High-Resolution Solid-State NMR Studies of Imidazole-Based Proton Conductors:  Structure Motifs and Chemical Exchange from ¹H NMR

Abstract

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 ( ) 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.

Interaction Tensors and Local Dynamics in Common Structural Motifs of Nitrogen: A Solid-State 14N NMR and DFT Study

“…A concentrated effort at fitting these line shapes, however, indicates the presence of two separate domains, in agreement with the findings of Goward and Spiess. 88,89 This is most easily illustrated by considering the simulations in Figure 9, which show how the 14 N powder pattern changes over a range of both jump rates and echo delays under the PH model. Two regimes can be distinguished.…”

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

“…The elucidation of the packing environment is often best investigated by looking at the proximities of the protons to each other using DQ filtered experiments. 6,7 Protons that are mobile on the timescale of the experiment are motionally averaged and filtered from the spectra. In the 1D DQF 30 kHz MAS NMR spectrum of fully dried choline DHP acquired using the Back-to-Back (BaBa) pulse sequence 6,18 at ambient temperature themselves (self-correlations E-E and F-F in Figure 6) and with each other (cross peaks E-F).…”

“…Proton double-quantum filtered ( 1 H DQF) experiments have been used in the study of a large number of systems for determination of the packing arrangement of the hydrogen-bonding motifs. [6][7][8][9][10][11] These experiments access the information inherent to through-space dipolar coupling and the relative strength of the measured dipolar coupling can be used as a probe of molecular mobility.…”

Proton Transfer in Heterocycle Crystals

“…High resolution NMR spectroscopy [5] has shown that, even below the melting point, the proton doped samples contain locally disordered domains with reduced dipolar interactions. These domains are believed to be associated with the local rearrangement of the hydrogen bonds (HBs), which, in turn, is related to the molecular motion and the proton diffusion.…”

“…These domains are believed to be associated with the local rearrangement of the hydrogen bonds (HBs), which, in turn, is related to the molecular motion and the proton diffusion. The fast time scale of the rearrangement of the hydrogen bonds' patterns has prevented the determination of the microscopic diffusion mechanism [5]. However, an understanding of the diffusion mechanism is essential not only because of the relevance of proton conduction in very many areas of science, but also for a rational design of proton conductors with improved properties.…”