| STUDY |
Nuclear magnetic resonance (NMR) spectra of disordered, noncrystalline solids such as glasses, polymers, and biological materials typically have peaks that are broad, relatively featureless, and of limited use for materials characterization. Now, using a two-dimensional NMR correlation approach, a team has obtained high-resolution NMR spectra of disordered solids. The new technique "could lead to the detailed structural characterization of these types of materials, which are central to many areas of chemical sciences. High-resolution NMR spectra can already be obtained routinely from ordered, homogeneous solids, using techniques like magic-angle spinning (MAS). But conventional solid-state NMR techniques don't help to narrow the broad distribution of NMR chemical shifts that arises in structurally disordered solids. "By disordered systems, I mean things like polymers, glasses, surface species, or catalysts, where the local environment changes from one subunit to another due to a change in geometry. "In silicate glasses, for example, the Si–O–Si bond angle changes slightly from one Si–O–Si unit to another. In a crystalline silicate, the Si–O–Si bond angle is always identical." In disordered solids, resonance frequencies of otherwise equivalent nuclei vary because of such microenvironment differences. These variations aren't averaged out by rapid molecular tumbling, as occurs in solution NMR, nor can they be removed by conventional solid-state techniques like MAS. When NMR spectra of disordered materials are obtained, "people try to interpret them as best they can because this is all they can get, but the broadening severely limits the range of materials where the spectra are useful." This has for a long time looked like an impossible problem," he says. Authors have now shown that by going from one- to two-dimensional NMR and by making pairwise correlations between neighboring atoms, high-resolution spectra of disordered solids can finally be obtained. The new method may not represent a universal route to highly resolved spectra of disordered solids because chemical shift distances between neighboring atoms are constant in some disordered materials, but not all. Nevertheless, the technique demonstrates that obtaining such spectra is feasible. Authors are now investigating the range of disordered solids and NMR-active nuclei to which the strategy is applicable and the range of correlation techniques that can carry it out |
| UPDATE | 04.03 |
| AUTHOR | This data is not available for free |
| LITERATURE REF. | This data is not available for free |
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