Spectroscopic Investigation of Atmospherically Significant Criegee Intermediates and Hydroxyl Radicals

Alkene ozonolysis generates Criegee intermediates (carbonyl oxides) that decay to several products including hydroxyl (OH) radicals, the primary oxidant of trace atmospheric species. In the laboratory, an alternative method allows for efficient generation of stabilized Criegee intermediates upon photolysis of synthesized diiodo alkane precursors and reaction with O2. Following supersonic expansion, the Criegee intermediates are photoionized with fixed vacuum ultraviolet radiation (VUV, 118 nm) and detected on their parent mass channels in a time-of-flight mass spectrometer (TOF-MS). Significant ultraviolet (UV) induced depletions of (CH3)2COO and CH3CH 2CHOO photoionization signals are attributed to strong characteristic π*←π transitions localized on the carbonyl oxide group. Using a similar approach, synthesis of a suitable precursor has enabled production of a vinyl-substituted Criegee intermediate, methylvinylketone oxide (MVK-OO), which would be formed upon isoprene ozonolysis in the atmosphere. MVK-OO is detected by VUV photoionization on the parent mass channel. OH radical products are cooled to X2Π 3/2 (v = 0) in the expansion and detected concurrently using a resonance-enhanced UV+VUV ionization scheme via the OH A2Σ+ (v = 1) state. Another study used tunable VUV radiation generated by four-wave mixing to examine the origin of the enhanced ionization efficiency observed for OH radicals prepared in specific A2Σ+ intermediate levels. The enhancement is shown to arise from resonant excitation to distinct rotational and fine structure levels of two newly identified 2Π Rydberg states with an A3Π cationic core and a 3d electron, followed by ionization. Finally, synthesis of a partially deuterated CD3CHI2 precursor is used to produce selectively deuterated syn-CD3CHOO Criegee intermediates in an analogous manner. Vibrational activation of syn-CD3CHOO is shown to result in deuterium atom transfer and release of OD radical products. Direct time-resolved measurement of the rate of appearance of OD products reveals a 10-fold slower rate for unimolecular decay of syn-CD3CHOO in the vicinity of the transition state barrier compared to syn-CH3CHOO. The equivalent kinetic isotope effect of ca. 10 is attributed primarily to the decreased probability of D-atom vs. H-atom transfer arising from tunneling