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

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 correlation is mainly achieved via cross polarization (CP) in solid samples, relying on the through-space dipolar couplings. As a result, such 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 1D and 2D HETCOR experiments, enabling sequential acquisition of 1D 13C and 2D HETCOR spectra of both rigid and mobile components in a single experiment, respectively. Particularly, CP and heteronuclear Overhauser effect is used for 1H13C polarization transfer in rigid and mobile components, respectively, both enabling achieving signal enhancement as well as remote heteronuclear chemical shift correlations. The proposed experiments were firstly demonstrated on the small molecular model system, glycine and adamantane mixture, and then on two typical polymer systems, including PMMA/PB blend (poly(methyl methacrylate)/polybutadiene) and PU (polyurethane). Due to the dynamic selectivity of this experiment, it can also be used for the fast chemical shift resonance assignment and dynamics based spectral editing of multiphase materials. We envisage that such an approach can be quite useful for the structural elucidation and thus revealing the interplay of structures and dynamics in multiphase materials.