Experimental Methods

Kinetics of Chlorine-Substituted Peroxy Radicals

3.2 Experimental Methods

82

spectroscopy.¹⁹ Since the ClRO2 radicals have different masses than the corresponding HORO2 (or other secondary RO2), the measured decays arose solely from the ClRO2 + NO reaction. However, since all of the ClRO2 isomers formed from the Cl-initiated oxidation of a VOC precursor have the same mass, no differences in chemical structure could be assessed with their technique. The measured rate constant was effectively an average across all ClRO2 isomers produced, weighted by the yields of each isomer and their sensitivities for detection by CIMS.

In the third part of this chapter, we present measurements of ClRO2 + NO rate constants across a range of ClRO2 structures derived from the chlorine-initiated oxidation of ethene (𝑘₇), propene, 1-butene, 2-butene, 1,3-butadiene, and isoprene. For the precursors with more than two carbons atoms, various ClRO2 isomers form upon oxidation.

We detected ClRO2 in the near-IR by probing their forbidden 𝐴̃ ← 𝑋̃ transition. Unlike the 𝐵̃ state, the 𝐴̃ state of RO2 radicals is bound, leading to detailed absorption spectra that vary with chemical structure.²⁰ This transition therefore has the potential to differentiate RO2 isomers in a complex oxidation system as well as discriminate first generation RO2

from later generation RO2 formed by radical recycling. We evaluate the practicalities of such measurements in this work. Furthermore, it is commonly assumed that RO2 + NO reactions proceed with approximately the same rate constant (𝑘 ≈ 9 x 10^-12 cm³ molc^-1 s^-1), with little sensitivity to structure. Possible evidence for a difference in rate constant between ClRO2 isomers derived from the same VOC precursor is discussed.

A schematic summary of the oxidations and peroxy radical reactions studied in this work is presented in Figure 1. For β-CEP, we have measured 𝑘₂, 𝑘₃, 𝑘₄, α, and 𝑘₇. For larger alkene precursors, we have measured ClRO2 + NO rate constants.

83 ringdown spectroscopy (CRDS) is a popular approach in which the probe light beam becomes resonant with an optical cavity formed by two highly reflective mirrors to generate long effective pathlengths.²² With each pass, the intracavity power drops as a small fraction of photons are lost at the mirrors or absorbed/scattered by the analyte gas.

A detector positioned behind one of the mirrors records the exponential decay of light intensity leaking out of the cavity (a “ringdown”). By comparing the lifetime of photons within the resonator both with and without the analyte gas present, the absorption can be deduced.

The near-IR spectrometer used in this work for pulsed CRDS absorption measurements has been described previously.^23,24 A schematic of the instrument is presented in Figure 2. Tunable visible radiation from a Nd:YAG (Continuum NY61) pumped dye laser (Lambda Physik FL3002) was focused into a 2 m long stainless steel Herriott cell filled with ~10 atm of hydrogen.²⁵ After nine passes, the stimulated Raman scattering was filtered for the second sequential Stokes shift in the near-IR spectral region.

The beam was injected into a 50 cm long optical cavity formed by two highly reflective mirrors (𝑅 ~ 99.99%). Transmitted radiation was focused onto an InGaAs photodetector (ThorLabs PDA10CS, 17 MHz bandwidth) and the ringdown signals were digitized by a 14-bit transient digitizer (GaGe CompuScope 4327). By using multiple ringdown mirrors and laser dyes, the spectrometer is capable of performing pulsed near-IR CRDS over 7000–

9000 cm^-1 with a 0.2 cm^-1 linewidth (set by the dye laser) and a ~1 mJ pulse energy. The typical noise-equivalent absorption (NEA) coefficient for this system is ~1.5 x 10^-8 cm^-1 Hz^-1/2. In practical terms, assuming an absorption pathlength of 10 cm, a cross section of σ ≈ 10^-21–10^-20 cm², and 𝑁 = 100 averages (5 Hz), this corresponds to a minimum detectable concentration of [RO2]min ~ 10¹¹–10¹² molc cm^-3.

An excimer laser (Lambda Physik LPF220, 5 Hz, 351 nm) initiated radical chemistry through photolysis of Cl2. The UV beam was expanded (~6 mJ cm^-2 fluence in the reactor) and propagated orthogonal to the axis of the optical cavity through two quartz windows mounted on the sides of the ringdown cell. A digital delay generator synchronized the firing of the Nd:YAG and excimer laser pulses and established a time axis for kinetic measurements. Ringdowns were acquired both with and without excimer laser photolysis to elucidate the absorption induced by radical chemistry.

84

Reactant gases were flown through the ringdown cell in the region enclosed by the photolysis windows. The ringdown mirrors were protected by a purge of N2 set at 10% of the total flow. The pressure was set to 300 Torr and experiments were conducted at room temperature (T ≈ 296 K). Specific concentrations in the reactor were varied to optimize conditions for measurement of different rate constants. Typically, our radical precursor was set to [Cl2] ≈ 5 x 10¹⁶ molc cm^-3 and yielded [Cl]0 ≈ 2 x 10¹⁴ atoms cm^-3 upon photolysis (~0.2%). The VOC precursor was set to great excess at ~10¹⁶–10¹⁷ molc cm^-3 to ensure that the Cl + VOC oxidation step was never rate-limiting to the measured ClRO2 kinetics.

Ethene, propene, 1-butene, 2-butene, and 1,3-butadiene were diluted from pure tanks into custom tanks of ~5–20% in N2. The vapor from liquid methanol and isoprene was used to prepare tanks of these species at ~5% in N2. For experiments measuring 𝑘₂, the [O2] was kept low and varied around ~10¹⁶ molc cm^-3 (< 1%) to ensure the pseudo first order association rates were slow enough to measure (the bath gas was almost entirely N2). For experiments measuring ClRO2 + NO rate constants, O2 was raised to ~5% to ensure that formation of ClRO2 was effectively instantaneous relative to its decay (~95% N2). [NO]

was kept at least an order of magnitude in excess of [Cl]0 and varied within ~10¹⁵–10¹⁶ molc cm^-3. For experiments measuring 𝑘₃, 𝑘₄, and α, O2 was raised to ~75% to minimize the impact of secondary chemistry that will be discussed in a later section (~25% N2). The total flow rate was maintained at ~4250 sccm to set the residence time in the flow cell at

~160 ms and ensure complete refreshment of the gases between adjacent photolysis pulses.

3.2.2 Calibration of the Initial Radical Concentration

To calibrate [Cl]0, we conducted experiments oxidizing methanol:

Cl + CH3OH → HCl + CH2OH (R9)

CH2OH + O2 → HCHO + HO2 (R10)

The conditions of these experiments were such that each Cl atom yielded one HO2 radical.

Loss of nascent HO2 was exclusively through its self reaction to form H2O2 (R6). Since the rate constant 𝑘₆ is well-known,²⁶ the bimolecular decay constant of HO2 absorption can be used to determine the initial HO2 concentration and [Cl]0.

The absorption of HO2 was monitored near the peak of its 𝐴̃ ← 𝑋̃ origin Q-branch

-127

85 arising from a vibrational overtone band.²⁸ Spectra of HO2 and H2O2 measured in this work are presented in Figure 3. The spectra measured with a 10 µs delay between the probe and photolysis pulses was recorded before the self reaction has occurred to any appreciable extent, and thus represents solely HO2. The spectrum measured with a 35 ms delay was recorded after many half-lives of the self reaction, and represents solely H2O2.

Since the rate of HO2 decay is linked to the rate of H2O2 production, integration of their rate laws shows that the absorption at 7041.64 cm^-1 can be modeled according to:

𝐴(𝑡) = 𝐴₀− 𝐴_∞

1 + 2𝑘₆[HO₂]₀𝑡+ 𝐴_∞ (Eqn. 1) The measured time-resolved absorption, collected over 0–10 ms with 0.1 ms steps, undergoes a three-parameter fit to Eqn. 1 to determine the initial HO2 absorption before the self reaction (𝐴₀), the H2O2 absorption after completion of the self reaction (𝐴_∞), and the bimolecular decay constant of HO2 (𝑘₆[HO₂]₀). Dividing the last parameter by the known value of 𝑘₆ gives [HO₂]₀, which we assume to be equal to [Cl]0. An example absorption signal and fit yielding [Cl]0 = 1.66 x 10¹⁴ molc cm^-3 is shown in Figure 4.

Each day, we performed calibrations of the [Cl]0 yielded from Cl2 photolysis. In the event that [Cl2] was changed from the concentration used in the calibration scan, we assumed that the Cl2 photolysis fraction was constant to determine [Cl]0 in the other scan.

The calibration was generally performed at the beginning and end of experiments to ensure that [Cl]0 had not drifted to any significant extent.