Strong coupling and flip-angle dependence of coupling patterns in heteronuclear shift correlation two-dimensional NMR spectra

Abstract

Heteronuclear shift correlation two-dimensional NMR spectroscopy (HETCOSY) has become a powerful technique for resonance assignments of carbon and proton spectra, and for structural information of organic and biomolecules. Coupled HETCOSY has lower signal intensity than decoupled HETCOSY due to distribution of intensities into several peaks of each multiplet but gains in intensity by avoiding refocusing delays, the optimum of which depends on various parameters. The coupling patterns contain information which leads to unambiguous resonance assignments and signs of coupling constants. However, strong coupling effects associated with flip-angle variations can give rise to unusual multiplet patterns which require detailed analysis. The multiplet patterns in the HETCOSY experiments usually show characteristic patterns for various types of carbons in the molecule. On the other hand, the patterns arising from a methylene carbon, with two nonequivalent protons, which are often strongly coupled, change significantly with variations of the chemical shift of the protons and their couplings. In particular the intensities of the central peaks along the carbon axis show significant variations as a function of these parameters.