Detection and Characterization of Products from Photodissociation of XCH₂CH₂ONO (X = F, Cl, Br, OH)

Alkyl nitrites have been used previously to produce alkoxy radicals, which are important intermediates in the oxidation of alkanes in atmospheric and combustion processes. Substituted alkoxy radicals, particulary hydroxyalkoxy radicals, are also important intermediates in the atmospheric oxidation of alkenes and combustion of alcohols. In order to produce substituted alkoxy radicals we have photolyzed at 351 nm substituted alkyl nitrites, XCH₂CH₂ONO (X = F, Cl, Br, OH). Using laser-induced fluorescence only in the case of X = F do we observe the spectrum of substituted alkoxy radical, XCH₂CH₂O; but we always observe the electronic transitions of formaldehyde, HCHO, and vinoxy radical, CH₂CHO. HCHO can be formed by the dissociation of XCH₂CH₂O in its ground state as the barrier to C–C bond dissociation is less than the photon energy remaining after O–NO bond breakage. However, the barrier along the reaction path directly leading from XCH₂CH₂O to CH₂CHO + HX is much higher than the available energy remaining after O–NO bond breakage. A roaming mechanism, involving a frustrated dissociation of X followed by HX extraction, might explain the apparent paradox. Under the conditions of our observations vinoxy retains considerable vibrational excitation but the observed rotational temperatures of both HCHO and CH₂CHO are ≲7 K

Observation of the Ã−X̃ Electronic Transition of the β-Hydroxyethylperoxy Radical

The Ã−X̃ electronic absorption spectrum of β-hydroxyethyl peroxy radical (β-HEP) has been recorded in the NIR by cavity ringdown spectroscopy. The precursor, β-hydroxyethyl radical, is generated by photolysis of 2-iodoethanol and by OH-initiated oxidation of ethene, in both cases followed by addition of O₂ to form the peroxy. Although electronic structure calculations predict that 13 conformers of β-HEP exist as minima on the potential energy surface, the experimental spectrum is rationalized in terms of the band origins and vibrational progressions of only the two most stable conformers, G₁G₂G₃ and G₁^′G₂G₃

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH₂CH₂O Radical from BrCH₂CH₂ONO Photolysis at 351 nm

We report a study of the unimolecular dissociation of BrCH₂CH₂O radicals produced from the photodissociation of BrCH₂CH₂ONO at 351/355 nm. Using both a crossed laser-molecular beam scattering apparatus with electron bombardment detection and a velocity map imaging apparatus with tunable VUV photoionization detection, we investigate the initial photodissociation channels of the BrCH₂CH₂ONO precursor and the subsequent dissociation of the vibrationally excited BrCH₂CH₂O radicals. The only photodissociation channel of the precursor we detected upon photodissociation at 351 nm was O–NO bond fission. C–Br photofission and HBr photoelimination do not compete significantly with O–NO photofission at this excitation wavelength. The measured O–NO photofission recoil kinetic energy distribution peaks near 14 kcal/mol and extends from 5 to 24 kcal/mol. There is also a small signal from lower kinetic energy NO product (it would be 6% of the total if it were also from O–NO photofission). We use the O–NO photofission <i>P</i>(<i>E</i>_T) peaking near 14 kcal/mol to help characterize the internal energy distribution in the nascent ground electronic state BrCH₂CH₂O radicals. At 351 nm, some but not all of the BrCH₂CH₂O radicals are formed with enough internal energy to unimolecularly dissociate to CH₂Br + H₂CO. Although the signal at <i>m</i>/<i>e</i> = 93 (CH₂Br⁺) obtained with electron bombardment detection includes signal both from the CH₂Br product and from dissociative ionization of the energetically stable BrCH₂CH₂O radicals, we were able to isolate the signal from CH₂Br product alone using tunable VUV photoionization detection at 8.78 eV. We also sought to investigate the source of vinoxy radicals detected in spectroscopic experiments by Miller and co-workers (J. Phys. Chem. A 2012, 116, 12032) from the photodissociation of BrCH₂CH₂ONO at 351 nm. Using velocity map imaging and photodissociating the precursor at 355 nm, we detected a tiny signal at <i>m</i>/<i>e</i> = 43 and a larger signal at <i>m</i>/<i>e</i> = 15 that we tentatively assign to vinoxy. An underlying signal in the time-of-flight spectra at <i>m</i>/<i>e</i> = 29 and <i>m</i>/<i>e</i> = 42, the two strongest peaks in the literature electron bombardment mass spectrum of vinoxy, is also apparent. Comparison of those signal strengths with the signal at HBr⁺, however, shows that the vinoxy product does not have HBr as a cofragment, so the prior suggestion by Miller and co-workers that the vinoxy might result from a roaming mechanism is contraindicated

Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH₃)₂COO

The Criegee intermediate acetone oxide, (CH₃)₂COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O₂ and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO₂ was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10^–11 cm³ s^–1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10^–10 cm³ s^–1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of <i>anti</i>-CH₃CHOO with SO₂, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N₂ from cavity ring-down decay of the ultraviolet absorption of (CH₃)₂COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10^–10 to (2.29 ± 0.08) × 10^–10 cm³ s^–1. Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, <i>k</i>_H/<i>k</i>_D = (0.53 ± 0.06), for reactions with SO₂, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD₃)₂COO with NO₂ has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10^–12 cm³ s^–1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of <i>anti</i>-CH₃CHOO with NO₂. Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH₃)₂COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s^–1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH₃)₂COO with SO₂ and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH₃)₂COO with NO₂ and with SO₂ suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products