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.