| Abstract | Monosaccharides having a ring proton replaced by a deuteron give ¹³C-n.m.r. spectra that differ from those of undeuterated sugars. Under routine conditions for acquisition of sugar spectra, the ¹³C signal of the appended carbon atom “disappears”, as originally observed in the spectra, obtained without proton decoupling of monosubstituted benzenes. The effect on the resonances of adjacent carbon atoms is less pronounced, but they generally undergo a small displacement, which is frequently upfield by 0.04-0.12 p.p,m. However, a downfield shift can also occur, as in the case of [²H₆]acetone.
The disappearance of the deuterated carbon resonances of monosubstituted benzenes is due to two factors, (a) their long spin-lattice relaxation times (T ₁), which require pulse-delay intervals far in excess of those usually used for the acquisition of the resonances of protonated carbon atoms, and (b) the occurrence of ¹³C-²H coupling. In the case of carbohydrate spectra, obtained under conditions of proton decoupling, one question remains unresolved. This is the extent of diminution of signals, if any, that could arise from the lack of ¹³C-¹H nuclear Overhauser effect (NOE). In low-molecular-weight molecules, this has a value close to the maximum of 2.98 (or NOE factor ɳ = 1.98). To measure the NOE, a series of partly deuterated methyl α-D-glucopyranosides were examined. |
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