Data preparation
Prior to modeling the spectral region between 1900 – 2400
cm-1 was deleted to remove contributions from
CO2. Timepoints during the induction period were also
removed from the data prior to analysis. The exact timepoints removed
for this purpose were determined by plotting the absorbance at 970
cm-1 as a function of time. A linear fit with slope
zero was applied to the initial data before autoxidation, and a second
linear fit was applied to the linear region of increasing peak
absorbance. The point at which these fits intersect was defined as t = 0
in the model. The first datapoint to fall on the linear region was the
first timepoint included in the model. A visual demonstration of this
procedure is provided in Fig. S1. Inclusion or exclusion of additional
timepoints were found to slightly decrease the overall quality of fit
found through global analysis but did not have a significant impact on
the species associated spectra (SAS) or rate constants (Fig. S2). Except
where otherwise indicated, all spectra shown have been generated from
data processed using this procedure.
For single wavelength modeling, the decrease in peak area at 3010
cm-1 for each set of data was found by determining the
peak area at each time point using OMNIC TQ Analyst software. Once each
area was found, the values were normalized so that the area at time 0
was equal to 1. The normalized areas were plotted vs. time and the
linear portion of the curve, indicating a steady decrease in area, was
identified. The slope of this region was found and determined to be the
value of k3010 for each experiment, corresponding to the
decrease in isolated cis double bonds.