Cavity ringdown spectra of theà 2 A −X 2 A electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both theà andX states of CH3O2·. An estimate for the absorption cross-section is determined from the CRDS absorption and the rate of radical-radical recombination.Low temperature oxidation of hydrocarbons is a pervasive process in both nature and technology. It is critically important for the environmental quality of our atmosphere. [1][2][3][4][5] Similarly it is critical to the efficiency and fuel economy of internal combustion engines. 6-8 Arguably the most important reaction 7 for low temperature oxidation is the production of peroxy radicals (RO 2 ·) from alkyl radicals (R·), i.e.,There are a number of important alkyl peroxy radicals defined by the nature of the R group, ranging 1 from the simplest species, methyl (R=CH 3 ) peroxy to complex species with R containing at least 8-10 carbon atoms with correspondingly many structural isomers. Because of their significance, much effort 1 has been expended studying the mechanisms and kinetics of peroxy radical production and their subsequent reactions. This work has largely been based upon monitoring of the peroxy radicals via theirB 2 A −X 2 A UV absorption. This is a strong transition, common to all the peroxies, that is centered around 240nm.Unfortunately this transition is broad and unstructured with a half-width of ≈40nm. The quasi-continuum nature of this transition has at least two clear disadvantages. It is unsuitable for obtaining rotational or vibrational information about the radical. Additionally the overlapping of the UV spectra of different RO 2 · radicals makes the identification of a specific alkyl peroxy radical, particularly from a mixture, a significant challenge.Experiments involving the peroxyà 2 A −X 2 A transition in the IR are sparse. There was an early report, 9 using low resolution modulated absorption spectroscopy, of the observation of theÃ−X IR transition of several RO 2 · radicals and more recently a report 10 of the observation of fragmentary spectra of ethyl peroxy using a cw absorption technique. There was also a report 11 of the detection of theÃ−X transition of CH 3 O 2 · by intracavity laser absorption spectroscopy, but few spectroscopic details were given. Based upon the recent observation 12 and analysis 13 of a well resolved spectrum of the hydroperoxy radical, HO 2 ·, we expect thẽ A −X IR transitions of the alkyl peroxy radicals to be well structured and observable using cavity ringdown spectroscopy (CRDS).Historically this IR transition has been viewed as difficult to study because of its small oscillator strength (the cross-section σ is ≈ 2 × 10 −21 cm −2 for the corresponding transition 13 in HO 2 ·) and the near IR spectral region (≈ 7000 − 8000 cm −1 ) in which the transition is located. However CRDS is a powerful technique 14-16 for dealing with these difficulties. T...

Cavity ringdown spectra of the near IR A 2 A − X 2 A electronic transition of acetyl peroxy radical and its perdeutero analogue are reported. The electronic origin for the CH3C(O)O2 is observed at 5582.5(5) cm −1 . Extensive ab initio calculations have been carried out to predict frequencies and help assign the electronic origin, observed vibrational hot bands and severalà state vibrational frequencies.

The conformational distribution and unimolecular decomposition pathways for the n-propylperoxy radical have been generated at the CBS-QB3, B3LYP/6-31+G and mPW1K/6-31+G levels of theory. At each of the theoretical levels, the 298 K Boltzmann distributions and rotational profiles indicate that all five unique rotamers of the n-propylperoxy radical can be expected to be present in significant concentrations at thermal equilibrium. At the CBS-QB3 level, the 298 K distribution of rotamers is predicted to be 28.1, 26.4, 19.6, 14.0, and 11.9% for the gG, tG, gT, gG', and tT conformations, respectively. The CBS-QB3 C-OO bond dissociation energy (DeltaH298 K) for the n-propylperoxy radical has been calculated to be 36.1 kcal/mol. The detailed CBS-QB3 potential energy surface for the unimolecular decomposition of the n-propylperoxy radical indicates that important bimolecular products could be derived from two 1,4-H transfer mechanisms available at T< 500 K, primarily via an activated n-propylperoxy adduct.

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Cavity ringdown spectra of the A-X electronic transition of the 1-propyl and 2-propyl peroxy radicals are reported. Spectroscopic assignments are facilitated by implementing several production mechanisms, either isomer-specific or not. Assignments of specific spectral lines to particular conformers of a given isomer are suggested. Observations on the temporal decay of the various species are reported.