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Techniques for enhancing the signals arising from low-γ, insensitive (I) nuclei are central to solid-state nuclear magnetic resonance. One of the leading and best-established methods to sensitize these unreceptive species is Hartmann–Hahn cross polarization (HH-CP), a polarization transfer mechanism often executed under MAS. Herein, we explore the possibility of utilizing the 1H dipolar order created via adiabatic demagnetization in the rotating frame (ADRF), to enhance the unreceptive spins under MAS. It is found that an efficient polarization transfer via ADRF-CPMAS is not only possible but can exceed, at least in some instances involving plastic crystals, the efficiency of an optimized HH-CPMAS transfer. The experiment requires low radiofrequency nutation fields on both the 1H- and the I-spin channels, and displays unusual matching conditions that are reminiscent of the zero- and double-quantum matching conditions arising under CPMAS, albeit centered at zero frequency and demanding the simultaneous involvement of several spins. The origin of these multi-spin transfer processes is analytically derived and numerically simulated in predictions that compare well with experimental 13C and 15N results collected on model compounds at different spinning speeds. These derivations start from descriptions that depart from traditional thermodynamic arguments, and treat instead the ADRF processes in static and spinning solids on the basis of coherent evolutions. The predictions of these analytical derivations are corroborated by numerical simulations. The effects of additional factors, including chemical shift anisotropies, J-couplings, and radiofrequency inhomogeneities, are also theoretically and experimentally explored. [ABSTRACT FROM AUTHOR]

The potential use of relaxation times toward the resolution of inequivalent chemical sites in the NMR spectroscopy of powdered or disordered samples is discussed. The proposal is motivated by the significant differences that could often be detected in the relaxation behavior of sites in solids.

A new scheme for the excitation of spins according to the joint values of their heteronuclear or homonuclear J couplings and of their chemical shifts, is proposed and demonstrated. The principles of the new pulses involved derive from those employed in NMR imaging for exciting arbitrary 2D spatial shapes, using so-called 'multidimensional' RF pulses. It is shown that if recast in a suitable spectroscopic framework, the distinction that π-pulses enable to establish between linear and bilinear interactions, support the selective excitation of coherences possessing arbitrary combinations of chemical shift and J-coupling values - a flexibility akin to that provided by a 2D J-resolved NMR spectrum. Details on the execution of the resulting 2D 'J-shift' RF pulses are given, and examples where excitation only addresses spins with particular chemical shift offsets fulfilling specific J-coupling displacements, are demonstrated. Additional instances where such pulses could be applied, as well as main limitations of this new approach, are briefly discussed. [ABSTRACT FROM AUTHOR]

Recent years have witnessed efforts geared at increasing the sensitivity of NMR experiments, by relying on the suitable tailoring and exploitation of relaxation phenomena. These efforts have included the use of paramagnetic agents, enhanced 1H-1H incoherent and coherent transfers processes in 2D liquid state spectroscopy, and homonuclear 13C-13C spin diffusion effects in labeled solids. The present study examines some of the opportunities that could open when exploiting spontaneous 1H-1H spin-diffusion processes, to enhance relaxation and to improve the sensitivity of dilute nuclei in solid state NMR measurements. It is shown that polarization transfer experiments executed under sufficiently fast magic-angle-spinning conditions, enable a selective polarization of the dilute low-γ spins by their immediate neighboring protons. Repolarization of the latter can then occur during the time involved in monitoring the signal emitted by the low-γ nuclei. The basic features involved in the resulting approach, and its potential to improve the effective sensitivity of solid state NMR measurements on dilute nuclei, are analyzed. Experimental tests witness the advantages that could reside from utilizing this kind of approach over conventional cross-polarization processes. These measurements also highlight a number of limitations that will have to be overcome for transforming selective polarization transfers of this kind into analytical methods of choice. [ABSTRACT FROM AUTHOR]

Two-dimensional nuclear magnetic resonance (2D NMR) provides contemporary tools available for the elucidation of molecular structure, function, and dynamics. Hundreds of different 2D NMR pulse sequences are used in almost all areas of experimental science, ranging from solid-state chemistry and physics, through inorganic, and biochemistry, and onto various areas of in vivo experimentation and diagnosis.

A demonstration that single- and double-quantum transitions of [.sup.14.N] nuclei could be detected indirectly in solid powdered samples is presented. The quadrupole tensor parameters, which reflect the local electronic charge distribution, could be obtained by analysis of the line shapes.

Line narrowing has been traditionally achieved in solid-state 1H NMR spectroscopy by applying pulse sequences that combine multiple-pulse operations with magic-angle spinning (MAS), to effectively average out the dipoledipole homonuclear Hamiltonian. The present study explores a new alternative that departs from the usual concept of directly acting on the strongly coupled spins with radiofrequency pulses; instead, we seek to achieve a net homonuclear dipolar decoupling in solids by exploring the reintroduction of MAS-averaged heteronuclear dipolar couplings between the 1H nuclei and directly bonded 13C or 15N nuclei. This recouplinganti-recoupling (RaR) scheme thus relies on the recoupling of the dipolar interaction with heteronuclear spins, which, under fast MAS, will exceed the strength and will not commute with the homonuclear 1H1H coupling one is intending to average out. Subsequent removal ('antiRecoupling') of these heteronuclear interactions can lead to narrowed 1H resonances, without ever pulsing on the aforementioned channel. The line-narrowing properties of RaR are illustrated with numerical simulations and with experiments on model organic solids. [ABSTRACT FROM AUTHOR]

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, 14N and 15N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio ( γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality 14N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance 14N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., 1H). The BRoadband Adiabatic INversion Cross- Polarization ( BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform- Rate Smooth- Truncation ( WURST) pulses for broadband refocusing, Carr- Purcell Meiboom- Gill ( CPMG) echo trains for T2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for 14N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where 14N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments. [ABSTRACT FROM AUTHOR]

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, (14)N and (15)N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality (14)N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance (14)N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., (1)H). The BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform-Rate Smooth-Truncation (WURST) pulses for broadband refocusing, Carr-Purcell Meiboom-Gill (CPMG) echo trains for T2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for (14)N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where (14)N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)