Measurement of J CH NMR Spin-Couplings in Saccharides: Non-First-Order Behavior in 1 H NMR Spectra Introduced by Selective 13 C Labeling

The incorporation of 13 C isotopes into saccharides introduces additional signal multiplicity into the 1 H NMR spectra caused by 13 C– 1 H spin-coupling to the highly abundant 13 C spins. Large 1 J CH values have been exploited to relieve non-first-order (strong coupling) effects in these spectra by spitting the signal of a 1 H spin that overlaps the signal arising from a mutually coupled 1 H spin. Using NMR spin simulation and complementary experimental studies, we show that this approach is valid only under specific spectral conditions. When the homonuclear J HH value between the strongly coupled hydrogens exceeds ∼6.5 Hz, the underlying multiplet exposed by 1 J CH splitting will experience a chemical shift offset due to strong coupling that complicates the measurement of J CH values if this hydrogen is also spin-coupled to the 13 C-labeled carbon. Since the magnitude of the offset scales with the magnitude of the 3 J HH 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 J CH values, especially those having dynamic ranges of