Using simple 13C NMR linewidth and relaxation measurements to make detailed chemical shift assignments in triacylglycerols and related compounds

Two simple experiments measuring the ¹³C linewidths ν_1/2 and spin–lattice relaxation times T₁ of each of the signals in the spectrum of trilinolein indicate that the ν_1/2 and T₁ values are consistent with the different degrees of motional freedom expected for the various ¹³C nuclei. However, for each chain, the ν_1/2 and T₁ measurements indicate a small reversal in mobility at C-10 relative to C-9 before motional freedom again steadily increases on each chain starting at C-11. The T₁ experiment allows unambiguous assignments of the C-8 signal and C-14 signal, which differ by only 0.010 ppm. Measurements of ¹³C ν_1/2 and T₁ values on tripalmitin provide secure assignments for the C-5 and C-6 signals, for which conflicting assignments have been reported. The T₁ measurements also show that among the tightly clustered C-8 through C-12 signals, the C-11 signals are the most downfield, while the C-12 signals are the most upfield, again contrary to a previous report. Similar measurements of ¹³C ν_1/2 and T₁ values on other triacylglycerols or related compounds may prove equally useful in making chemical shift assignments and detecting any discontinuities in motional freedom along a chain. The benefits and possible limitations of ultrahigh field NMR for studying triacylglycerols and related compounds are discussed.