on a model system of bisphosphinoamine, we demonstrate that the existence and the spatial range of small amplitude disorder can be probed using quantitative statistical analyses of 2D NMR line shapes obtained from through-space correlation experiments collected using variable mixing times. We show how low-energy vibration modes calculated from first principles can be conveniently used not as a cause of disorder but, instead, to generate a basis set of physically plausible local distortions to describe candidate static distributions of local geometries. Calculations of (31)P NMR isotropic chemical shifts are then used for the first time to simulate 2D correlation lineshapes associated with these distortions, which permit their evaluation as a potential source of disorder by comparison to experimental 2D cross-peaks between phosphorus sites. This new type of structural constraints allows the identification of changes in the bonding geometry that most likely contribute to the local structural disorder. We thus identify at least one type of structural deformation that is compatible with the experimental 2D NMR data and is also within the order of magnitude of the "thermal ellipsoids" associated with the uncertainties on the atomic positions of the X-ray diffraction structure.

Two-dimensional 43Ca– 1H correlation solid-state NMR spectroscopy

Calcium-43 (nuclear spin, S=7/2) is an NMR insensitive low- γ quadrupolar nucleus and up until recently only one-dimensional solid-state 43Ca NMR spectra have been reported. Through-space correlation experiments are challenging between spin- 1 2 and low- γ quadrupolar nuclei because of the intrinsically weak dipolar interaction and the often-low natural abundance of the quadrupolar nucleus. Rotary-resonance recoupling ( R 3) has recently been used to re-introduce hetero-nuclear dipolar interactions for sensitive high- γ quadrupolar nuclei, but has not yet been applied in the case of low- γ half-integer quadrupolar nuclei. Here an effective and robust 2D 1H- 43Ca NMR correlation experiment combining the R 3 dipole-recoupling scheme with 2D HMQC is presented. It is demonstrated that the weak 43Ca– 1H dipolar coupling in hydroxyapatite and oxy-hydroxyapatite can be readily re-introduced and that this recoupling scheme is more efficient than conventional cross-polarization transfer. Moreover, three 43Ca– 1H dipolar coupled calcium environments are clearly resolved in the structurally unknown oxy-hydroxyapatite. This local information is not readily available from other techniques such as powder XRD and high resolution electron microscopy. R 3-HMQC is also a desirable experiment because the set-up is simple and it can be applied using conventional multi-resonance probes.

High-resolution 14N-edited 1H– 13C correlation NMR experiment to study biological solids

It was recently shown that nuclear magnetic resonance (NMR) spectra of nitrogen-14 (spin I = 1) can be obtained by indirect detection via spin S = 1/2 nuclei in powders spinning at the magic angle. An increased number of solid-state NMR methods are now available to tailor sequences for specific purposes, e.g., hetero-nuclear dipolar recoupling or homo-nuclear dipolar decoupling schemes. Here, we combine the latest recoupling and decoupling techniques to obtain high-resolution 1H– 13C through-space correlation spectra, where only the correlation peaks of those carbons close to nitrogen nuclei are observed. The experiment is demonstrated on a 13C enriched l-histidine. HCl·H 2O powder sample.

2D Multinuclear NMR, Hyperpolarized Xenon and Gas Storage in Organosilica Nanochannels with Crystalline Order in the Walls

The combination of 2D 1H-13C and 1H-29Si solid state NMR, hyperpolarized 129Xe NMR, synchrotron X-ray diffraction, together with adsorption measurements of vapors and gases for environmental and energetic relevance, was used to investigate the structure and the properties of periodic mesoporous hybrid p-phenylenesilica endowed with crystalline order in the walls. The interplay of 1H, 13C, and 29Si in the 2D heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, revealed the spatial relationships (<5 angstroms) among various spin-active nuclei of the framework. Indeed, the through-space correlations in the 2D experiments evidenced, for the first time, the interfaces of the matrix walls with guest molecules confined in the nanochannels. Organic-inorganic and organic-organic heterogeneous interfaces between the matrix and the guests were identified. The open-pore structure and the easy accessibility of the nanochannels to the gas phase have been demonstrated by highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution. Two-dimensional exchange experiments showed the exchange time to be as short as 2 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the nanochannel space and surface, a physisorption energy of 13.9 kJ mol-1, and the chemical shift value of xenon probing the internal surfaces. These results prompted us to measure the high storage capacity of the matrix towards benzene, hexafluorobenzene, ethanol, and carbon dioxide. Both host-guest, CH...pi, and OH...pi interactions contribute to the stabilization of the aromatic guests (benzene and hexafluorobenzene) on the extended surfaces. The full carbon dioxide loading in the channels could be detected by synchrotron radiation X-ray diffraction experiments. The selective adsorption of carbon dioxide (ca. 90 wt %) vs that of oxygen and hydrogen, together with the permanent porosity, high thermal stability, and high degree of order, makes this a suitable matrix for purifying hydrogen in clean-energy generation.

A new strategy for structure determination of large proteins in solution without deuteration

So far high-resolution structure determination by nuclear magnetic resonance (NMR) spectroscopy has been limited to proteins <30 kDa, although global fold determination is possible for substantially larger proteins. Here we present a strategy for assigning backbone and side-chain resonances of large proteins without deuteration, with which one can obtain high-resolution structures from (1)H-(1)H distance restraints. The strategy uses information from through-bond correlation experiments to filter intraresidue and sequential correlations from through-space correlation experiments, and then matches the filtered correlations to obtain sequential assignment. We demonstrate this strategy on three proteins ranging from 24 to 65 kDa for resonance assignment and on maltose binding protein (42 kDa) and hemoglobin (65 kDa) for high-resolution structure determination. The strategy extends the size limit for structure determination by NMR spectroscopy to 42 kDa for monomeric proteins and to 65 kDa for differentially labeled multimeric proteins without the need for deuteration or selective labeling.

Improved methods for1H-3H heteronuclear shift correlation

A better understanding of the structure of complex 3H-labeled molecules can be obtained by complete assignment of their 1H and 3H solution-state NMR spectra. The assignment process is aided by the detection of heteronuclear chemical shift correlations between 1H and 3H nuclei. Heteronuclear correlation (HETCOR) experiments previously applied to this task exhibit several drawbacks caused by the nature of both the pulse sequences and 1H-3H spin systems. The range of J-couplings involved in 1H-3H coupling networks make it challenging to perform correlation experiments using methods that rely on coherences created during free precession periods and interrupted by transfer pulses. Two alternative HETCOR experiments are demonstrated for 1H-3H systems in the present work and are shown to have advantages over earlier methods. The first experiment is known as hetero-TOCSY and correlates heteronuclear chemical shifts using J-cross polarization. This experiment achieves both homonuclear and heteronuclear mixing and connects the chemical shifts of all 1H and 3H nuclei in a coupling network. A second HETCOR experiment uses the heteronuclear Overhauser effect to obtain through-space correlations between nearby nuclei. The 1H-3H HETCOR experiments are phase sensitive and typically contain more correlations than other methods, which is beneficial for assignment purposes, while being sensitive enough to be applicable to routine analytical samples. The experiments were used to analyze 3H incorporation in sub-milligram quantities of 3H-labeled pharmaceutical derivatives with complex labeling schemes.

Characterization of the Room-Temperature Structure of SnP2O7 by 31P Through-Space and Through-Bond NMR Correlation Spectroscopy

The room-temperature structure of SnP2O7 has been investigated using various one- and two-dimensional (1- and 2D) magic-angle spinning (MAS) NMR experiments. We show that 31P homonuclear through-bond and through-space correlation NMR spectroscopies allow us to discriminate between the 12 possible space group symmetries of the structure. In particular, the 31P 2D refocused INADEQUATE experiment allows us to resolve an impressive number of distinct 31P resonances, showing that the asymmetric unit contains at least 49 different P2O7 groups with two inequivalent P sites. The results suggest that the symmetry of the SnP2O7 room-temperature structure is monoclinic with space group P21 or Pc.