Homonuclear Dipole-dipole Coupling Research Articles

Cross-term Splittings Due to the Orientational Inequivalence of Proton Magnetic Shielding Tensors: Do Water Molecules Trapped in Crystals Hop or Tunnel?

Water molecules trapped in crystals of barium chlorate monohydrate have been investigated by magic-angle spinning (MAS) proton NMR spectroscopy in the temperature range 110-300 K. At high temperatures, a single spinning sideband pattern is observed. Below 150 K, however, two interleaved spinning sideband manifolds appear, with distinct centerbands that do not coincide with the average isotropic chemical shift seen at high temperatures. This hitherto unknown "cross-term splitting" results from the interplay of the homonuclear dipole-dipole coupling and two anisotropic proton shielding tensors that have identical principal components but nonequivalent orientations. The resulting cross terms cannot be averaged out by rotation about the magic angle. The analysis of the exchange-induced broadening, coalescence, and narrowing of the cross-term splitting in MAS spectra allows one to estimate the rate of exchange of the two protons between 140 and 190 K. The experimental data is compared with 2H and 1H NMR studies of the same sample reported in the literature. Density functional theory methods are utilized to estimate the thermal activation energy for a 2-fold hopping process of proton exchange about the H-O-H bisector. The Bell-Limbach model allows one to take into account contributions due to incoherent quantum tunneling in the low-temperature regime.

Comparison of double-quantum NMR normalization schemes to measure homonuclear dipole-dipole interactions.

A recent implementation of a double-quantum (DQ) recoupling solid-state NMR experiment, dubbed DQ-DRENAR, provides a quantitative measure of homonuclear dipole-dipole coupling constants in multispin-1/2 systems. It was claimed to be more robust than another, previously known experiment relying on the recording of point-by-point normalized DQ build-up curves. Focusing on the POST-C7 and BaBa-xy16 DQ pulse sequences, I here present an in-depth comparison of both approaches based upon spin-dynamics simulations, stressing that they are based upon very similar principles and that they are largely equivalent when no imperfections are present. With imperfections, it is found that DQ-DRENAR/POST-C7 does not fully compensate for additional signal dephasing related to chemical shifts (CS) and their anisotropy (CSA), which over-compensates the intrinsic CS(A)-related efficiency loss of the DQ Hamiltonian and leads to an apparent cancellation effect. The simulations further show that the CS(A)-related dephasing in DQ-DRENAR can be removed by another phase cycle step or an improved super-cycled wideband version. Only the latter, or the normalized DQ build-up, are unaffected by CS(A)-related signal loss and yield clean pure dipolar-coupling information subject to unavoidable, pulse sequence specific performance reduction related to higher-order corrections of the dipolar DQ Hamiltonian. The intrinsically super-cycled BaBa-xy16 is shown to exhibit virtually no CS(A) related imperfection terms, but its dipolar performance is somewhat more challenged by CS(A) effects than POST-C7, which can however be compensated when applied at very fast MAS (>50 kHz). Practically, DQ-DRENAR uses a clever phase cycle separation to achieve a significantly shorter experimental time, which can also be beneficially employed in normalized DQ build-up experiments.

Robust NMR Approaches for the Determination of Homonuclear Dipole–Dipole Coupling Constants in Studies of Solid Materials and Biomolecules

This review addresses the NMR spectroscopy study of molecular structure and dynamics by way of homonuclear dipole-dipole couplings by relying on their orientation and direct distance dependence. The study of homonuclear couplings as opposed to heteronuclear couplings poses specific challenges. On the one hand, two like spins cannot be independently manipulated easily, which means that simple shift-refocusing concepts by using hard π pulses cannot be used to cope with potentially large chemical-shift dispersions at the high fields used today. On the other hand, the noncommutativity of the different pair Hamiltonians in a multispin system leads to complications associated with the isolation of specific pair couplings while minimizing the influence of the other spins. In particular, the so-called dipolar-truncation effect challenges the observation of weak couplings of interest in the presence of stronger ones. Recent advances in determining homonuclear dipole-dipole coupling constants are reviewed, stressing the use of double-quantum spectroscopy approaches and their similarity to the popular heteronuclear rotational-echo double-resonance experiment. Particular emphasis is put on corrections for the influence of transverse relaxation effects on the measured data, and the handling of distribution effects as well as potential dynamic heterogeneities in complex substances.

DQ-DRENAR: A new NMR technique to measure site-resolved magnetic dipole-dipole interactions in multispin-1/2 systems: Theory and validation on crystalline phosphates

A new solid state NMR technique is described for measuring homonuclear dipole-dipole interactions in multi-spin-1∕2 systems under magic-angle spinning conditions. Re-coupling is accomplished in the form of an effective double quantum (DQ) Hamiltonian created by a symmetry-based POST-C7 sequence consisting of two excitation blocks, attenuating the signal (intensity S'). For comparison, a reference signal S0 with the dipolar re-coupling absent is generated by shifting the phase of the second block by 90° relative to the first block. As in rotational echo double resonance, the homonuclear dipole-dipole coupling constant can then be extracted from a plot of the normalized difference signal (S0 - S')∕S0 versus dipolar mixing time. The method is given the acronym DQ-DRENAR ("Double-Quantum-based Dipolar Re-coupling effects Nuclear Alignment Reduction"). The method is analyzed mathematically, and on the basis of detailed simulations, with respect to the order and the geometry of the spin system, the dipolar truncation phenomenon, and the influence of the chemical shift anisotropy on experimental curves. Within the range of (S0 - S')∕S0 ≤0.3-0.5 such DRENAR curves can be approximated by simple parabolae, yielding effective squared dipole-dipole coupling constants summed over all the pairwise interactions present. The method has been successfully validated for (31)P-(31)P distance determinations of numerous crystalline model compounds representing a wide range of dipolar coupling strengths.

Towards measurement of homonuclear dipolar couplings in 1H solid-state NMR: recoupling with a rotor-synchronized decoupling scheme

We describe a magic-angle spinning NMR experiment for (1)H-(1)H homonuclear dipole-dipole coupling estimations in organic solids. The methodology involves reintroducing dipolar interactions with rotor-synchronized homonuclear decoupling pulse sequences. Frequency-selective DANTE pulses are used to isolate a specific spin pair from a natural isotopic abundance sample. The coupling of interest, between the selected spin pair, may be extracted by a non linear least-squares fit of the experimentally observed modulation of the signal intensity to an exact analytical formula. The experiment is demonstrated on natural isotopic abundance glycine and alanine powder samples.

Selective internuclear coupling estimation in the solid-state NMR of multiple-spin systems

A new solid-state NMR method is presented for estimating homonuclear dipole-dipole couplings for selected groups of nuclear spins in a multiple-spin coupled network. The methodology combines off-magic-angle spinning, frequency selective spin echoes, and multiple quantum filtering. The new method is insensitive to incoherent relaxation effects and may be used to estimate weak couplings. Internuclear (13)C-(13)C couplings are estimated in uniformly (13)C-labelled l-Histidine·HCl·H(2)O. Weak intermolecular couplings between (13)C nuclei separated by distances exceeding 6 Å are estimated.

Recoupling of native homonuclear dipolar couplings in magic-angle-spinning solid-state NMR by the double-oscillating field technique

A new solid-state NMR method, the double-oscillating field technique (DUO), that under magic-angle-spinning conditions produces an effective Hamiltonian proportional to the native high-field homonuclear dipole-dipole coupling operator is presented. The method exploits one part of the radio frequency (rf) field to recouple the dipolar coupling interaction with a relatively high scaling factor and to eliminate offset effects over a reasonable bandwidth while in the recoupling frame, the other part gives rise to a sufficiently large longitudinal component of the residual rf field that averages nonsecular terms and in addition ensures stability toward rf inhomogeneity and rf miscalibration. The capability of the DUO experiment to mediate transfer of polarization is described theoretically and compared numerically and experimentally with finite pulse rf driven recoupling and experimentally with dipolar-assisted rotational resonance. Two-dimensional recoupling experiments were performed on antiparallel amyloid fibrils of the decapeptide SNNFGAILSS with the FGAIL fragment uniformly labeled with (13)C and (15)N.

Cation environments and spatial distribution in Na2O–B2O3 glasses: New results from solid state NMR

The spatial distribution of the sodium ions in sodium borate glasses with composition (Na2O)x(B2O3)1+x (0 < or = x < or = 0.30) has been studied by complementary high resolution and dipolar solid state NMR experiments. 23Na-23Na homonuclear dipole-dipole couplings measured via 23Na spin echo decay spectroscopy indicate the absence of cation clustering at all compositions. Consistent with this result, 11B{23Na} rotational echo double resonance (REDOR) measurements indicate that the trigonal BO3/2 groups and the four-coordinate BO4/2(-) units experience 23Na local dipolar fields of similar magnitudes. Both experiments suggest a sodium spatial distribution that is essentially statistical. The dipolar coupling data are modelled consistently on the basis of a cubic NaCl-type lattice, from which an appropriate number of sodium ions have been randomly removed to reproduce the Na+ number density of the glass under consideration. In addition, at very low Na+ concentrations (x < 0.12) a compositionally independent local sodium environment is formed, which is characterized by a significant Na+ BO4/2(-) pair correlation involving an internuclear distance of 316 pm. For higher sodium oxide contents (x > 0.20) the 23Na-23Na dipole-dipole couplings suggest a gradual transition from a random towards a more homogeneous sodium distribution.

Dipolar coupling information in multispin systems: application of a compensated REDOR NMR approach to inorganic phosphates. Rotational echo double resonance.

An experimental strategy has been developed for measuring multiple dipole-dipole interactions in inorganic compounds using the technique of rotational echo double resonance (REDOR) NMR. Geometry-independent information about the dipole couplings between the observe nuclear species S (arbitrary quantum number) and the heteronuclear species I (spin-12) can be conveniently obtained from the experimental curve of DeltaS/S(0) versus dipolar evolution time by limiting the analysis to the initial data range 0 < delta S/S(0) < 0.30. Numerical simulations have been carried out on a three-spin system of type SI(2) in order to assess the effect of the I-I homonuclear dipole-dipole coupling and the influence of experimental imperfections such as finite pulse length and misadjustments of the 180 degrees pulses applied to the I-spin species. The simulations show further that within the initial data range the effects of such misadjustments can be internally compensated by a modified sequence having an additional 180 degrees pulse on the I channel in the middle of the dipolar evolution periods. Experimental (27)Al¿(31)P¿ REDOR results on the multispin systems Al(PO(3))(3), AlPO(4), [AlPO(4)](12)(C(3)H(7))(4)NF, and Na(3)PO(4) confirm the general utility of this approach. Thus, for applications to unknown systems the compensation strategy obviates calibration procedures with model compounds.

Approaches to high-resolution 1H NMR in solids

Principles and selected applications of solid-state 1H NMR are reviewed, with emphasis on the role of local fields and particularly the magnetic dipole-dipole couplings. The strong dipolar fields of the hydrogen nuclei compound the usual difficulties of solid-state NMR, notably the line broadening due to spatial anisotropy, and largely determine what information can be obtained from a spectrum and how. Methods designed to defeat both the 1H- 1H interactions and the orientational broadening, such as a combination of multiple-pulse averaging and magic-angle spinning, may yield highly resolved spectra displaying the isotropic chemical shifts. Although still far from routine, techniques to eliminate homonuclear dipole-dipole couplings have been applied successfully to polymers, zeolites, coals, and surface systems, among others. Yet these same dipolar interactions, if used to advantage, have spectroscopic value as well precisely because they do establish definite relationships among the coupled nuclei according to a well-defined geometrical dependence. Potential uses include the determination of cluster sizes and the measurement of H-X internuclear distances in powders to high accuracy. Some examples of these techniques, plus related methods such as zero-field NMR and proton imaging, are mentioned to suggest the wide range of possible chemical applications.

NMR studies of chalcogenide glasses: The system phosphorus-selenium

The feasibility of modern pulsed high-field high resolution solid state NMR techniques in non-oxide chalcogenide glasses is demonstrated for the system phosphorus-selenium. Isotropic and anisotropic chemical shieldings as well as 31P-31P dipolar couplings from 31P MAS and spin-echo NMR are reported on the glasses (10 to 50 mole% P), and the crystalline model compounds P4Se3 and P4Se4. Measurement of spin-echo decays show a monotonic increase of 31P-31P homonuclear dipole-dipole couplings with increasing phosphorus contents. The latter result suggests that the structure is more random than implied by the cluster model previously suggested.