Experimental
Four model molecules: methyl oleate, trans-7-tetradecene, oleic acid and
stearic acid were purchased from Fluka Aldrich. Each substance was
studied pure (dissolved in CDCl3) and after oxidation at
210°C for 3 hours in containers open to air with stirring as previously
.
NMR samples were prepared by dissolving 60 mg of substance in 60 mg of
CDCl3 and contained in 5 mm tubes. NMR spectra were
collected with a 12 Tesla Varian-500 spectrometer. Quantitative13C observation at 125 MHz was made with 300 ppm
spectral window, 30° pulse (3.2 us), inverse-gated 1H
decoupling during 0.7 s acquisition time, 12 s relaxation delay and 128
to 2 k repetitions. 1H observation at 500 MHz was made
with 20 ppm spectral window, 40° pulse (6.0 μs), acquisition time of 0.8
s, relaxation delay of 10 s and 12 repetitions. Chemical shifts are
referenced to TMS (tetramethylsilane) at 0 ppm using the solvent signal
as secondary reference: 1H (CHCl3 at
7.25 ppm) 13C (CDCl3 triplet centered
at 77.0 ppm).
A standard composed of H2O-D2O (1:1
vol), with known D (2.1 x 10-9 m2/s
at 25°C ) was used to calibrate the gradient strength and confirm
reproducible measurements. Gcal value = 0.00190 G/cm/DAC. Our D values
are consistent with others (Table S1 ). DOSY parameters are
reported in figure captions. Big delta (observation time) was usually
fixed at 140 ms. Small delta (pulsed gradient time) was also fixed but
choosing it (from 2 to 30 ms) to realize a full change in echo intensity
given the arrayed gradient strengths (G) incremented as detailed inSupporting Information. “Doneshot” was used in all cases
except for thermally treated stearic acid studied at 50°C. Convection
correction was applied in this case using Varian’s “Dbppste-cc” pulse
sequence.
Since D is concentration dependent, we reason in terms of relative
values (large changes) in D. Pure model molecules, dissolved in
CDCl3, exhibited single D values. Oxidized oil molecules
did not always exhibit mono-exponential behavior in chloroform, due to
inter-molecular attractive interactions. This also slowed the diffusion
of unreacted molecules in oxidized solutions.
Thermal oxidation of model molecules to reveal vegetable oil
polymerization studied by NMR spectroscopy and self-diffusion
M. Barani, R. Bonetti, W. Parker
Physical Chemistry Department, Eni, San Donato (MI), Italy
Table S 1: Self-diffusion
coefficients (D) of untreated model molecules (at 25°C, except stearic
acid) determined by pulsed-field gradient and DOSY NMR spectroscopy.