Heteronuclear Dipole Research Articles

Dipolar NMR Strategies for Multispin Systems Involving Quadrupolar Nuclei: 31P{23Na} Rotational Echo Double Resonance (REDOR) of Crystalline Sodium Phosphates and Phosphate Glasses

A new procedure for the site-selective measurement of heteronuclear magnetic dipole−dipole interactions involving quadrupolar nuclei has been developed. Particular attention is given to the case of S{I} rotational echo double resonance (REDOR) with S = 1/2 and I = 3/2, where the nuclear electric quadrupole splitting affects the efficiency of the π(I) pulses in recoupling the S−I dipolar interactions. Detailed two- and three-spin simulations using the SIMPSON code indicate that the REDOR response is sensitively affected by the quotient of the nutation frequency and the nuclear electric quadrupole frequency (ν1/νq). Multispin systems can be conveniently analyzed using a curvature determination of REDOR data in the limit of short dipolar evolution times (ΔS/S0 < 0.2), to yield dipolar second moment (M2) values. The apparent M2 values measured in this fashion include a scaling factor f1, the magnitude of which can be predicted if ν1/νq is known. Based on this finding, a robust method has been developed to analyze experimental REDOR data in multispin systems, in terms of average dipolar M2 values, which are shown to be affected by distributions of magnetic dipolar and nuclear electric quadrupolar coupling constants only to a minor extent. Validation experiments on suitable model compounds indicate that dipolar M2 values can be measured within 10% accuracy in this fashion for crystalline compounds. For glasses, the error may be higher, because of possible cumulative errors that may occur from unknown geometries and distribution effects. Experiments conducted on glassy sodium phosphates indicate that the 31P−23Na dipolar interaction strengths in these systems are differentiable for the various Q(n) sites and increase as the charge of the phosphate moiety increases.

New Developments of NMR Spectroscopy Applied to Zeolite Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Expanding the Frequency Range of the Solid-State T1 ρ Experiment for Heteronuclear Dipolar Relaxation

Solid-state spin–lattice relaxation in the rotating frame permits the investigation of dynamic processes with correlation times in the range of microseconds. The relaxation process in organic solids is driven by the fluctuation of the local magnetic field due to the dipole–dipole interaction of the probe nuclei ( 13C, 15N) with 1H in close proximity. However, its effect is often hidden by a competing relaxation process due to the contact between the rotating frame 13C/ 15N Zeeman and 1H dipolar reservoirs. In most cases the latter process becomes superior for the commonly applied low and moderate spin-lock fields and practically does not provide information about the molecular dynamics. To suppress this undesired process and to expand the dynamic range of T 1 ρ experiments, we present two approaches. The first one uses a resonance offset of the frequency of the spin-lock irradiation, which leads to a significant enhancement of the effective spin-lock frequency without the application of destructive high transmitter powers. We derive the theory and demonstrate the applicability of the method on various model compounds. The second approach utilizes heteronuclear 1H decoupling during the 13C/ 15N spin-lock irradiation which disrupts the contact between the 13C/ 15N Zeeman and 1H dipolar reservoirs. We demonstrate the method and discuss the results qualitatively.

Rotary resonance recoupling for half-integer quadrupolar nuclei in solid-state nuclear magnetic resonance spectroscopy

Investigations were made of rotary resonance recouplings (R3) of chemical shift anisotropy (CSA), heteronuclear dipolar (HTD), and homonuclear dipolar (HMD) couplings involving half-integer quadrupolar nuclei under magic-angle sample spinning condition. Under rotary resonance conditions provided by a low amplitude rf field and a high spinning speed, the spectrum of the central transition coherence of half-integer quadrupolar nuclei shows recouplings of CSA, HTD, and HMD interactions that depend on the ratio of the rf field to the spinning speed. These new properties can be used to extract electronic and structural information about the sample that are otherwise difficult to extract in the presence of a dominant quadrupolar interaction. An average Hamiltonian theory is used to explain the recoupling properties of various interactions. Experimental implementations of the R3 are demonstrated on model compounds with spin-3/2 systems.

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 XH n 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.

The Effect of RF Inhomogeneity on Heteronuclear Dipolar Recoupling in Solid State NMR: Practical Performance of SFAM and REDOR

Abstract Practical heteronuclear dipolar recoupling performances under magic angle spinning for SFAM and REDOR have been investigated under well-defined rf inhomogeneity environments with variation of resonance offsets for the irradiated nucleus. The heteronuclear dipolar recoupling efficiencies were quantitatively determined based on the experimentally obtained rf homogeneity. As a result, SFAM retains higher recoupling efficiency (>95%) at an 85% effective nutation frequency, and its recoupling efficiency is gradually reduced at lower effective nutation frequencies. On the other hand, although REDOR retains higher recoupling (>95%) efficiency at high (>92%) effective nutation frequency with an XY-8 compensation pulse sequence, the recoupling efficiency is dramatically decreased when the effective nutation frequency is below 90%. Over all, SFAM has significant advantages for insensitivity to carrier frequency offset and rf inhomogeneity.

Heteronuclear H1–13C multiple-spin correlation in solid-state nuclear magnetic resonance: Combining rotational-echo double-resonance recoupling and multiple-quantum spectroscopy

High-resolution magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy exploiting the dipole–dipole coupling between unlike spins is a powerful tool for the study of structure and dynamics. In particular, the rotational-echo double-resonance (REDOR) technique has established itself as a method for probing heteronuclear dipole–dipole couplings in isotopically dilute systems of low-γ nuclei. In organic substances it is, however, particularly advantageous to consider heteronuclear spin-pairs such as H1–13C, on account of the high natural abundance of H1 and thus a much wider range of possible applications, such as the determination of order parameters in liquid crystals and polymer melts. We describe the possibility of performing C13-observed REDOR in H1–13C systems, where very-fast MAS with spinning frequencies of up to 30 kHz is used to successfully suppress the perturbing homonuclear couplings among the protons, which would usually be expected to hamper a proper data analysis. Simple modifications of the REDOR experiment are presented which lead to a two-dimensional experiment in which heteronuclear multi-spin multiple-quantum modes are excited, the evolution of which is monitored in the indirect frequency dimension. The existence of higher quantum orders in the proton subspace of these heteronuclear coherences is proven by performing a phase-incremented spin-counting experiment, while a phase cycle can be implemented which allows the observation of specific selected coherence orders in the indirect dimension of two-dimensional shift correlation experiments. The significance of the heteronuclear approach to spin counting is discussed by comparison with well-known homonuclear spin-counting strategies. For the shift correlation, the high resolution of H1 chemical shifts in the indirect dimension is achieved by the use of high B0 fields (ωLH1/2π=700.13 MHz) combined with very-fast MAS, and dipolar coupling information can be extracted by analyzing either peak intensities or spinning-sideband patterns in the indirect frequency dimension. The method is termed dipolar heteronuclear multiple-spin correlation (DIP-HMSC).

NMR Structure Determination of Saccharose and Raffinose by Means of Homo- and Heteronuclear Dipolar Couplings

Residual dipolar couplings have dramatically improved the accuracy and precision of high-resolution NMR structures during the last years. This was first demonstrated for proteins. In this article, we describe, with raffinose and saccharose as examples, that dipolar couplings improve the precision of structures of carbohydrates for which usually very few structural parameters are available. The relative orientation as well as the dynamics of the monosaccharide moieties with respect to each other can be determined with the help of 13C,1H and 1H,1H dipolar couplings, which can easily be measured. Significant differences between the solution and the X-ray crystal structure exist. These results indicate that residual dipolar-coupling data may provide a more complete and dynamic model of carbohydrates in particular, and small molecules in general.

Aluminum Incorporation and Interfacial Structures in MCM-41 Mesoporous Molecular Sieves

Two-dimensional (2D) solid-state NMR spectroscopy has been used to identify interfacial species and establish framework locations of aluminum atoms incorporated in aluminosilicate MCM-41 mesophases and mesoporous solids. In these experiments, chemical shifts of protons in the material are correlated with the chemical shifts of nearby (ca. 1 nm) 13C, 27Al, or 29Si species via their respective heteronuclear dipole−dipole couplings. For aluminosilicate MCM-41 mesophases prepared at room temperature, 2D heteronuclear chemical shift correlation NMR spectra show that tetrahedrally coordinated aluminum and silicon species are in close spatial proximity to the trimethylammonium head groups of the cationic surfactant species in the as-synthesized materials and to ammonium cations following calcination and ion exchange. For MCM-41 materials synthesized under hydrothermal conditions, 2D heteronuclear correlation NMR measurements show that the appearance of six-coordinate aluminum species results from strongly bound water molecules coordinated to aluminum atoms that are also proximate to the surfactant species. The detection of couplings between 27Al or 29Si species and protons associated with the structure-directing surfactant molecules or exchangeable ammonium counterions establishes that a significant fraction of the aluminum atoms are present in the inorganic frameworks of these materials.

Measurement of Heteronuclear Dipolar Interactions between Quadrupolar and Spin-1/2 Nuclei in Solids by Multiple-Quantum REDOR NMR

Measurement of Heteronuclear Dipolar Interactions between Quadrupolar and Spin-1/2 Nuclei in Solids by Multiple-Quantum REDOR NMR

Wide frequency range 31P relaxation in the ion conducting glass LiPO 3

Spin–lattice relaxation studies on the stationary 31P nucleus have been performed in order to investigate to which extent the dynamics of the mobile lithium ions are reflected in the behavior of the glassy network. The temperature dependence of the 31P relaxation, which is governed by the heteronuclear dipole–dipole interaction between lithium and phosphorus, can be described in terms of a Gaussian distribution of activation energies and that over a wide frequency range from about 34 kHz to 81 MHz. A relaxation rate maximum, which provides useful information about correlation times and activation energies of the lithium-ion diffusion process, could only be observed in the rotating frame relaxation measurements.

ChemInform Abstract: Measurement of Heteronuclear Dipolar Interactions by Rotational‐Echo, Double‐Resonance Nuclear Magnetic Resonance

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Frequency-selective heteronuclear dephasing by dipole couplings in spinning and static solids

A compensated pulse sequence for the spectrally selective reintroduction of heteronuclear dipole–dipole interactions (frequency-selective dipolar recoupling) into solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments is described and shown to provide frequency-selective dipolar dephasing in weakly coupled spin systems. The experimental dipolar spin evolution is interpreted via analytical and numerical calculations, which include a simple model for the observed losses of spin coherence in the multiple pulse experiments. In the peptide glycylglycine, the selective dipolar evolution of two spins is observed while the influence of larger internuclear couplings is suppressed. This approach is aimed at obtaining several quantitative internuclear distances independently in dipolar ‘‘recoupling’’ MAS experiments by reducing multiple spin effects in the observed dipolar evolution. Similar frequency-selective dephasing experiments are also introduced for static solids, where an efficient application to measuring relative tensor orientations in powdered samples is demonstrated.

REDOR NMR: approaching structural elucidation of hydrated layers in silicate electrode glasses

Rotational echo double resonance (REDOR) is proposed as an alternative method for the structural determination of hydrated glasses, in particular of the incorporation of protons into the structure. First 23N 1H and 27Al 1H REDOR nuclear magnetic resonance data are presented for hydrated layers of electrode glasses of the system Na 2OAl 2O 3SiO 2 showing distinct differences of both the equilibrium amplitudes of the REDOR signals and the heteronuclear dipole couplings. The potential of this approach is demonstrated for kaolinite in terms of quantitative results.

Heteronuclear Dipolar Cross-Correlated Cross Relaxation for the Investigation of Side-Chain Motions

A new NMR technique is introduced and applied for the characterization of side-chain motion in biopolymers. It takes advantage of cross-correlated cross relaxation between 4SzI1zI2z and Sz spin orders in 13CH2 and 13CH3 subsystems. The basic process, called SIIS cross relaxation, relies on the motional cross correlation of two CH dipolar interactions. In a particular range of intramolecular motional correlation times, it allows the distinction of different models of restricted and unrestricted intramolecular motion based on the sign of the cross peaks in a 2D cross-relaxation spectrum. The technique is applied to the cyclic decapeptide antamanide and to the protein human ubiquitin.

NMR Measurement of Resolved Heteronuclear Dipole Couplings in Liquid Crystals and Lipids

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTNMR Measurement of Resolved Heteronuclear Dipole Couplings in Liquid Crystals and LipidsK. Schmidt-Rohr, D. Nanz, L. Emsley, and A. PinesCite this: J. Phys. Chem. 1994, 98, 27, 6668–6670Publication Date (Print):July 1, 1994Publication History Published online1 May 2002Published inissue 1 July 1994https://doi.org/10.1021/j100078a002RIGHTS & PERMISSIONSArticle Views309Altmetric-Citations87LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Multiple-pulse NQR dynamics of spin systems with strong heteronuclear coupling

The results are presented of a theoretical consideration of the nuclear quadrupole resonance (NQR) and spin—spin relaxation for a paramagnetic body containing nuclei of two different sorts coupled by the strong homonuclear and heteronuclear dipole—dipole interactions and influenced by an external multiple-pulse radiofrequency magnetic field acting only on nuclei of one sort. Kinetic equations were obtained giving the possibility of finding the time dependence of the magnetization of the body and the kinetic coefficients calculated as a function of the multiple-pulse field parameters. The possibilities of using the results in question for molecular structure and molecular dynamics investigations are briefly surveyed.

Removal of olefinic impurities from H-containing chlorofluorocarbons: R.M. Jansen Hoechst A.G. Eur. Pat. Appl. 511,612, Nov. 4, 1992; Ger. Appl. Apr. 27, 1991

New and powerful solid-state NMR techniques have been developed in the last 10–15 years. This paper reports on some applications on the location and structure of acid sites in zeolites. The distribution and location of acid sites is one of the fundamental questions. By means of the 13C-11B and 13C-27Al heteronuclear dipole interaction, an analysis of the spatial orientation of structure-directing agents towards the B and Al atoms in the framework of a variety of zeolites (CIT-1, SSZ-24, Beta, SSZ-13, SSZ-16, ITQ-3, ITQ-4, ZSM-12, Octadecasil) has been studied. This, leads to interesting information about the mechanism of structure-direction and the location of heteroatoms, implanted into the zeolite structure. Charge ordering between the negative framework charge (B or Al sites) and quaternary ammonium cations was a general result observed by these dipolar NMR methods. 11B and 27Al quadrupolar interactions in the activated acid forms of boron or aluminum zeolites are generally large due to a strong bond distortion around the heteroatom. These quadrupolar interactions are correlated with coordination symmetries and structural distortions. The dipolar interaction is also a useful probe to characterize the proton sites associated with distorted B or Al framework atoms, and to investigate the nature of adsorbed species and their distances to the framework. A crucial advantage of the dipolar interaction compared to other NMR interactions is that it can be analyzed directly without the need of model assumptions.

Measurement of Heteronuclear Dipolar Coupling by Transferred-Echo Double-Resonance NMR

A magic-angle spinning experiment called transferred-echo double resonance (TEDOR) has been introduced recently to measure the I-S dipolar coupling of heteronuclear I-S pairs of spin- 1 2 nuclei while eliminating unwanted background signals from uncoupled I and S spins via a coherence-transfer process. In this paper, a quantitative description of the TEDOR experiment is given in terms of the evolution of the density matrix for a pair of heteronuclear spins. The resulting equations provide a theoretical basis for evaluating the selectivity and sensitivity of TEDOR and suggest strategies for determining dipolar coupling constants directly from TEDOR data. Experimental examples illustrating these aspects of TEDOR are provided by studies performed on a range of 13C- 15N dipolar couplings found in double-labeled asparagine, alanine, glycine, and the linear peptide antibiotic, gramicidin.

Measurements of the dc electric dipole polarizabilities of the alkali dimer molecules, homonuclear and heteronuclear

The average static (dc) electric dipole polarizabilities of the NaK and KCs heteronuclear alkali dimers have been measured, and those of the homonuclear molecules remeasured using molecular beam deflection in an inhomogeneous electric field. A congruent magnetic field enabled removal of the monomer constituents of the beam. The resulting deflection curves reflect both the permanent electric dipole force (for the heteronuclear dimers only), and the induced (polarizability) dipole force. It is assumed that the molecular constituents of the beam possess modified Maxwell–Boltzmann velocity and rotational state distributions. A Mach number is introduced to describe the sonic beam properties. The data analysis consists of a two-parameter least squares best fit of the experimental deflection curves to computed ones, the parameters being the polarizability and the Mach number. Values of the permanent heteronuclear dipole moments are taken from the literature. The obtained static electric dipole polarizabilities in units of (Å3) are 34±3 for Li2 (948 K), 40±3 for Na2 (676 K), 77±6 for K2 (542 K), 79±6 for Rb2 (527 K), 104±8 for Cs2 (480 K), 51±3 for NaK (612 K), and 89±5 for KCs (488 K). Comparison with recent calculations as well as earlier measurements of the homonuclear species are given. Two empirical rules are proposed for determining polarizabilities and dipole moments of heteronuclear molecules.