Luke A. O’Dell

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state 14N NMR and DFT study.

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.

Static solid-state 14N NMR and computational studies of nitrogen EFG tensors in some crystalline amino acids

The recently reported direct enhancement of integer spin magnetization (DEISM) methodology for signal enhancement in solid-state NMR of integer spins has been used to obtain static (14)N powder patterns from alpha-glycine, L-leucine and L-proline in relatively short experimental times at 9.4 T, allowing accurate determination of the quadrupolar parameters. Proton decoupling and deuteration of the nitrogen sites were used to reduce the (1)H-(14)N dipolar contribution to the transverse relaxation time allowing more echoes to be acquired per scan. In addition, ab initio calculations using molecular clusters (Gaussian 03) and the full crystal lattice (CASTEP) have been employed to confirm these results, to obtain the orientation of the electric field gradient (EFG) tensors in the molecular frame, and also to correctly assign the two sets of parameters for L-leucine. The (14)N EFG tensor is shown to be highly sensitive to the surrounding environment, particularly to nearby hydrogen bonding.

Multinuclear Solid-State Nuclear Magnetic Resonance and Density Functional Theory Characterization of Interaction Tensors in Taurine

A variety of experimental solid-state nuclear magnetic resonance (NMR) techniques has been used to characterize each of the elements in 2-aminoethane sulfonic acid (taurine). A combination of (15)N cross-polarization magic angle spinning (CPMAS), (14)N ultrawideline, and (14)N overtone experiments enabled a determination of the relative orientation of the nitrogen electric field gradient and chemical shift tensors. (17)O spectra recorded from an isotopically enriched taurine sample at multiple magnetic fields allowed the three nonequivalent oxygen sites to be distinguished, and NMR parameters calculated from a neutron diffraction structure using density functional theory allowed the assignment of the (17)O parameters to the correct crystallographic sites. This is the first time that a complete set of (17)O NMR tensors are reported for a sulfonate group. In combination with (1)H and (13)C MAS spectra, as well as a previously reported (33)S NMR study, this provides a very broad set of NMR data for this relatively simple organic molecule, making it a potentially useful structure on which to test DFT calculation methods (particularly for the quadrupolar nuclei (14)N, (17)O, and (33)S) or NMR crystallography approaches.

Crystal structure based design of signal enhancement schemes for solid-state NMR of insensitive half-integer quadrupolar nuclei.

A combination of density functional and optimal control theory has been used to generate amplitude- and phase-modulated excitation pulses tailored specifically for the (33)S nuclei in taurine, based on one of several reported crystal structures. The pulses resulted in significant signal enhancement (stemming from population transfer from the satellite transitions) without the need for any experimental optimization. This allowed an accurate determination of the (33)S NMR interaction parameters at natural abundance and at a moderate magnetic field strength (11.7 T). The (33)S NMR parameters, along with those measured from (14)N using frequency-swept pulses, were then used to assess the accuracy of various proposed crystal structures.

New opportunities for quantitative and time efficient 3D MRI of liquid and solid electrochemical cell components: Sectoral Fast Spin Echo and SPRITE.

The ability to image electrochemical processes in situ using nuclear magnetic resonance imaging (MRI) offers exciting possibilities for understanding and optimizing materials in batteries, fuel cells and supercapacitors. In these applications, however, the quality of the MRI measurement is inherently limited by the presence of conductive elements in the cell or device. To overcome related difficulties, optimal methodologies have to be employed. We show that time-efficient three dimensional (3D) imaging of liquid and solid lithium battery components can be performed by Sectoral Fast Spin Echo and Single Point Imaging with T1 Enhancement (SPRITE), respectively. The former method is based on the generalized phase encoding concept employed in clinical MRI, which we have adapted and optimized for materials science and electrochemistry applications. Hard radio frequency pulses, short echo spacing and centrically ordered sectoral phase encoding ensure accurate and time-efficient full volume imaging. Mapping of density, diffusivity and relaxation time constants in metal-containing liquid electrolytes is demonstrated. 1, 2 and 3D SPRITE approaches show strong potential for rapid high resolution (7)Li MRI of lithium electrode components.

Comparison of various NMR methods for the indirect detection of nitrogen-14 nuclei via protons in solids

We present an experimental comparison of several through-space Hetero-nuclear Multiple-Quantum Correlation experiments, which allow the indirect observation of homo-nuclear single- (SQ) or double-quantum (DQ) (14)N coherences via spy (1)H nuclei. These (1)H-{(14)N} D-HMQC sequences differ not only by the order of (14)N coherences evolving during the indirect evolution, t1, but also by the radio-frequency (rf) scheme used to excite and reconvert these coherences under Magic-Angle Spinning (MAS). Here, the SQ coherences are created by the application of center-band frequency-selective pulses, i.e. long and low-power rectangular pulses at the (14)N Larmor frequency, ν0((14)N), whereas the DQ coherences are excited and reconverted using rf irradiation either at ν0((14)N) or at the (14)N overtone frequency, 2ν0((14)N). The overtone excitation is achieved either by constant frequency rectangular pulses or by frequency-swept pulses, specifically Wide-band, Uniform-Rate, and Smooth-Truncation (WURST) pulse shapes. The present article compares the performances of four different (1)H-{(14)N} D-HMQC sequences, including those with (14)N rectangular pulses at ν0((14)N) for the indirect detection of homo-nuclear (i) (14)N SQ or (ii) DQ coherences, as well as their overtone variants using (iii) rectangular or (iv) WURST pulses. The compared properties include: (i) the sensitivity, (ii) the spectral resolution in the (14)N dimension, (iii) the rf requirements (power and pulse length), as well as the robustness to (iv) rf offset and (v) MAS frequency instabilities. Such experimental comparisons are carried out for γ-glycine and l-histidine.HCl monohydrate, which contain (14)N sites subject to moderate quadrupole interactions. We demonstrate that the optimum choice of the (1)H-{(14)N} D-HMQC method depends on the experimental goal. When the sensitivity and/or the robustness to offset are the major concerns, the D-HMQC sequence allowing the indirect detection of (14)N SQ coherences should be employed. Conversely, when the highest resolution and/or adjusted indirect spectral width are needed, overtone experiments are the method of choice. The overtone scheme using WURST pulses results in broader excitation bandwidths than that using rectangular pulses, at the expense of reduced sensitivity. Numerically exact simulations also show that the sensitivity of the overtone (1)H-{(14)N} D-HMQC experiment increases for larger quadrupole interactions.

Anisotropic MRI contrast reveals enhanced ionic transport in plastic crystals

Organic ionic plastic crystals (OIPCs) are attractive as solid-state electrolytes for electrochemical devices such as lithium-ion batteries and solar and fuel cells. OIPCs offer high ionic conductivity, nonflammability, and versatility of molecular design. Nevertheless, intrinsic ion transport behavior of OIPCs is not fully understood, and their measured properties depend heavily on thermal history. Solid-state magnetic resonance imaging experiments reveal a striking image contrast anisotropy sensitive to the orientation of grain boundaries in polycrystalline OIPCs. Probing triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) samples with different thermal history demonstrates vast variations in microcrystallite alignment. Upon slow cooling from the melt, microcrystallites exhibit a preferred orientation throughout the entire sample, leading to an order of magnitude increase in conductivity as probed using impedance spectroscopy. This investigation describes both a new conceptual window and a new characterization method for understanding polycrystalline domain structure and transport in plastic crystals and other solid-state conductors.

Optimisation of excitation schemes for 14N overtone MAS NMR using numerically exact simulations

Numerically exact simulations of the 14N overtone (14NOT) MAS NMR experiment are used to investigate the effects of the applied magnetic field strength as well as three types of excitation pulse. The results show that both the resolution and sensitivity of 14NOT MAS NMR increase linearly with the applied static magnetic field strength. Standard RF excitation pulses are compared with frequency-swept WURST pulses as well as several composite pulses. WURST pulses are demonstrated to provide the largest bandwidths, while the direction of the frequency sweep is shown to be important when these pulses are used for the direct observation of 14NOT signals. A composite pulse is shown to provide the most efficient excitation overall, but only when applied on resonance. WURST excitation pulses are therefore the best option when studying a sample with unknown 14N NMR parameters.

The influence of the size and symmetry of cations and anions on the physicochemical behavior of organic ionic plastic crystal electrolytes mixed with sodium salts

The phase behaviour, ionic conductivity, electrochemical stability and diffusion coefficients of mobile components in three organic ionic plastic crystals (OIPCs): triisobutylmethylphosphonium bis(fluorosulphonyl)amide (P1i444FSI), triisobutylmethylphosphonium bis(trifluromethanesulphonyl)amide (P1i444NTf2) and trimethylisobutylphosphonium bis(trifluoromethanesulphonyl)amide (P111i4NTf2) are compared to study the effect of the anions and cations on phase behaviour and dynamics. The FSI-based OIPC shows lower melting point and higher conductivity values most likely because of the higher degree of charge distributions and weaker ion-ion interactions compared to NTf2 anion-based OIPCs. Cyclic voltammetry of electrolytes consisting of these OIPCs with 70 mol% sodium salt incorporated indicates stable sodium plating/stripping behaviour at 70 and 50 °C for all samples. The magnitude of the peak currents, however, are much higher for the FSI-based electrolyte.

Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy

Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy has been used to study an ionic liquid salt solution (N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide, C3mpyrFSI, containing 1.0 m lithium bis(fluorosulfonyl)imide, 6LiFSI) in its glassy state at a temperature of 92 K. The incorporation of a biradical to enable DNP signal enhancement allowed the proximities of the lithium to the individual carbon sites on the pyrrolidinium cation to be probed using a 13C-6Li REDOR pulse sequence. Distributions in Li-C distances were extracted and converted into a 3D map of the locations of the Li+ relative to the C3mpyr that shows remarkably good agreement with a liquid-phase molecular dynamics simulation.