Exchange experiments improving resolution

Recent improvements in high resolution NMR of solutions have added a new dimension to the structural study of silicon compounds. Many new methods in measuring Si NMR have been reported. The low sensitivity of Si can be circumvented by isotopic enrichment and/or the use of very high fields. For aqueous Si-enriched (99%) sodium silicate solutions, experiments at high field (11.7 T) have been reported by Harris and coworkers. Furthermore, a large number of species have been identified using two-dimensional (2D) experiments such as COSY for observation of Si- Si correlations in chemical exchange studies. 

Judeinstein et al have conducted direct measurement of through-space NMR interactions that provide definitive evidence for spatial proximity of different species. Dipole-dipole interactions can be measured in principle between any NMR active nuclei with heteronuclear correlation experiments in the liquid or solid state." The dipole-dipole interactions decay quickly with the internuclear distances (r ), and are difficult to evaluate for long-range distances and even more difficult when exchange, conformation, or motion phenomena are present. However, the measurement of the nuclear Overhauser method" based on the dipole-dipole-induced crossrelaxation, was proposed to successfully measure intermolecular interactions" and the formation of ion pairs." " In agreement with recent studies, the pulsed field gradient enhanced inverse HOESY (heteronuclear Overhauser enhancement spectroscopy) sequence is usually preferred because it is more sensitive for isotope pairs H- Li and also improves the digital resolution in the H crowded spectrum." ... 

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