Fig. 3
Possible multiplet components for the central carbon (carbon 2) of lactate. Clockwise (a–d), the four possible multiplet components are shown. The spectrum shown in a contains 13C in position 2 only, whereas positions 1 and 3 carry a 12C nucleus. In both b and c, one of the adjacent carbon nuclei carries an additional 13C nucleus in the same molecule. Therefore, the NMR signal is split into two resonance lines (doublet). Because of the different chemistry of carbons 1 and 3, the amount of splitting depends on which neighbour carries the 13C nucleus. This is why splittings can be used to uniquely assign which neighbouring carbon is labelled even though it may be impossible to directly observe that carbon, such as carbon 1, in a HSQC spectrum. The multiplet for a fully labelled lactate molecule is plotted in d. Because both neighbouring carbons are 13C, the signal is now split into four peaks (doublet of doublets). Under normal circumstances, multiplets will contain a mixture of those patterns. Such a mixture is shown in e. Despite the rather complex appearance of this signal, it can be fully quantified by a quantum-mechanical signal deconvolution