Referencing 13C Chemical Shifts

Any heteronuclear signal of a solvent or an added reference substance can be used for referencing I3C shifts. For example, 13C shifts can be directly measured relative to a deuterium signal of the deuterated solvent usually required for field/frequency stabilization. However, homonuclear shift references such as the l3C signals of tetramethylsilane (TMS), carbon disulfide, benzene, cyclohexane, 1,4-dioxane or the easily localizable mul-tiplet signals of deuterated solvents (Fig. 2.22) are predominantly applied in 13C NMR. 

For 13C shift/structure correlations and for tabulations of ppm values one generally accepted reference should be used. Carbon disulfide, which appears in the low field region of, 3C spectra, was widely used in the early literature [73a, b]. Later, tetramethylsilane (TMS), known from proton NMR, became the generally accepted carbon-13 shift reference, particularly because of some parallels in the behavior of H and 13C shifts. 

Neither CS2 nor TMS are ideal standards. The 13C signals of CS2 and carbonyl carbons overlap, as do the 13C signals of cyclopropane and some methyl carbons with TMS (Fig. 3.3). Furthermore, the 13C resonance of TMS has been shown to suffer from solvent shifts of the order of + 0.1 to 1.5 ppm in common NMR solvents, even at infinite dilution [74]. This must be considered if 13C shifts relative to TMS of one compound in different solvents are to be compared. There are two alternative methods to overcome this problem one is to use cyclohexane as the internal reference cyclohexane was shown to have 13C solvent shifts lower than + 0.5 ppm [74], The other alternative is to use TMS as an external reference (Sections 1.9.3 and 2.8.5) and to make bulk susceptibility shift corrections according to eq. (1.44). 

Since medium effects may be expected for any resonance, the solvent must be mentioned when 13C shift values are tabulated. In this work, all 13C shifts are given relative to TMS. Those shifts which were originally reported relative to other references (e.g. carbon disulfide, 5CS2, or 1,4-dioxane, 5C4hho2) have been converted into shifts relative to TMS (t TMs) using the known shift difference between common reference substances, as 

Compound C-D signal multi- plicity C-D coupling constants Hz 13C shifts Isotopic H compound  

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