Oligomers and acid fragments
The major oxidation product of oleic acid gives a methine carbon bonded to oxygen with 13C and 1H signals at 74 and 4.9 ppm, respectively. In addition, we know the hydrogen of this methine group is three-bonds away from the carboxylate carbon of the ester group (supplementary information ref ). Thus, the methine bonded to oxygen is not an ether. This information, combined with the size increase found by DOSY, allows the oligomerization mechanism to be hypothesized.
Initially, in theory, oxidation of oleic acid can produce several types of “monomeric” species (M1-M3, Figure 10, neglecting peroxy species). However, NMR shows none of these species (M1-M3) are present in large amounts (Figure 8 ). M1, with its methine (CH-O) carbon giving a 13C signal at 67 ppm, is missing. No epoxide13C signals (57, 59 ppm) for M2 were observed. M3 is not present either since there is no 1H methineCH-O signal at 3.5 ppm (Figure S16 ). Thus, the monomeric oxidization product of oleic acid is reactive and was consumed before NMR observation. Also, hydroperoxy species, which give13C signals near 80 ppm (Figure S17 ), were not observed.
It is important to note that M1 type species (alpha-hydroxy olefin), widely accepted oxidation products in the literature, were not observed. Such chemical groups are easily distinguished using 2D13C-1H HMBC NMR spectroscopy. This inverse detection method reveals “long range” scalar coupling between hydrogens and carbon nuclei 3 bonds away. The olefinic hydrogens in all samples studied here do not exhibit correlations with carbons bonded to oxygen (e.g. Figure S18 ).