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 ).