The incorporation of 13C isotopes into saccharides introduces additional signal multiplicity into the 1H NMR spectra caused by 13C-1H spin-coupling to the highly abundant 13C spins. Large 1JCH values have been exploited to relieve non-first-order (strong coupling) effects in these spectra by spitting the signal of a 1H spin that overlaps the signal arising from a mutually coupled 1H spin. Using NMR spin simulation and complementary experimental studies, we show that this approach is valid only under specific spectral conditions. When the homonuclear JHH value between the strongly coupled hydrogens exceeds ∼6.5 Hz, the underlying multiplet exposed by 1JCH splitting will experience a chemical shift offset due to strong coupling that complicates the measurement of JCH values if this hydrogen is also spin-coupled to the 13C-labeled carbon. Since the magnitude of the offset scales with the magnitude of the 3JHH between the strongly coupled hydrogens after a threshold of ∼6.5 Hz is reached, antiperiplanar hydrogens in saccharide pyranosyl rings are particularly vulnerable to this complication. The offset is not eliminated when spectral data are collected in two (and presumably higher) dimensions. This heretofore under-appreciated effect causes significant errors in the measurement of JCH values, especially those having dynamic ranges of 13C enrichment at one or more carbons when multidimensional spectra are collected to improve the spectral resolution.
Yoon et al. (Wed,) studied this question.