Paul Hodgkinson

Homonuclear dipolar decoupling in solid-state NMR using continuous phase modulation

Abstract We present a theoretical framework for the use of continuously phase modulated radio-frequency pulses for homonuclear decoupling in solid-state NMR. Within this framework, we have derived new families of decoupling sequences using numerical optimization. One of these sequences is tested experimentally on an ordinary organic solid, and its performance is compared with standard multiple-pulse sequences.

Chemical shift computations on a crystallographic basis: some reflections and comments.

Computations for chemical shifts of molecular organic compounds using the gauge‐including projector augmented wave method and the NMR‐CASTEP code are reviewed. The methods are briefly introduced, and some general aspects involving the sources of uncertainty in the results are explored. The limitations are outlined. Successful applications of the computations to problems of interpretation of NMR results are discussed and the range of areas in which useful information is obtained is illustrated by examples. The particular value of the computations for comparing shifts between resonances where the same chemical site is involved is emphasised. Such cases arise for shifts between different crystallographically independent molecules of the same chemical species, between polymorphs and for shift anisotropies and asymmetries. Copyright

Spectroscopic and Structural Characterization of the CyNHC Adduct of B2pin2 in Solution and in the Solid State

The Lewis base adduct of B(2)pin(2) and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Numerical simulation of solid-state NMR experiments

Reference EPFL-ARTICLE-204466doi:10.1016/S0079-6565(99)00019-9View record in Web of Science Record created on 2015-01-08, modified on 2017-12-03

The reliability of the determination of tensor parameters by solid-state nuclear magnetic resonance

The accuracy of determination of tensor parameters measured using solid-state nuclear magnetic resonance is investigated. In particular, the reliability of the determination of the anisotropy and the asymmetry parameter of the chemical shift is calculated using the Cramer-Rao lower bounds. Minimizing this measure of the error as a function of an experimental parameter (in this case the spinning speed of the sample) enables the optimization of any given experiment. Hence, an optimum number of sidebands is found for which the determination of the anisotropy is most reliable. Comparision to the static limit shows that the reliability of the determination of the anisotropy is always greater in spinning experiments than in static experiments. An analogous analysis for the asymmetry parameter shows it to be consistently more reliably determined from a static spectrum. The sensitivity of the fitting to the simulation algorithm is found to become pronounced with slower speeds and static spectra, and a discussion of such systematic errors is provided. The reliability of the determination of dipolar or first-order quadrupolar tensors is also calculated, and we find that this presents a surprisingly different situation to that of the chemical shift

Computation and NMR crystallography of terbutaline sulfate

This article addresses, by means of computation and advanced experiments, one of the key challenges of NMR crystallography, namely the assignment of individual resonances to specific sites in a crystal structure. Moreover, it shows how NMR can be used for crystal structure validation. The case examined is form B of terbutaline sulfate. CPMAS 13C and fast MAS 1H spectra have been recorded and the peaks assigned as far as possible. Comparison of 13C chemical shifts computed using the CASTEP program (incorporating the Gauge Including Projector Augmented Wave principle) with those obtained experimentally enable the accuracy of the two distinct single‐crystal evaluations of the structure to be compared and an error in one of these is located. The computations have substantially aided in the assignments of both 13C and 1H resonances, as has a series of two‐dimensional (2D) spectra (HETCOR, DQ‐CRAMPS and proton–proton spin diffusion). The 2D spectra have enabled many of the proton chemical shifts to be pinpointed. The relationships of the NMR shifts to the specific nuclear sites in the crystal structure have therefore been established for most 13C peaks and for some 1H signals. Emphasis is placed on the effects of hydrogen bonding on the proton chemical shifts. Copyright

Improved heteronuclear decoupling schemes for solid-state magic angle spinning NMR by direct spectral optimization

It is shown that a direct spectral optimization scheme can be implemented to obtain improved heteronuclear dipolar decoupling schemes for solid-state magic-angle-spinning NMR experiments. The resulting schemes, which turn out to have a particularly simple form, are shown to be applicable over the whole range of commercially available spinning speeds (from 5 to 35 kHz), and are shown to improve on the performance of the best existing sequences.

Understanding two-pulse phase-modulated decoupling in solid-state NMR

A theoretical description of the two-pulse phase-modulated (TPPM) decoupling sequence in magic-angle spinning NMR is presented using a triple-mode Floquet approach. The description is formulated in the radio-frequency interaction-frame representation and is valid over the entire range of possible parameters leading to the well-known results of continuous-wave (cw) decoupling and XiX decoupling in the limit of a phase change of 0 degrees and 180 degrees , respectively. The treatment results in analytical expressions for the heteronuclear residual coupling terms and the homonuclear spin-diffusion terms. It also allows the characterization of all resonance conditions that can contribute in a constructive or a destructive way to the residual linewidth. Some of the important resonance conditions are described for the first time since they are not accessible in previous treatments. The combination of the contributions from the residual couplings and the resonance conditions to the effective Hamiltonian, as obtained in a Floquet description, is shown to be required to describe the decoupling behavior over the full range of parameters. It is shown that for typical spin system and experimental parameters a (13)C linewidth of approximately 12 Hz can be obtained for TPPM decoupling in an organic solid or a protein. This is a major contribution to the experimentally observed linewidths of around 20 Hz and indicates that decoupling techniques are still one of the limiting factors in the achievable linewidths.

Quantification of homonuclear dipolar coupling networks from magic-angle spinning 1H NMR

Numerical simulations of magic-angle spinning (MAS) spectra of dipolar-coupled nuclear spins have been used to assess different approaches to the quantification of dipolar couplings from 1H solid-state NMR. Exploiting the translational symmetry of periodic spin systems allows extended networks with ‘realistic’ numbers of spins to be considered. The experimentally accessible parameter is shown to be the root-sum-square of the dipolar couplings to a given spin. The effectiveness of either fitting the resulting spinning sideband spectra to small spin system models, or using analyses based on moment expansions, has been examined. Fitting of the spinning sideband pattern is found to be considerably more robust with respect to experimental noise than frequency domain moment analysis. The influence of the MAS rate and system geometry on robustness of the quantification is analysed and discussed.

High-Resolution 1H NMR Spectroscopy of Solids

Abstract Driven both by technological and methodological advances, major progress has been made over the past decade in improving the quality of 1 H NMR spectra in the solid state. Traditionally high-resolution solid-state 1 H NMR involved specialised and often challenging techniques, but the advent of fast magic-angle spinning (MAS) probes, higher magnetic fields, and RF decoupling schemes that complemented, rather than competed with, MAS has considerably simplified the task of obtaining useful 1 H spectra from solids. After introducing the factors that determine resolution in 1 H NMR and the approaches used to narrow 1 H linewidths, the relevant literature over the past decade is reviewed. The final section presents a selection of applications, highlighting the different areas to which resolved 1 H NMR spectra can now be applied.