Alkylperoxy Radicals (RO2)
Under high-NOX conditions, the dominant fate of the RO2 radical is reaction with NO. While the main products of this reaction are known to be alkoxy radicals and NO2, some fraction may form alkyl nitrates, RONO2. Studies indicate that nitrate formation is a minor channel for small R groups, but branching can reach as high as 50% for larger species [Lightfoot et al., 1992]. The branching ratio is crucial to the efficiency of the tropospheric HOX cycle, as nitrate formation is a radical-termination step. Thus tropospheric ozone production depends critically on the importance of this channel.
RO2 + NO → [ROONO]* → RO + NO2
+ M → RONO2 +M
The mechanism is believed to proceed through an excited ROONO intermediate, which then rearranges to form RONO2 [Donahue et al., 2001b]. Thus, the yield of nitrate is expected to be a strong function of temperature, pressure, and the size of the R group, though data over a wide range of pressure and temperature remain scarce. Furthermore, despite multiple observations of the RONO2end product, the ROONO intermediate has never been observed.
By use of IR CRDS for RO2 detection and high-resolution FTIR for the detection of alkyl nitrates, we propose to study the reaction kinetics and products, in order to better understand branching ratios and reaction mechanism. Whereas the many product studies examine the behavior of RO2 during hydrocarbon oxidation, we propose to prepare RO2 directly (by alkyl halide photolysis in the presence of O2), simplifying available reaction pathways significantly. Detection of RO2 (at ~1500 cm-1 [Fink and Ramsay, 1997]) with CRDS will allow for direct measurement of the kinetics of these reactions. Spectra of RO2 radicals will be characterized beforehand in our jet, and interferences by RO2 self-reaction will be minimized by keeping NO in great excess (as is typical for our kinetics studies).
In addition, we will use high-resolution FTIR to measure nitrate yields as a function of pressure, temperature, and reaction time, using reaction modulation spectroscopy [Donahue et al., 1996b]. Ideally, at low temperatures and high pressures the ROONO intermediate may be detected; again, prior identification in the jet expansion with CRDS is essential. Coupled with RRKM/master equation modeling of the reaction dynamics, the temperature- and pressure-dependences of the rate should afford excellent insight into the mechanism of nitrate formation.
Study of the RO + NO2 reaction may also improve our understanding of the RO2 + NO reaction mechanism. RO + NO2 is analogous to OH + NO2, though because of the very short lifetime of alkoxy radicals, it is of minor atmospheric importance. However, the reaction occurs on the same potential energy surface as RO2 + NO; because reactants are at a somewhat lower energy (~ 7 kcal/mol), stabilization of ROONO may be more important and ROONO detection easier. Pressure- and temperature-dependent rate measurements and ROONO yields may help constrain the RO2 + NO reaction mechanism significantly.