Solid-state NMR with fast MAS and proton detection for the structural characterization of organic-inorganic hybrid materials

1H isotope presents a great challenge in solid state NMR, due to high-order dipolar interaction Hamiltonian terms (either multi-spin dipolar interactions or cross-terms involving dipolar coupling and magnetic shielding tensors), which do not vanish completely, but rather become smaller with increasing MAS frequency. As a result, protons are rarely used, in spite of the desperate need for the study of a variety of systems that are not suitable for high resolution investigation using other techniques, such as X-ray crystallography and solution NMR spectroscopy. This scenario is changing with the increasing popularity of fast (> 60 kHz) MAS probes, which are now commercially available [1]. Such high spinning rates allows the suppression of higher-order dipolar terms, improving the proton spectral resolution. Although the internuclear interactions, important for correlation experiments, are suppressed by ultra-fast MAS, they can be reintroduced by appropriate pulse sequences. In the present discussion, we will show the combination of fast MAS with the plethora of experiments available in solid and liquid state NMR for the characterization of H chemical environments in some amorphous organic-inorganic systems. These results are combined with conventional solid-state NMR techniques giving complementary structural insight. The following systems will be discussed: (i) amorphous metal-organic compounds with Eu-based luminescence; (ii) Organosilica materials for organic electronic applications; (iii) Photopolymerizable Aluminum-Phosphate-Silicate Sol–Gel Hybrid Materials for Additive Manufacturing; (iv) Highly luminescent organosilicate xerogel doped with Ir(III) complex.
[1] Nishiyama, Y. Solid-State NMR Under Ultrafast MAS Rate of 40 – 120 KHz. In Experimental Approaches of NMR Spectroscopy: Methodology and Application to Life Science and Materials Science; Japan, T. N. M. R. S. of, Ed.; Springer Singapore: Singpore, 2018; pp 171–195.