J cross polarization in magic-angle-spinning NMR

Unravelling the mechanism of polarization transfer from spin-1/2 to spin-1 system in solids

An analytic theory based on the concept of "effective-fields" is proposed to explain the mechanism of polarization transfer from spin I = 1/2 to spin S = 1 in non-rotating (static) solids. Employing an isolated two-spin model system, the matching conditions responsible for polarization transfer in cross-polarization (CP) experiments are identified and described in terms of the single-transition operators. In contrast to other existing treatments, the polarization transfer among spins is quantified through analytic expressions highlighting the individual contributions emerging from all plausible CP matching conditions. The interplay between the CP matching conditions observed in experiments is outlined in both isotropic and anisotropic systems and verified through comparison between simulations based on analytic and exact numerical methods. The predictions emerging from the analytic theory are verified over a wide range of experimentally relevant parameters and could be beneficial in the optimization of the CP experiments.

Sensitivity considerations in polarization transfer and filtering using dipole-dipole couplings: implications for biomineral systems

The robustness and sensitivities of different polarization-transfer methods that exploit heteronuclear dipole-dipole couplings are compared for a series of heterogeneous solid systems, including polycrystalline tetrakis(trimethylsilyl)silane (TKS), adamantane, a physical mixture of doubly (13)C,(15)N-enriched and singly (13)C-enriched polycrystalline glycine, and a powder sample of siliceous marine diatoms, Thalossiosira pseudonana. The methods were analyzed according to their respective frequency-matching spectra or resultant signal intensities. For a series of (13)C{(1)H} cross-polarization experiments, adiabatic passage Hartmann-Hahn cross-polarization (APHH-CP) was shown to have several advantages over other methods, including Hartmann-Hahn cross-polarization (HHCP), variable-amplitude cross-polarization (VACP), and ramped-amplitude cross-polarization (RACP). For X-Y systems, such as (13)C{(15)N}, high and comparable sensitivities were obtained by using APHH-CP with Lee-Goldburg decoupling or by using the transferred-echo double resonance (TEDOR) experiment. The findings were applied to multinuclear (1)H, (13)C, (15)N, and (29)Si CP MAS characterization of a powder diatom sample, a challenging inorganic-organic hybrid solid that places high demands on NMR signal sensitivity.

Multiple-quantum cross polarization in quadrupolar spin systems during magic-angle spinning

We describe the concept of multiple-quantum cross polarization (CP) between an I = 32 and an I = 12 spin during magic-angle spinning. Experimental and theoretical results for (23)Na-(1)H pairs are presented that elucidate the transfer mechanism and the beneficial effect of adiabatic amplitude modulations of the CP field. The multiple-quantum CP approach is shown to be beneficial for improving the sensitivity of CP-MQMAS experiments and for detecting dipolar correlations.

A comparison of NMR distance determinations in the solid state by cross polarization, REDOR, and TEDOR techniques

Solid-state NMR distance determinations using Hartmann-Hahn cross polarization between spin-1/2 nuclei (19F/29Si) are reported. 19F –> 29Si polarization transfer for the T-1 silicon site in the clathrasil octadecasil shows an oscillatory behaviour as a function of the contact time. These oscillations were observed for non-spinning powder samples and also for different spinning speeds at the various MAS sideband matching conditions. Three analytical functions that allow efficient nonlinear least-square regression analyses of the experimental data to determine the internuclear distances for non-spinning powder samples as well as at the ±1 and ±2 MAS sideband matching conditions are reported. Using these functions, a 19F-29Si distance of 2.53 ± 0.04 Å was determined for the T-1 silicon in octadecasil from fitting of the oscillatory behaviour. This distance is in good agreement with that known from the X-ray structure and the previous 19F/29Si REDOR and TEDOR distance measurements. The advantages and limits of the different dipolar-based NMR techniques for heteronuclear distance determinations are examined and discussed.Key words: cross polarization (CP), NMR distance measurements, octadecasil, 19F-29Si distances, REDOR, TEDOR.

13C solid-state NMR of gramicidin A in a lipid membrane

The natural-abundance 13C NMR spectrum of gramicidin A in a lipid membrane was acquired under magic-angle spinning conditions. With fast sample spinning (15 kHz) at approximately 65 degrees C the peaks from several of the aliphatic, beta-, alpha-, aromatic, and carbonyl carbons in the peptide could be resolved. The resolution in the 13C spectrum was superior that observed with 1H NMR under similar conditions. The 13C linewidths were in the range 30-100 Hz, except for the alpha- and beta-carbons, the widths of which were approximately 350 Hz. The beta-sheet-like local structure of gramicidin A was observed as an upfield shift of the gramicidin alpha and carbonyl resonances. Under slow sample spinning (500 Hz), the intensity of the spinning sidebands from 13C in the backbone carbonyls was used to determine the residual chemical shift tensor. As expected, the elements of the residual chemical shift tensor were consistent with the single-stranded, right-handed beta6.3 helix structure proposed for gramicidin A in lipid membranes.

Application of rotor-synchronized amplitude-modulated cross-polarization in a 13C-1H spin pair under fast magic-angle spinning

Rotor-synchronized amplitude-modulated Hartmann-Hahn cross-polarization has been applied under fast magic-angle spinning to a powder sample of ferrocene. The influence on the cross-polarization process of the heteronuclear double quantum (flop-flop) transitions induced by the amplitude modulation for low ratios of the radiofrequency-field strength to the spinning speed is studied in details. The experimental data are in good agreement with theoretical calculations for a two-spin <SGMLEXP TYPE="INLINE">12 system, although the intensity of the double quantum transitions is observed to be significantly smaller than expected. Moreover, it is shown that an efficient polarization transfer at the Hartmann-Hahn centerband matching condition as well as a broadening of the matching conditions is obtained by an appropriate partial scaling of the effective radiofrequency fields which minimizes the destructive effect of double quantum cross-polarization. Copyright 1998 Academic Press.

High-speed magic-angle spinning 13C MAS NMR spectra of adamantane: self-decoupling of the heteronuclear scalar interaction and proton spin diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH2-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of the J-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH2-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).