Reduced dimensionality hyphenated NMR experiments for the structure determination of compounds in mixtures

Abstract

For the structure determination of molecules in mixtures using NMR spectroscopy, the dispersion of ¹³C chemical shifts provides much needed separation of resonances in the indirectly detected dimension of 2D heterocorrelated NMR experiments. This separation is crucial for establishing networks of coupled spins by hyphenated techniques that combine hetero- and homonuclear polarisation transfers. However, as the sample complexity increases, ¹³C chemical shifts stop being unique, hindering spectral interpretation. The resulting ambiguities can be removed by adding another dimension to these experiments. However, the spectra obtained from complex samples are riddled with overlapped signals, meaning that another dimension will only reduce the spectral resolution and prevent structure determination. A promising solution is to stay in two dimensions and use the combined ¹³C and ¹H chemical shifts to separate signals. We have developed a suite of (3,2)D reduced dimensionality hyphenated NMR experiments that preserve the information content of 3D spectra but offer all of the advantages of 2D spectra – high resolution and ease of manipulation with only a mild sensitivity penalty. The proposed experiments complement the existing (3,2)D HSQC-TOCSY and include a (3,2)D HSQC-NOESY/ROESY, (3,2)D HSQC-CLIP-COSY and (3,2)D HSQC-HSQMBC. The new experiments represent a set of NMR techniques typically employed in the structure determination of complex compounds and have been adopted here for use on mixtures. The resolving power of these experiments is illustrated on the analysis of hot water extracts of green tea.