GEMSTONE: ultra-selective NMR methods for fast analysis of complex spectra

Identification and structure elucidation of organic molecules are generally done by 2D NMR, which can provide extensive structural, conformational and dynamic information. However, such experiments are often time-consuming as they in general cover a large frequency range and provide information about all spin systems present. In many cases, it is just information originating from a single system that is needed to solve the problem at hand. Here 1D 1H selective experiments can often extract the key information required for structural and conformational analysis in a much shorter time. Such experiments are indeed often employed and can be immensely useful. Unfortunately, though, in 1H NMR the narrow range of chemical shifts often causes multiplets to overlap, so that traditional selective excitation methods cannot select a single chemical site. Potential solutions include the use of doubly selective experiments such as 1D COSY-TOCSY and HSCQ-TOCSY, but these require prior knowledge about the spin systems and/or have a steep cost in sensitivity. The chemical shift selective filter (CSSF)1 is a more general solution and does allow selection of a single chemical shift in an overlapped region, but is time-consuming because multiple increments are required to achieve selectivity.
Here, a new family of 1D ultra-selective experiments, GEMSTONE2 (gradient-enhanced multiplet-selective targeted-observation NMR experiment), is described and demonstrated. GEMSTONE allows selective excitation of a single multiplet even in the presence of severe overlap and requires only a single scan to achieve multiplet selectivity, saving significant time compared to CSSF. GEMSTONE versions of a range of 1D selective methods will be presented2,3, providing unambiguous through-space and through-bond correlations for individual selected signals.

References
[1] L. D. Hall, T. J. Norwood, J. Magn. Reson. 1988, 76, 548-554.
[2] P. Kiraly, M. Nilsson, G. A. Morris, R. W. Adams, Chem. Commun. 2021, 57, 2368-2371.
[3] P. Kiraly, N. Kern, M. P. Plesniak, M. Nilsson, D. J. Procter, G. A. Morris, R. W. Adams, Angew. Chemie-Int. Ed. 2021, 60, 666-669.

Acknowledgements: The work was funded by the Engineering and Physical Sciences Research Council, Royal Society of Chemistry, Johnson Matthey, Bruker, JEOL, and AstraZeneca