Stochastic dipolar recoupling in nuclear magnetic resonance of solids

I describe a nuclear magnetic resonance (NMR) technique, called stochastic dipolar recoupling (SDR), that permits continuous experimental control of the character of spin dynamics between coherent and incoherent limits in a system of magnetic dipole-coupled nuclei. In the fully incoherent limit of SDR, spin polarization transfers occur at distance-dependent rates without the quantum mechanical interferences among pairwise dipole-dipole couplings that often limit the feasibility or precision of structural studies of solids by NMR. In addition to facilitating structural studies, SDR represents a possible route to experimental studies of effects of decoherence on the dynamics of quantum many-body systems.     

Homonuclear zero-quantum recoupling in fast magic-angle spinning nuclear magnetic resonance

Solid-state magic-angle-spinning NMR pulse sequences which implement zero-quantum homonuclear dipolar recoupling are designed with the assistance of symmetry theory. The pulse sequences are compensated on a short time scale by the use of composite pulses and on a longer time scale by the use of supercycles. (13)C dipolar recoupling is demonstrated in powdered organic solids at high spinning frequencies. The new sequences are compared to existing pulse sequences by means of numerical simulations. Experimental two-dimensional magnetization exchange spectra are shown for [U-(13)C]-L-tyrosine.     

An analysis of phase-modulated heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance

The design of variants of the swept-frequency two-pulse phase modulation sequence for heteronuclear dipolar decoupling in solid-state NMR is reported, their performance evaluated, and compared with other established sequences like TPPM and SPINAL. Simulations performed to probe the role of the homonuclear (1)H-(1)H bath show that the robustness of the decoupling schemes improves with the size of the bath. In addition, these simulations reveal that the homonuclear (1)H-(1)H bath also leads to broad baselines at high MAS rates. Results from a study of the SPINAL decoupling scheme indicate that optimisation of the starting phase and phase increment improves its performance and efficiency at high MAS rates. Additionally, experiments performed on a liquid crystal display the role of the initial phase in SPINAL-64 and sequences in the SW(f)-TPPM family.     

Observation of intermolecular double-quantum coherence signal dips in nuclear magnetic resonance

The correlated spectroscopy revamped by asymmetric Z-gradient echo detection (CRAZED) sequence is modified to investigate intermolecular double-quantum coherence nuclear magnetic resonance signal dips in highly polarized spin systems. It is found that the occurrence of intermolecular double-quantum coherence signal dips is related to sample geometry, field inhomogeneity and dipolar correlation distance. If the field inhomogeneity is refocused, the signal dip occurs at a fixed position whenever the dipolar correlation distance approaches the sample dimension. However, the position is shifted when the field inhomogeneity exists. Experiments and simulations are performed to validate our theoretic analysis. These signal features may offer a unique way to investigate porous structures and may find applications in biomedicine and material science.     

Improvement of homonuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance utilising radiofrequency imperfections

The often annoying imperfections in the phases and pulses of typical radiofrequency multiple-pulse irradiation schemes for homonuclear dipolar decoupling are revisited and analysed here. The analysis is with respect to one such multiple-pulse sequence, namely, the windowed phase-modulated Lee-Goldburg sequence. The error terms in the Hamiltonian due to pulse imperfections may lead to effective rotation of the spins around the z-axis giving rise to image free and high-resolution 1H spectra. Certain precautions to be taken with regard to scale factor estimation are also detailed. The analysis also points out the range of off-set values where the best homonuclear dipolar decoupling performance of a particular pulse scheme may be obtained.     

Heteronuclear polarization transfer by symmetry-based recoupling sequences in solid-state NMR

We demonstrate a new set of methods for transferring spin polarization between different nuclear isotopes in magic-angle-spinning solid-state NMR. The technique employs symmetry-based recoupling sequences on one irradiation channel and a simple sequence of between one and three strong radiofrequency pulses on the second channel. A phase shift of the recoupling sequences is applied at the same time as a pi/2 pulse on the second channel. The trajectory of the transferred polarization may be used to estimate heteronuclear distances. The method is particularly attractive for nuclei with low gyromagnetic ratios or for those experiencing strong anisotropic spin interactions, where conventional Hartmann-Hahn cross-polarization is difficult to apply. We demonstrate the method on 1H-13C, 1H-15N and 19F-109Ag systems.     

Dipolar broadening of double-quantum nuclear magnetic resonances in solids

Expressions are derived for the dipolar second moment of double-quantum nuclear magnetic resonances in solids. For like nuclei, double-quantum resonances are found to be narrower than single-quantum resonances because of a smaller “flip-flop” contribution.     

Heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance under ultra-high magic-angle spinning

We compare in this communication several heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance experiments under a magic-angle spinning frequency of 60 kHz. The decoupling radiofrequency field amplitudes considered are 190 and 10 kHz. No substantial difference was found among the sequences considered here in performance barring the difference in the optimisation protocol of the various schemes, an aspect that favours the use of swept-frequency two pulse phase modulation (SW(f)-TPPM).     

Experimental aspect of solid-state nuclear magnetic resonance studies of biomaterials such as bones

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is increasingly becoming a popular technique to probe micro-structural details of biomaterial such as bone with pico-meter resolution. Due to high-resolution structural details probed by SSNMR methods, handling of bone samples and experimental protocol are very crucial aspects of study. We present here first report of the effect of various experimental protocols and handling methods of bone samples on measured SSNMR parameters. Various popular SSNMR experiments were performed on intact cortical bone sample collected from fresh animal, immediately after removal from animal systems, and results were compared with bone samples preserved in different conditions. We find that the best experimental conditions for SSNMR parameters of bones correspond to preservation at −20 °C and in 70% ethanol solution. Various other SSNMR parameters were compared corresponding to different experimental conditions. Our study has helped in finding best experimental protocol for SSNMR studies of bone. This study will be of further help in the application of SSNMR studies on large bone disease related animal model systems for statistically significant results.     

Characterization of polyalkylvinyl ether phases by solid-state and suspended-state nuclear magnetic resonance investigations

Pure organic polyalkylvinyl ether phases were synthesized by suspension polymerization using different ratios and compositions of n-butylvinyl ether (C₄VE) and n-octadecylvinyl ether (C₁₈VE) with triethylene glycol divinyl ether or divinylbenzene as crosslinkers, respectively. These phases were investigated by means of solid-state ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and ¹H high-resolution magic angle spinning (HR MAS) NMR spectroscopy in suspended-state. A comparison of these two methods showed the substantial advantages of ¹H HR MAS NMR measurements. Structure elucidation was achieved using a 2D H,H-COSY NMR experiment performed under MAS conditions enabling full peak assignment of the ¹H NMR spectra of these phases. The dynamic behavior of the polyalkylvinyl ether phases was determined by employing temperature-dependent measurements of spin–lattice relaxation times (T₁) as well as accumulation of a 2D wide line separation NMR spectrum.     

Multidimensional dipolar exchange-assisted recoupling measurements in solid-state NMR

Aseries of uni- and multidimensional variants of the dipolar exchange-assisted recoupling (DEAR) NMR experiment is described and applied to determinations of (13)C-(14)N dipolar local field spectra in amino acids and dipeptides. The DEAR protocol recouples nearby nuclei by relying on differences in their relative rates of longitudinal relaxation, and has the potential to give quantitative geometric results without requiring radiofrequency pulsing on both members of a coupled spin pair. One- and two-dimensional variants of this recoupling strategy on generic I-S pairs are discussed, and measurements of (13)C-(14)N distances and 2D local field experiments sensitive to the relative orientation of CN vectors with respect to the (13)C shielding tensor are presented. Since these measurements did not involve pulsing on the broad nitrogen resonance, their results were independent of the quadrupolar parameters of this nucleus. High-resolution 3D NMR versions of the 2D experiments were also implemented in order to separate their resulting local field patterns according to the isotropic shifts of inequivalent (13)C sites. These high-resolution 3D acquisitions involved collecting a series of 2D DEAR NMR data sets on rotating samples as a function of spinning angle, and then subjecting the resulting data to a processing akin to that involved in variable-angle correlation NMR. Once successfully tested on l-alanine this experiment was applied to the analysis of a series of dipeptides, allowing us to extract separate local field (13)C-(14)N spectra from this type of multisite systems.     

Near-zero-field nuclear magnetic resonance

We investigate nuclear magnetic resonance (NMR) in near zero field, where the Zeeman interaction can be treated as a perturbation to the electron mediated scalar interaction (J?coupling). This is in stark contrast to the high-field case, where heteronuclear J?couplings are normally treated as a small perturbation. We show that the presence of very small magnetic fields results in splitting of the zero-field NMR lines, imparting considerable additional information to the pure zero-field spectra. Experimental results are in good agreement with first-order perturbation theory and with full numerical simulation when perturbation theory breaks down. We present simple rules for understanding the splitting patterns in near-zero-field NMR, which can be applied to molecules with nontrivial spectra.     

Supercycled symmetry-based double-quantum dipolar recoupling of quadrupolar spins in MAS NMR: I. Theory

Using average Hamiltonian (AH) theory, we analyze recently introduced homonuclear dipolar recoupling pulse sequences for exciting central-transition double-quantum coherences (2QC) between half-integer spin quadrupolar nuclei undergoing magic-angle-spinning. Several previously observed differences among the recoupling schemes concerning their compensation to resonance offsets and radio-frequency (rf) inhomogeneity may qualitatively be rationalized by an AH analysis up to third perturbation order, despite its omission of first-order quadrupolar interactions. General aspects of the engineering of 2Q-recoupling pulse sequences applicable to half-integer spins are discussed, emphasizing the improvements offered from a diversity of supercycles providing enhanced suppression of undesirable AH cross-terms between resonance offsets and rf amplitude errors.     

Supercycled SW(f)-TPPM sequence for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

The performance of a supercycled SW(f)-TPPM sequence for heteronuclear dipolar decoupling in solid-state NMR is analyzed here. The decoupling performance of this sequence with respect to experimental parameters, such as, the phase angle, proton offset and MAS frequency is studied. A comparison is made with two other commonly used decoupling schemes in solid-state NMR namely, SPINAL-64 and SW(f)-TPPM, on a sample of U-13C-labeled tyrosine. Our results show that supercycled SW(f)-TPPM performs better than the former sequences. Also, numerical spin dynamics studies are presented which support the experimentally observed efficiency in the decoupling.     

P and N solid-state nuclear magnetic resonance spectroscopic study of some monophospha-λ-azenes

³¹P Cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of eight monophospha-λ⁵-azenes were measured at room temperature. Three of these compounds, with 2-OH substituents, are in principle capable of taking part in the equilibrium process. However, only the open form, tautomer A, is found in the present study. ¹⁵N CP-MAS NMR spectra support the results. The most shielded component of the ³¹P chemical shift tensor lies in the direction of the P=N bond.     

Two-dimensional nuclear magnetic resonance petrophysics

Two-dimensional nuclear magnetic resonance (2D NMR) opens a wide area for exploration in petrophysics and has significant impact to petroleum logging technology. When there are multiple fluids with different diffusion coefficients saturated in a porous medium, this information can be extracted and clearly delineated from CPMG measurements of such a system either using regular pulsing sequences or modified two window sequences. The 2D NMR plot with independent variables of T2 relaxation time and diffusion coefficient allows clear separation of oil and water signals in the rocks. This 2D concept can be extended to general studies of fluid-saturated porous media involving other combinations of two or more independent variables, such as chemical shift and T1/T2 relaxation time (reflecting pore size), proton population and diffusion contrast, etc.