A fast two-dimensional switching-angle sample-spinning method for separating chemical-shift powder patterns

Abstract

Some years ago, a two-dimensional NMR method incorporating the technique of switching the spinner axis direction was introduced by Terao et al. (I) and Maciel et al. (2), independently, to unravel the overlapping chemical-shift powder patterns due to inequivalent nuclei. The former named the technique switching-angle sample spinning (SASS). This type of measurement has the advantages of not requiring the skilled adjustment of the experimental parameters and of yielding lineshapes containing no serious distortions, in comparison with other methods (3-8). Today, a commercial probe for SASS measurements (9) is available, so the technique may soon be more commonly used. However, one disadvantage of the SASS method still remains; that is, it is often time-consuming because of the requirements of 2D spectroscopy. In this study, we propose a simple modification of the previous 2D chemical-shift SASS method which makes the measuring time much shorter. We report here the chemicalshift principal values for the individual carbons of 2,4-dimethoxyacetophenone separately obtained with this new method. The previous 2D SASS method yields a spectrum which reflects the anisotropic and isotropic parts of chemical-shift interactions on one axis ( W, ), and only the isotropic part on the other (02); the chemical-shift powder patterns appear on the o, cross sections, separated with respect to the isotropic shifts in the o2 dimension. The powder patterns on the w, cross sections are situated at the individual isotropic-shift positions. Since the isotropic values are spread over a wide frequency range, as is often the case with 13C NMR solid-state spectra, a large w1 spectral width is required to cover simultaneously all the powder patterns without the occurrence of confusing aliasing; in particular, this condition is critical under the high magnetic field which is necessary to obtain well-resolved resonance lines in the w2 dimension. It follows that one is often forced to perform the experiments one by one with a small tI -increment value in the 2D spectroscopy. This fact is one reason why the previous 2D SASS method is often time-consuming when we wish to obtain all the powder patterns at once. Inherently, however, only small portions of such a wide frequency range in the w1 dimension are necessary to show the individual powder patterns, because they are scaled down around the respective isotropic shifts owing to the off-magic-angle spinning (OMAS) ( 1,2, 10); the widths of the powder patterns are reduced as much as possible to achieve high sensitivity taking care that their singularities are not missed because of the lower resolution. Hence, we can employ a much smaller spectral width in the