Solid state NMR characterization of lithium conducting ceramics

⁷Li solid state NMR has been used to characterize lithium aluminium titanium phosphate and lithium lanthanum titanate ceramics. Both materials have high ionic mobilities at room temperature and this is reflected in their static ⁷Li powder patterns. In the case of the phosphate based ceramic, a narrow Lorentzian peak is observed above 300 K, which narrows further with increasing temperature. The accompanying quadrupolar structure, with C_Q (quadrupolar coupling constant) ~ 40 kHz, suggests that the lithium ions are hopping rapidly between equivalent, high electric field gradient sites. The ²⁷Al and ³¹P magic angle spinning (MAS) spectra reveal an asymmetric phosphorus peak and two distinct aluminium resonances. The room temperature powder pattern of Li_0.33La_0.57TiO₃ shows a dipolar broadened peak which narrows quite suddenly at 310 K revealing quadrupolar satellites with C_Q ~ 900 Hz. A second lithium site is also observed in this material, as indicated by a further, weaker quadrupolar structure (C_Q ~ 40 kHz).

Solid state NMR analysis of polypyrrole grown in a phosphonium ionic liquid

³¹P, ¹⁹F and ¹³C solid state NMR analysis has been used to investigate the intercalation/de-intercalation of both anions and cations in electrochemically synthesized polypyrrole films. Use of a phosphonium-based ionic liquid, tri(hexyl)(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide, allows the separate detection of the cation and anion by analysis of the phosphorous and fluorine resonances, respectively. Initial results indicate the incorporation of both cations and anions during film growth in the ionic liquid. There is a notable change in the ³¹P chemical shift of the cation on incorporation into the film, consistent with a significant change in environment compared to the pure ionic liquid. Despite its large size, the phosphonium cation can be completely expelled from the film by oxidation.

Investigation of proton dynamics and the proton transport pathway in choline dihydrogen phosphate using solid-state NMR

Choline dihydrogen phosphate has previously been shown to be a good ionic conductor as well as an excellent host for acid doping, leading to high proton conductivities required for e.g., electrochemical devices including proton membrane fuel cells and sensors. A combination of variable-temperature ¹H solid-state NMR and 2D NMR pulse sequences, including ³¹P and ¹³C CODEX and ¹H BaBa, show that the proton conduction mechanism primarily involves assisted transport via a restricted three-site motion of the phosphate unit around the P–O bond that is hydrogen bonded to the choline and exchange of protons between these anions. In other words, proton transport at ambient temperatures appears to occur most favorably along the crystallographic b axis, from phosphate dimer to dimer. At elevated temperatures exchange between the protons of the hydroxyl group on the choline cation and the hydrogen-bonded dihydrogen phosphate groups also contributes to the structural diffusion of the protons in this solid state conductor.

Solid-state NMR as a probe of anion binding: molecular dynamics and associations in a [5]polynorbornane bisurea host complexed with terephthalate

A range of solid-state NMR techniques is used to characterise a molecular host:guest complex consisting of a [5]polynorbornane bisurea host binding a terephthalate dianion guest. Detailed information is obtained on the molecular dynamics and associations from the point of view of both the host and guest molecules. The formation of the complex in the solid state is confirmed using (1)H 2D exchange NMR, and the 180° flipping of the (2)H-labelled terephthalate guest and its eventual expulsion from the complex at elevated temperatures are quantified using variable-temperature (2)H spin-echo experiments. Two-dimensional (1)H-(13)C HETCOR spectra obtained under fast magic angle spinning conditions (60 kHz) show a high resolution despite the poor crystallinity of the solid complex, and clearly reveal changes in the rigidity of the host molecule when complexed. Short-range intra- and intermolecular (1)H-(1)H proximities are also detected using 2D SQ-DQ correlation methods, providing insight into the molecular packing in the solid phase.

Ultra-wideline 14N solid-state NMR as a method for differentiating polymorphs: glycine as a case study

Nitrogen-14 solid-state NMR (SSNMR) is utilized to differentiate three polymorphic forms and a hydrochloride (HCl) salt of the amino acid glycine. Frequency-swept Wideband, Uniform Rate, Smooth Truncated (WURST) pulses were used in conjunction with Carr-Purcell Meiboom-Gill refocusing, in the form of the WURST-CPMG pulse sequence, for all spectral acquisitions. The ¹⁴N quadrupolar interaction is shown to be very sensitive to variations in the local electric field gradients (EFGs) about the ¹⁴N nucleus; hence, differentiation of the samples is accomplished through determination of the quadrupolar parameters CQ and ηQ, which are obtained from analytical simulations of the ¹⁴N SSNMR powder patterns of stationary samples (i.e., static NMR spectra). Additionally, differentiation of the polymorphs is also possible via the measurement of ¹⁴N effective transverse relaxation time constants, T^eff2(¹⁴N). Plane-wave density functional theory (DFT) calculations, which exploit the periodicity of crystal lattices, are utilized to confirm the experimentally determined quadrupolar parameters as well as to determine the orientation of the ¹⁴N EFG tensors in the molecular frames. Several signal-enhancement techniques are also discussed to help improve the sensitivity of the ¹⁴N SSNMR acquisition method, including the use of selective deuteration, the application of the BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) technique, and the use of variable-temperature (VT) experiments. Finally, we examine several cases where ¹⁴N VT experiments employing Carr-Purcell-Meiboom-Gill (CPMG) refocusing are used to approximate the rotational energy barriers for RNH3⁺ groups.

Structural description of humic substances from subtropical coastal environments using elemental analysis, FT-IR and C-13-solid state NMR data

Elemental composition and spectroscopic properties (FT-IR and CP/MAS C-13-NMR) of sedimentary humic substances (HS) from aquatic subtropical environments (a lake, an estuary and two marine sites) are investigated. Humic acids (HA) are relatively richer in nitrogen and in aliphatic chains than fulvic acids (FA) from the same sediments. Conversely, FA are richer in carboxylic groups and in ring polysaccharides than HA. Nitrogen is mostly present as amide groups and for lake and marine HS the FT-IR peaks around 1640 cm(-1) and 1540 cm(-1) identify polypeptides. Estuarine HS exhibit mixed continental-marine influences, these being highly influenced by site location. Overall, the data suggest that aquatic and mixed HS are more aliphatic than has been proposed in current models and also that amide linkages form an important part of their structural configuration.