Search

High-resolution solid-state ¹H NMR under fast magic angle spinning is used for the first time to study proton conductivity. The materials of interest, ethylene oxide tethered imidazole heterocycles (Imi-nEO), are characterized by variable temperature experiments, as well as 2D homonuclear double quantum (DQ) NMR and 2D exchange spectroscopy. Quantum chemical calculations provide a full assignment and understanding of the ¹H chemical shifts, based on a single-crystal structure obtained for Imi-2EO. Three types of hydrogen-bonded N-¹H resonances are observed by ¹H MAS NMR at 30 kHz. Double quantum NMR experiments identify those hydrogen-bonded protons that are mobile on the time scale of the experiment, and thereby, those which are able to participate in charge transport. Characterized by their spin−spin relaxation (%@mt;sys@%T%@sx@%2%@be@%*%@sxx@%%@mx@% ) behavior, the local mobility of these protons as a function of temperature is compared to the conductivity of the materials. Homonuclear ¹H 2D DQ MAS spectra provide evidence for locally ordered domains within all the Imi-nEO materials. Disordered (mobile) and ordered components in Imi-2EO dramatically differ in their ¹H spin−lattice relaxation times. 2D NOESY spectra show no evidence of chemical exchange processes between the ordered and disordered domains. These results indicate that the highly ordered regions of the materials do not (or only poorly) contribute to proton conductivity, which is rather taking place in the disordered regions. Molecules in the disordered domains are in a state of dynamic or fluctuating hydrogen-bonding, allowing for Grotthus mechanism proton transport, while molecules in the ordered domains do not experience exchange, and do not participate in long-range proton conductivity. At the interface between these regimes a small number of molecules undergo slow exchange. With increasing temperature, this exchange becomes fast on the NMR time scale, and the final chemical shift of 12.5 ppm in Imi-5EO implies the persistence of strongly and weakly hydrogen-bonded domains, which reorganize rapidly to support the proton transport process.

The solid columnar discotic and liquid crystalline (LC) phases formed by alkyl-substituted hexabenzocoronene (HBC) mesogens have been investigated by a combination of X-ray scattering and a variety of advanced solid-state NMR methods. Correlations between chemical structure, molecular packing and dynamics are established. Maximum charge carrier mobility is observed for a crystal-like stacking of the disks in the column with optimized π−π interactions. For systems with lower intracolumnar order the charge carrier mobility is slightly lower and slow axial motion of the disks around the column axes occurs.

Site specific ¹³C labeling of anhydrous glucose is used to study the time scale and geometry of reorientational motion of the exocyclic CH2OH group in relation to the main glucose ring. By comparison of 2D echo decay NMR experiments with Monte Carlo simulations a bimodal distribution of jump angles, a 75% fraction of 1−2° jumps and a 25% of 7−8° jumps, is found to describe the geometry of the reorientational processes of the main ring. For the CH2OH group the average jump angle of the larger jump process is somewhat larger. The jump rates for both the CH2OH group and the ring are similar. The apparent activation energy determined for the rotational motion of the CH2OH group and the ring is 480 ± 40 kJ/mol, which is very similar to an earlier determination using viscometry. It is concluded that the glucose ring and the exocyclic CH2OH-group mobility are strongly correlated and that the rotational freedom of the CH2OH group should not be used to explain the faster β-relaxation process also found for glucose.

Recently, 31P two-dimensional (2D) NMR has been proposed as a powerful tool for direct investigations of the connectivities between the Qnunits in glasses, i.e. the medium-range order. In this paper, the principles of these experiments and applications to both polycrystalline Mg2P2O7and a Na2O-P205glass are demonstrated, including a theoretical description and modelling of the polarization transfer between the two inequivalent phosphorus sites in the magnesium pyrophosphate.

Single crystals of KHCO3and KHSO4have been investigated by solid state high resolution proton magnetic resonance techniques. These compounds may be considered as model systems for compounds that contain abundant quadrupolar nuclei (39K) with small magnetogyric ratio in ad-dition to the protons. Despite considerable line broadening resulting from 1H-39K dipole-dipole coupling, all four possible proton shielding tensors in KHSO4, and an averaged shielding tensor for the two different proton sites in KHCO3could be measured accurately. The results are analysed to determine the degree of ionization of the protons in the three types of hydrogen bonds occuring in KHCO3and KHSO4.

NMR wideline and pulse techniques have been used to study orientation and molecular motions of the intercalation compounds of niobium disulfide with monolayers of mono-, di-, and trimethylamine, tetramethylammonium and ammonia. Specific information has been obtained from the 1H and 2D spectra and from spin-lattice relaxation measurements of the selectively deuterated monomethylamines. Models for the arrangement of the various molecules within the layers and for the motions have been derived. One-dimensional rotation of CH3and NH3groups, rapid intermolecular exchange of NH, NH2and NH3protons, as well as translational and reorientational motions of whole molecules are of importance. Interpretation of spin-lattice relaxation rates at various temperatures and frequencies yields details of some of these motion and numerical values for the activation energies.

Differential scanning calorimetry, the temperature dependence of proton and deuteron lineshapes and spin-lattice relaxation times are reported for the isotopic species CH3NH3ClO4, CD3NH3ClO4and CH3ND3ClO4of monomethylammonium Perchlorate. The data confirmed the existence of three different phase modifications in monomethylammonium Perchlorate and its selectively deuterated analogues. In addition they were used to identify the molecular motions occurring in the respective phases and to determine their activation parameters. In the low temperature phase III, stable below 320 K. the CH3and NH3groups reorient about their threefold symmetry axes C3with different frequencies. In the low-temperature range of this phase the deuteron quadrupole coupling constant indicates N - H ... O hydrogen bonds between the monomethylammonium and the Perchlorate ions. In the intermediate phase II, between 320 K and 451 K for CH3NH3ClO4and CD3NH3ClO4and 320 K and 437 K for CH3ND3ClO4, the methylammonium ions reorient about an axis inclined at an angle of 18 degrees to the C3axis. The analysis of the entropy changes, associated with the III- II transitions indicates that the ClO4-ions have a large motional freedom in phase II, presumably because of breaking or weakening of N - H . . . O hydrogen bonds. In phase I the monomethylammonium ions undergo isotropic motion along with translational diffusion between different sites of the primitive cubic unit cell.