Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds
Chapter 9: Heteronuclear Shift Correlation Sequences
Published:26 Sep 2018
Special Collection: 2018 ebook collectionSeries: New Developments in NMR
2018. "Heteronuclear Shift Correlation Sequences", Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds, Darcy C Burns, William F Reynolds
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This chapter begins with a discussion of the older 13C shift correlation sequences. While significantly less sensitive than the newer corresponding 1H-detected sequences, the earlier versions can still be useful in cases where high resolution is required along both axes of a 2D spectrum. The first 1H-detected sequence discussed is the HMQC sequence. This was originally the sequence of choice for obtaining 1-bond correlation spectra but it has now been entirely replaced by the HSQC sequence. The numerous improvements that have been made to HSQC are discussed and evaluated. Finally, the ASAP-HMQC and ASAP-HSQC sequences that allow very rapid (<1 minute) 2D spectral acquisition are compared. The preferred sequence for obtaining 2-bond and longer 13C–1H correlation spectra is the HMBC sequences. Two different ways of acquiring and processing HMBC spectra are described, with one approach strongly favored. Different pulse sequences that are designed to give stronger correlation peaks due to very small 13C/1H couplings or to better distinguish between 2-bond and longer-range correlations are described, along with reasons why we regard some as valuable while others are not. The chapter ends with specific recommendations for acquisition and processing methods for spectra obtained with the recommended sequences, followed by a summary of the key recommendation from the chapter.