This figure shows the results of an FTIR observation of the reaction HO + C3F6 -> products. The reaction was chosen because it is reasonably fast and because both the reactants and products have high oscillator strengths and are consequently easy to detect with IR absorption. The experiment capitalizes on attributes of our High Pressure Flow System to yield high sensitivity, which in turn allows us to study the reaction under conditions that minimize unwanted secondary reactions.
In the experiment shown here, C3F6 is mixed with N2 carrier well upstream of the reaction zone at a number density of 5.5e13/cc in 10 torr of N2. Given a room temperature rate constant of 2e-12 cc/sec, OH radicals introduced into this mixture are removed in 0.01 sec. We generate OH radicals in a sidearm and inject them into the center of the 10 m/sec carrier flow roughly 50 cm upstream of a 15 cm base path, 44 pass White cell. The White cell is oriented perpendicular to the carrier flow and thus samples a volume with a roughly 0.05 sec reaction time. Therefore, the initial C3F6 + OH reaction shown here has gone to completion well upstream of the White cell. Furthermore, the system contains ample background O2 and the OH radicals are produced in the sidearm via a titration reaction (H + NO2 -> NO + OH), which produces a sufficient quantity of NO to rapidly react with any peroxy radicals generated in the reaction. Combined, these conditions make it probable that the radical products of the initial reaction are oxidized to produce two aldehydes: COF2 and CF3CFO.
Because the experiment is conducted in a flow system, it is in a steady state. We can thus improve our IR sensitivity by conducting long scans. However, in order to reduce the effect of various long term drifts, we chop our signal by modulating the reaction. We do this by modulating the flow of H2, which is the precursor for the H atoms used in the OH source. The H2 is turned on and off roughly every 10 minutes. After waiting a short while (~ 1 minute) for conditions to stabilize, we collect FTIR interferograms continuously (at 1/cm resolution in this case) and coadd them for the duration of the cycle. The resulting broad band spectrum from one period with the OH source turned on is divided by the average of the two spectra immediately preceeding and following it for which the OH source was turned off. The resulting transmittance spectra are then averaged for the duration of the experiment, giving the red curve in the figure (shown as optical depth, or -ln(transmittance)). Because the only change to the system is this modulation of the radical precursor (whose flow rate is a tiny fraction of the total flow rate in the system), the only changes we expect to see in the spectra are from either the changing source chemistry or the reaction under study. In particular:
In the figure we show the raw data (ln (OH off)/(OH on)) in red. Note the small changes in optical depth of no more than 0.001, with a noise level of roughly 2e-5. This experiment lasted for roughly 5 hours. Negative signal (lowered optical depth with the H atoms on) corresponds to species consumed when the OH source is on, while positive signal corresponds to species produced when the OH source is off. We have analyzed the spectrum with a multi component self consistent correlation technique, which we will not describe here. The various labeled and colored spectra show the results of this analyis. They are scaled to match their calculated contribution to the observed spectrum. Compounds removed when the OH source is turned on are offset below the raw data, while those produced are offset above the raw data. Calculated concentrations and errors are shown for important species.