Overtone NMR Research Articles

Selective 1H-14N Distance Measurements by 14N Overtone Solid-State NMR Spectroscopy at Fast MAS.

Accurate distance measurements between proton and nitrogen can provide detailed information on the structures and dynamics of various molecules. The combination of broadband phase-modulated (PM) pulse and rotational-echo saturation-pulse double-resonance (RESPDOR) sequence at fast magic-angle spinning (MAS) has enabled the measurement of multiple 1H-14N distances with high accuracy. However, complications may arise when applying this sequence to systems with multiple inequivalent 14N nuclei, especially a single 1H sitting close to multiple 14N atoms. Due to its broadband characteristics, the PM pulse saturates all 14N atoms; hence, the single 1H simultaneously experiences the RESPDOR effect from multiple 1H-14N couplings. Consequently, no reliable H-N distances are obtained. To overcome the problem, selective 14N saturation is desired, but it is difficult because 14N is an integer quadrupolar nucleus. Alternatively, 14N overtone (OT) NMR spectroscopy can be employed owing to its narrow bandwidth for selectivity. Moreover, owing to the sole presence of two energy levels (m = ± 1), the 14N OT spin dynamics behaves similarly to that of spin-1/2. This allows the interchangeability between RESPDOR and rotational-echo double-resonance (REDOR) since their principles are the same except the degree of 14N OT population transfer; saturation for the former whereas inversion for the latter. As the ideal saturation/inversion is impractical due to the slow and orientation-dependent effective nutation of 14N OT, the working condition is usually an intermediate between REDOR and RESPDOR. The degree of 14N OT population transfer can be determined from the results of protons with short distances to 14N and then can be used to obtain long-distance determination of other protons to the same 14N site. Herein, we combine the 14N OT and REDOR/RESPDOR to explore the feasibility of selective 1H-14N distance measurements. Experimental demonstrations on simple biological compounds of L-tyrosine.HCl, N-acetyl-L-alanine, and L-alanyl-L-alanine were performed at 14.1 T and MAS frequency of 62.5 kHz. The former two consist of a single 14N site, whereas the latter consists of two 14N sites. The experimental optimizations and reliable fittings by the universal curves are described. The extracted 1H-14N distances by OT-REDOR are in good agreement with those determined by PM-RESPDOR and diffraction techniques.

14N overtone NMR under MAS: Signal enhancement using cross-polarization methods.

Polarization transfer methods are widely adopted for the purpose of correlating different nuclear species as well as to achieve signal enhancement. Polarization transfer from 1H to the 14N overtone transition (Δm = 2) can be achieved using cross polarization methods under magic-angle spinning conditions, where spin locks of the order of several milliseconds can be obtained on common bio-solids (α-glycine and N-acetylvaline). Signal enhancement factors up to 4.4 per scan, can be achieved under favorable conditions, despite MHz-sized quadrupolar interaction. Moreover, we present a detailed theoretical treatment and accurate numerical simulations which are in excellent agreement the unusual experimental matching conditions observed for cross-polarization to 14N overtone.

Indirect detection of 10B (I = 3) overtone NMR at very fast magic angle spinning

The application of overtone nuclear magnetic resonance (OT NMR) to symmetric spin transitions of integer quadrupolar nuclei is of considerable interest since this transition is immune to the first-order quadrupolar interaction, thus resulting in narrow NMR lines. Owing to its roles in nature and its high natural abundance, 14N (I = 1) OT NMR has been explored, in which the indirect and/or direct acquisitions of 14N OT were experimentally demonstrated. However, other than 14N nucleus, no OT NMR observation of other integer quadrupolar nuclei has been reported in the literature. In this work, we extend the application of OT NMR to another integer quadrupolar nucleus, namely 10B (I = 3). However, this is not straightforward owing to the unfavorable characteristics of 10B isotope. Here, for the first time, we present the selective acquisition of 10B central (−1 ↔ +1) OT NMR via detection of 1H nuclei on perborate monohydrate sample. Numerical calculations are in a good agreement with the experimental results. Both show that the optimal sensitivity is achieved when the carrier frequency is applied at the second OT spinning sideband, i.e. an offset of twice of the spinning frequency from the center band.

(14)N overtone NMR under MAS: signal enhancement using symmetry-based sequences and novel simulation strategies.

Overtone (14)N NMR spectroscopy is a promising route for the direct detection of (14)N signals with good spectral resolution. Its application is currently limited, however, by the absence of efficient polarization techniques for overtone signal enhancement and the lack of efficient numerical simulation techniques to aid in both the development of new methods and the analysis and interpretation of experimental data. In this paper we report a novel method for the transfer of polarization from (1)H to the (14)N overtone using symmetry-based R-sequences that overcome many of the limitations of adiabatic approaches that have worked successfully on static samples. Refinement of these sequences and the analysis of the resulting spectra have been facilitated through the development of an efficient simulation strategy for (14)N overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker-Planck equations.

Dynamic nuclear polarisation enhanced (14)N overtone MAS NMR spectroscopy.

Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning (14)N(OT) (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the (14)N(OT) signal. The crystalline solids were impregnated with biradical solutions of organic liquids that do not dissolve the crystalline phase. The bulk phase was then polarized via(1)H spin diffusion from the highly-polarized surface (1)H nuclei, resulting in (1)H DNP signal enhancements of around two orders of magnitude. Cross polarisation from (1)H nuclei directly to the (14)N overtone transition is demonstrated under magic angle spinning, using a standard pulse sequence with a relatively short contact time (on the order of 100 μs). This method can be used to acquire (14)N overtone MAS powder patterns that match closely with simulated line shapes, allowing isotropic chemical shifts and quadrupolar parameters to be measured. DNP enhancement also allows the rapid acquisition of 2D (14)N(OT) heteronuclear correlation spectra from natural abundance powder samples. (1)H-(14)N(OT) HETCOR and (13)C-(14)N(OT) HMQC pulse sequences were used to observe all single-bond H-N and C-N correlations in histidine hydrochloride monohydrate, with the spectra obtained in a matter of hours. Due to the high natural abundance of the (14)N isotope (99.6%) and the advantages of observing the overtone transition, these methods provide an attractive route to the observation of C-N correlations from samples at natural isotopic abundance and enable the high resolution measurement of (14)N chemical shifts and quadrupolar interaction parameters.

Proton transport behaviour and molecular dynamics in the guanidinium triflate solid and its mixtures with triflic acid

Knowledge of the proton transport behaviour in electrolyte materials is crucial for designing and developing novel solid electrolytes for electrochemical device applications such as fuel cells or batteries. In the present work, high proton conductivity (approximately 10−3 S cm−1) was observed in the triflic acid (HTf) containing guanidinium triflate (GTf) composites. The proton transport mechanism in the composite was elucidated by comparing the diffusion coefficients obtained from NMR and conductivity measurements. Several orders of magnitude enhancement of conductivity is observed upon addition of HTf to the organic solid, and this appears to follow percolation behaviour with a percolation threshold of approximately 2% HTf. The data support a structural diffusion (or Grotthuss) mechanism of proton transport with a calculated Haven ratio significantly less than unity. 13C SUPER and 14N overtone NMR experiments were used to study the mobility and symmetry of the triflate anion and guanidinium cation respectively at a molecular level. The former experiment shows that the CF3 group in the anion displays fast and isotropic motion at room temperature. In contrast to the high mobility of the anion group, the 14N overtone experiments indicate that the guanidinium cation is static in both the pure and the acid-containing GTf samples at room temperature. It is anticipated that these solid-state NMR techniques may be also applied to other organic solid state electrolyte materials to achieve a better understanding of their transport mechanisms and molecular dynamics.

14N overtone NMR spectra under magic angle spinning: Experiments and numerically exact simulations

It was recently shown that high resolution (14)N overtone NMR spectra can be obtained directly under magic angle spinning (MAS) conditions [L. A. O'Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)]. Preliminary experimental results showed narrowed powder pattern widths, a frequency shift that is dependent on the MAS rate, and an apparent absence of spinning sidebands, observations which appeared to be inconsistent with previous theoretical treatments. Herein, we reproduce these effects using numerically exact simulations that take into account the full nuclear spin Hamiltonian. Under sample spinning, the (14)N overtone signal is split into five (0, ±1, ±2) overtone sidebands separated by the spinning frequency. For a powder sample spinning at the magic angle, the +2ω(r) sideband is dominant while the others show significantly lower signal intensities. The resultant MAS powder patterns show characteristic quadrupolar lineshapes from which the (14)N quadrupolar parameters and isotropic chemical shift can be determined. Spinning the sample at other angles is shown to alter both the shapes and relative intensities of the five overtone sidebands, with MAS providing the benefit of averaging dipolar couplings and shielding anisotropy. To demonstrate the advantages of this experimental approach, we present the (14)N overtone MAS spectrum obtained from L-histidine, in which powder patterns from all three nitrogen sites are clearly resolved.

QUEST—QUadrupolar Exact SofTware: A fast graphical program for the exact simulation of NMR and NQR spectra for quadrupolar nuclei

We present a new program for the exact simulation of solid-state NMR spectra of quadrupolar nuclei in stationary powdered samples which employs diagonalization of the combined Zeeman-quadrupolar Hamiltonian. The program, which we call QUEST (QUadrupolar Exact SofTware), can simulate NMR spectra over the full regime of Larmor and quadrupolar frequency ratios, which encompasses scenarios ranging from high-field NMR to nuclear quadrupole resonance (NQR, where the Larmor frequency is zero) and does not make use of approximations when treating the quadrupolar interaction. With the use of the fast powder averaging scheme of Alderman, Solum, and Grant [51], exact NMR spectral simulations are only marginally slower than the second-order perturbation theory counterpart. The program, which uses a graphical user interface, also incorporates chemical shift anisotropy and non-coincident chemical shift and quadrupolar tensor frames. The program is validated against newly-acquired experimental data through several examples including: the low-field 79/81Br NMR spectra of CaBr2, the 14N overtone NMR spectrum of glycine, the 187Re NQR spectra of Re2(CO)10, and lastly the 127I overtone NQR spectrum of SrI2, which, to the best of our knowledge, represents the first direct acquisition of an overtone NQR spectrum for a powdered sample.

14N magic angle spinning overtone NMR spectra

We present the first 14N overtone NMR spectra obtained directly under magic angle spinning conditions. Line-broadening contributions from chemical shift anisotropy and heteronuclear dipolar couplings are significantly reduced, and, despite theoretical predictions, spinning sidebands are minimal, leaving second-order quadrupolar powder patterns with narrow discontinuities that can be observed using a single excitation pulse. We also show that the limited bandwidth and offset-dependent nutation rates exhibited by standard RF pulses can be overcome using frequency-swept pulses, allowing the direct acquisition of solid-state 14N NMR spectra in which multiple nitrogen sites are well-resolved.

Feasibility of the overtone NMR for half-integer quadrupolar nuclei

We present a theoretical description and computer simulations for the overtone NMR of half-integer quadrupolar nuclei in solids. It is shown that the overtone NMR has the ability to produce high-resolution solid NMR spectra for the quadrupolar nuclei under magic angle spinning conditions. We demonstrate that when polycrystalline 23Na 2MoO 4 is placed in the static magnetic field of 7.05 T, the direct detection of the overtone NMR is possible. It becomes, however, apparent that indirect detection schemes are required to obtain the usual NMR spectra that have separated resonance lines corresponding to chemically non-equivalent sites.

A density matrix description of 14N overtone nuclear magnetic resonance in static and spinning solids

Overtone NMR is an experiment introduced by LeGros, Bloom, Tycko, and Opella, capable of providing 14N powder spectra devoid of first-order quadrupole broadenings by irradiation and observation of the nuclear spins at twice their Larmor frequency. This technique constitutes one of the most promising alternatives for the acquisition of high resolution solid 14N NMR spectra from random powders, particularly if it can be combined with strategies capable of removing the substantial second-order quadrupole broadenings remaining in the overtone line shapes. In order to facilitate the search for these averaging manipulations, we present here a theoretical description of the overtone experiment based on the time-domain propagation of density matrices. It is shown that by combining perturbation methods with appropriate rotating-frame transformations and diagonalizations, overtone spin-1 phenomena can be described using a single set of fictitious spin-1/2 operators. By contrast to conventional spin-1/2 irradiation and detection processes, however, overtone manipulations involve an unusual angular dependence on the azimuthal angle defining rotations about the main Zeeman magnetic field. This behavior introduces unexpected complications toward the narrowing of overtone resonances by conventional sample spinning techniques. Nevertheless, it can still be shown that the removal of all spin-1 anisotropies by certain forms of dynamic-angle spinning overtone NMR remains feasible.

Off-resonance proton-decoupling in overtone NMR spectroscopy of solids

An off-resonance proton-decoupling method is presented for the measurement of H–N dipolar couplings in 14N-overtone NMR spectroscopy of solids. This one-dimensional method employs magic-angle radio-frequency irradiation of protons in order to suppress the 1H– 1H dipole interactions during the 14N-overtone signal acquisition. The one-dimensional dipolar-overtone-shift spectrum obtained allows the direct measurement of N–H dipolar couplings for the structure determination of single crystalline or uniaxially oriented peptides. Results are presented from experiments on a single crystal sample of N-acetyl-D,L-valine. The proposed method is easy to implement experimentally and the results are simple to interpret. This experiment is less time-consuming and therefore useful for the study of biological solids as compared to two-dimensional separated-local-field methods.

Cross-polarization in 14N overtone NMR spectroscopy of solids

Several versions of cross-polarization (CP) sequences are analyzed in order to improve the sensitivity of 14N overtone NMR of biological solids. Optimum conditions are determined from experiments on a single crystalline sample of a model peptide, N-acetyl- d, l-valine, for an efficient CP from 1H to 14N overtone frequency. Experimental results infer that a CP sequence consisting of an amplitude modulated spin-locking of 14N overtone frequency during the CP period followed by a composite 90° postpulse is efficient in transferring the proton dipolar order to the observable overtone signal.

A composite pulse sequence for the broadband excitation in overtone 14N NMR spectroscopy

In this Letter, a composite π/2 pulse sequence, 45 135, for the broadband excitation in overtone NMR spectroscopy of spin S=1 quadrupolar nuclei is proposed. The performances of single and composite π/2 pulses against resonance offset and rf field inhomogeneity are compared based on the results from 14N overtone experiments on a single crystal sample of N-acetyl-D,L-valine. The results reveal that the 45 135 composite π/2 pulse is less sensitive to the pulse imperfections and it will be useful in the design of multidimensional overtone NMR experiments.

A historical account of NMR in the solid state

This article attempts to chronicle major milestones in the application of NMR in the solid state over the first 50 years since the discovery of NMR in 1945. After recording the earliest experiments with solids, we discuss in approximate chronological order the following topics: structure of solids, dynamics of solids including molecular reorientation, selfdiffusion and conformational motion, quadrupole effects, NQR, metals, negative temperatures, rotating frame temperatures, magnets and experimental methods, Fourier transform methods, magic angle spinning, multiple pulse NMR, CP/MAS, CRAMPS, double resonance, Overhauser effect, magnetic materials, ordered nuclear spin systems, phase transitions in solids, surfaces of solids, zero field NMR, 2D NMR, multiple quantum and overtone NMR in solids, NMR in high temperature superconductors, NMR in fullerenes, NMR imaging of solids, NMR force spectroscopy.

Determinationof the 14N quadrupole coupling tensor of an ℓ-alanine single crystal by overtone NMR

The overtone NMR approach was examined for determination of the quadrupole coupling tensor of the 14N nucleus in a single crystal and demonstrated for ℓ-alanine.

Demonstration of indirect detection of 14N |δm| = 2 (overtone) NMR transitions

Demonstration of indirect detection of 14N |δm| = 2 (overtone) NMR transitions

Overtone NMR spectroscopy

The theoretical background and experimental implementation of overtone NMR spectroscopy of quadrupolar nuclei, i.e., the direct observation of NMR signals at multiples of the Larmor frequency, are described. Perturbation theory is used to derive expressions for the dependence of overtone NMR frequencies, transition moments, and dipole–dipole splittings on molecular orientation for a nucleus with arbitrary spin. Experimental techniques for overtone cross-polarization and for two-dimensional spectroscopic measurements of transition moments and dipole–dipole splittings are demonstrated with 14N overtone spectra of single crystals. Overtone powder pattern line shapes are analyzed, both for spin S=1 and spin S=3/2 nuclei, and examples of 14N overtone powder patterns are presented. Overtone spectroscopy can be used to characterize quadrupole and dipole–dipole interactions in polycrystalline samples. The sensitivity, resolution, and spectral width characteristics of overtone NMR spectra are discussed and contrasted with those of conventional NMR spectra. The sensitivity and resolution of overtone spectra are shown to be comparable to those of conventional spectra, while the reduced spectral width of overtone spectra is a significant advantage in studies of single crystals and, in particular, polycrystalline materials.

High-resolution nitrogen-14 overtone spectroscopy: an approach to natural abundance nitrogen NMR of oriented and polycrystalline systems

In this paper, the authors present a novel approach to nitrogen NMR spectroscopy of both oriented and polycrystalline samples. The method employs the direct excitation and detection of /sup 14/N overtone NMR transitions at approximately twice the /sup 14/N Larmor frequency. New approaches to nitrogen NMR are needed both because of the chemical and biochemical importance of nitrogen and because of the limitations of current methods. In the solid state, /sup 15/N NMR typically requires isotopic labeling to achieve sufficient sensitivity. Through cross-polarization, good sensitivity is achieved in /sup 14/N NMR of single crystals. The large quadrupole interactions of /sup 14/N nuclei provide high resolution in single-crystal spectra but at the expense of a spectral width of several megahertz. Since the experimentally observable spectral range is only about 200 kHz, it is impossible to acquire a complete spectrum without repeated returning of the spectrometer. /sup 14/N powder pattern spectra have only been obtained with indirect detection or when the quadrupole couplings are unusually small. Large quadrupole couplings also limit the applicability of double-quantum and zero-field NMR techniques. For these reasons, the authors become interested in the insightful proposal by Bloom, and the experimental demonstration by Legros and Bloom, that /supmore » 14/N overtone NMR transitions can be directly excited and detected in the solid state.« less