Quantifying Site-Specific Proton Dynamics in Phosphate Solid Acids by ¹H Double Quantum NMR Spectroscopy

Abstract

Solid-state magic angle spinning (MAS) NMR was used to investigate changes in proton dynamics in phosphate solid acids that exhibited increased proton conductivity between room temperature and 110 °C. Double quantum dipolar recoupling methods were used to quantify site-specific changes in proton−proton dipolar coupling as a function of temperature. The static dipolar coupling and motionally induced changes to it were compared. This was accomplished by calculating (from crystal structures) and measuring (from the initial parts of the DQ recoupling curves) the root-sum-square of the dipolar coupling, a geometry-independent measure of dipolar coupling strength referred to as the "apparent dipolar coupling", D app . The analysis of KH 2 PO 4 and RbH 2 PO 4 showed that the experimentally determined apparent dipolar couplings were reduced from the calculated values at increased temperatures in dynamic systems. Higher proton conductivity was associated with greater reduction of the apparent dipolar coupling as measured by dipolar recoupling NMR methods. Most interestingly, in its monoclinic phase, RbH 2 PO 4 has two chemically distinct proton environments, one disordered and one ordered, which are resolved by 1 H MAS NMR. These sites exhibit different dipolar coupling responses as a function of temperature, revealing that proton conduction in this temperature range arises from motions involving only one of the sites. This site-specific dynamics is measured directly for the first time, using a combination of MAS to resolve the 1 H sites and dipolar recoupling experiments to probe the temperature dependence of the 1 H− 1 H dipolar interactions.

Use of Solid-State NMR Spectroscopy for the Characterization of Molecular Structure and Dynamics in Solid Polymer and Hybrid Electrolytes

“…The intensity of cross polarization spectra can also be used to gauge ion mobility as increased mobility tends to reduce the strength of the dipolar coupling interaction [ 121 ]. 1 H- 13 C cross polarization NMR has been used to compare the segmental mobilities of different polymer segments in PEG-based co-polymers.…”

Designing UiO-66-Based Superprotonic Conductor with the Highest Metal–Organic Framework Based Proton Conductivity

“…Meanwhile, a lot of effort has also been devoted within last decade to contribute to the understanding of proton conduction mechanism. [28][29][30][31][32][33][34][35][36] The most common method of proton conduction is solvent assisted proton transfer. 1,2 Similar mechanism has been found to operate in most of the Brønsted acids, common protonic solvents and functionalized polymers.…”

“…1,2 Similar mechanism has been found to operate in most of the Brønsted acids, common protonic solvents and functionalized polymers. 29,31,33,34 Apparently, protonic solvents and Brønsted acids have the simplest structures among all; but, on the contrary, they are found to follow complicated conductive mechanism. 3,29- 31 Since the last few decades, organic polymeric compounds (e.g.,Nafion) are well established as the supreme proton conductors.…”

“…29,31,33,34 Apparently, protonic solvents and Brønsted acids have the simplest structures among all; but, on the contrary, they are found to follow complicated conductive mechanism. 3,29- 31 Since the last few decades, organic polymeric compounds (e.g.,Nafion) are well established as the supreme proton conductors. They possess conductivity of the order of 10 -1 Scm -1 under humidification.…”

“…But they are less popular as proton conductors due to low efficiency at ambient conditions. [30][31][32] In this scenario, metal organic frameworks (MOFs) stand as a connecting bridge between inorganic cluster containing compounds and organic polymers in terms of detailed structural information from crystal structure and efficiency of proton conduction. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] These MOFs, being polymeric hybrid of organic linker molecule and metal ions, cultivate some of the useful properties of both.…”