Spectroscopy and Photo-induced Chemistry of Atmospherically Significant Criegee Intermediates and Hydroxyl Radicals
Criegee intermediates, carbonyl oxide species important in alkene ozonolysis reactions in the atmosphere, have eluded characterization until very recently. Four prototypical Criegee intermediates, CH2OO, CH3CHOO, (CH3)2COO and CH3CH2CHOO, were generated by photolysis of gem-diiodo compounds and subsequent reaction with oxygen in a quartz capillary tube reactor prior to free jet expansion. The Criegee intermediates were ionized using fixed-frequency vacuum ultraviolet (VUV) radiation and detected in a time-of-flight mass spectrometer. Ultraviolet (UV) excitation of the Criegee intermediates on very strong pi*-pi transitions localized on the COO group induced significant depletion of the ion signals near the peak of broad absorption profiles, indicating rapid excited state dynamics. The UV depletion spectra and corresponding absorption cross sections were evaluated, and utilized with the solar actinic flux to estimate the atmospheric lifetimes of Criegee intermediates with respect to photodissociation. Combining the UV and VUV excitation enabled resonance enhanced multiphoton ionization detection of hydroxyl (OH) radicals via the A2Σ+ state. The intensities of OH A-X (1,0) transitions detected by ionization have been compared with those obtained by laser-induced fluorescence (LIF) to calibrate the new method for state-selective detection of OH radicals. Energized Criegee intermediates produced in alkene ozonolysis are known to be a significant source of OH radicals in the atmosphere. The underlying mechanism for 1,4-hydrogen transfer from alkyl-substituted Criegee intermediates to vinyl hydroperoxide, and subsequent dissociation to OH radicals has been examined in detail. Infrared excitation was used to activate CH3CHOO and (CH3)2COO Criegee intermediates and drive the hydrogen transfer process leading to OH products, which were detected. Infrared excitation in the CH stretch overtone region yielded infrared action spectra of the Criegee intermediates, an upper limit for the effective barrier heights to reaction, and insights on the unimolecular decay dynamics to OH products. Lastly, stabilized vinyl hydroperoxide species have been generated via a carboxylic acid-catalyzed tautomerization of alkyl-substituted Criegee intermediates. Using deuterated formic or acetic acid, three prototypical vinyl hydroperoxides, CH2=CHOOD, CH2=C(CH3)OOD and CH3CH=CHOOD, were detected directly for the first time by means of VUV photoionization mass spectrometry.