The 13C-NMR spectra of long-chain fatty saturated acids show six easily recognised signals at about 180.6, 34.2, 24.8, 32.1, 22.8 and 14.1 ppm for the C1 to 3 and ω1 to 3 carbon atoms, respectively. The remaining signals, usually close together in the region 29.3-29.8 ppm, are described as the 'methylene envelope'. Some of these can be identified in single compounds as can be seen in Table 2 below, but generally these are not useful signals.
In an interesting paper concerned with medium- and long-chain saturated acids, Bengsch et al. (1986) placed the chemical shifts for each chain length in decreasing order. For the C14-C18 acids, they gave the sequence shown below. This is often useful in making chemical shift assignments in more complex acids.
|18:0||2 ω-3 12 ω-6 11 10 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1|
|16:0||2 ω-3 - - - - - --11 10 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1|
|14:0||2 ω-3 - - - - - - - - - - - 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1|
Acids and methyl esters give slightly different shifts for carbon atoms close to the acyl group, especially for C1 and C3 with smaller differences for C2 and C4-6. Glycerol esters give two signals for each of the first 6 or 7 carbon atoms, but only those for C1 and C2 are observed in most spectra. These have relative intensities of 2:1 and correspond to the α and β chains, respectively. Carbon atoms in the α-chain have the higher shift for C1 and C4, and the lower shift for other carbon atoms. Although the chemical shifts of the atoms in the α and β chains vary a little from spectrum to spectrum, their differences are fairly constant at values around 0.41, 0.17 and 0.04 ppm for the C1-3 atoms. In saturated chains, the ω1-3 signals do not change between acids and esters or between the α and β chains of glycerol esters. They may be different in unsaturated acids and in short- and medium-chain saturated acids. The characteristic shifts for short- and medium-chains are significant in the spectra of butter fat and of lauric oils (see Table 3 below)
Several authors have given values for the effect that end groups have on the chemical shifts of nearby carbon atoms (Table 1):
|Gunstone et al. (1976)
|Bus et al.||COOH||-||-||-||-||-|
|Johns et al.||COOH||+4.4||-5.0||-0.6||-0.45||-0.25|
|(These figures are changes from the chemical shift of mid-chain CH2 groups in saturated acids: Gunstone et al., 29.80 acids, 29.84 esters; Bus et al., 29.75 for methyl esters; Johns et al., 29.8 for acids).|
Knothe and Nelson (1998) have developed an equation for predicting 13C-NMR shifts for the saturated carbon atoms C18 acids, which also contain other functional groups such as cis and trans double bonds or oxo, hydroxy, acetoxy, and epoxy functions.
Howarth et al. (1995<) have also discussed the shifts of saturated carbon atoms on unsaturated acids and esters.
Table 2. Chemical shifts (ppm) for 14:0 and 18:0 acids, methyl esters, and glycerol esters (Gunstone et al. (1976), acids and methyl esters. Bus et al. (1976), glycerol esters)
|Acids||Methyl esters||Glycerol esters|
|14:0||18:0||14:0||18:0||14:0 α||14:0 β||18:0 α||18:0 β|
Table 3. Chemical shifts of glycerol esters of butyric (4:0), caproic (6:0), and caprylic (8:0) acids (Bus et al., 1976).
|Tributyrin (C4)||Tricaproin (C6)||Tricaprylin (C8)|
- Bengsch, E., Perly, B., Deleuze, C. and Valero, A. A general rule for the assignment of the carbon-13 NMR peaks in fatty acid chains. J. Mag. Res., 68, 1-13 (1986).
- Bus, J., Sies, I. and Lie Ken Jie, M.S.F. 13C-NMR of methyl, methylene, and carbonyl carbon atoms of methyl alkenoates and alkynoates. Chem. Phys. Lipids, 17, 501-518 (1976).
- Gunstone, F.D., Pollard, M.R., Scrimgeour, C.M., Gilman, N.W. and Holland, B.C. 13C Nuclear magnetic resonance studies of acetylenic acids. Chem. Phys. Lipids, 17, 1-13 (1976).
- Howarth, O.W., Samuel, C.J. and Vlahov, G. The σ-inductive effect of C=C and C≡C bonds: predicting NMR shifts of sp2 carbon in non-conjugated polyenoic acids, esters, and glycerides. J. Chem. Soc. Perkin Trans. 2, 2307-2310 (1995).
- Johns, S.R., Leslie, D.R., Willing, R.J. and Bishop, D.G. Relaxation times of carboxylic acids. Austral. J. Chem., 30, 813-822 (1977).
- Knothe, G. and Nelson, T.C. Evaluation of the 13C-NMR signals of saturated carbon atoms in some long-chain compounds. J. Chem. Soc. Perkin Trans. 2, 2019-2026 (1998).