2D HETCOR Solid-State NMR Spectroscopy for Multiphase Materials with Mobility Contrast

Abstract

Two-dimensional heteronuclear chemical shift correlation experiments (HETCOR) under magic angle spinning (MAS) can provide the isotropic chemical shift and the proximity of different nuclei. The establishment of heteronuclear correlations is mainly achieved via cross-polarization (CP) in solid samples, relying on the through-space dipolar couplings. As a result, such an experiment typically suffers from severe limitations for the multiphase materials containing rigid and mobile components with significant mobility contrast, where the signals of mobile components in HETCOR spectra are often lost due to the efficient averaging of dipolar couplings by the fast molecular motions. Herein, we propose novel one-dimensional (1D) and two-dimensional (2D) HETCOR experiments, enabling sequential acquisition of 1D 13C and 2D HETCOR spectra of both rigid and mobile components in a single experiment, respectively, leading to significant experimental time saving. Particularly, CP and the heteronuclear Overhauser effect are used for 1H → 13C polarization transfer in rigid and mobile components, respectively, both enabling signal enhancement and particularly remote heteronuclear chemical shift correlations, in addition to the chemical bonding information. The proposed experiments were first demonstrated on a small-molecular model system, glycine and adamantane mixture, and then on two typical polymer systems, including a poly(methyl methacrylate)/polybutadiene (PMMA/PB) blend and polyurethane (PU). Due to the dynamic selectivity of this experiment, it can also be used for the fast chemical shift resonance assignments and dynamics-based spectral editing of multiphase materials. We envisage that such an approach can be quite useful for structural elucidation and thus reveal the interplay of structures and dynamics in multiphase materials.