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.
Supporting information
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.