Spectroscopy of the Simplest Criegee Intermediate CH 2 OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra (original) (raw)

2012, Chemistry - A European Journal

These are not the final page numbers! ÞÞ &1& FULL PAPER tants, and influences the formation of aerosols. Therefore, it has a critical impact on air quality and climate. The OH radical plays a pivotal role in contributing to the oxidising capacity, being responsible for initiating oxidation of most volatile organic compounds. It is mainly formed in the troposphere during the day by photolysis of O 3 to give O( 1 D) followed by reaction of O( 1 D) with water. However, this OH generation mechanism depends on the presence of sunlight and water vapour. Recent field measurements have shown that OH levels in urban environments in winter and summer are often very similar. In winter the efficiency of ozone photolysis drops dramatically (by 50 % or more) and it is believed that winter production of OH via ozonolysis of alkenes makes up for this difference. In UK-based field mea-A C H T U N G T R E N N U N G surements, which were combined with experimentally constrained modelling studies, it has been shown that ozonealkene reactions dominate night-time OH production, with some contribution from NO 3 chemistry. In addition, these model studies have indicated that the OH generated by ozone-alkene reactions is dominated by Criegee decomposition chemistry. Also, it has been shown that reactions of Criegee intermediates with carbonyl containing molecules may be important for secondary aerosol formation. However, until direct measurements are made, the exact mechanism and products of a reaction of a Criegee intermediate with a reactant of atmospheric importance will not be known and will be open to speculation. Recently, however, the first kinetics study of a Criegee intermediate in the gasphase by direct measurement has been made. Welz and coworkers used the PIMS method to monitor CH 2 OO and carry out direct kinetics measurements of its reactions with SO 2 , NO 2 , NO and H 2 O. In that work, stabilised CH 2 OO was prepared as a product of the reaction of CH 2 I with O 2 , in higher yields than obtained in the earlier work where CH 2 OO was produced by the chlorine atom initiated oxidation of DMSO, that is, the H-abstraction reaction Cl + CH 3 SOCH 3 and then the reaction CH 3 SOCH 2 + O 2 . This study was followed by measurement of the room temperature rate coefficient of CH 2 OO with acetone, acetaldehyde and hexafluoroacetone (HFA), with direct kinetic mea-A C H T U N G T R E N N U N G surements of the Criegee intermediate being carried out using PIMS. For the reaction of CH 2 OO with acetaldehyde, no secondary ozonide (SOZ) was observed but acetic acid was identified as a reaction product. In contrast, for the acetone and HFA reactions, SOZs and products of ozonide isomerization were observed. For both CH 2 OO and the SOZs, obtained on reaction of CH 2 OO with acetone and PFA, the experimental photoionization curves agree with those computed using electronic structure calculations combined with Franck-Condon factor calculations. These calculations involved computing the minimum energy geometries of the ground states of the molecule and its cation, as well as their harmonic vibrational frequencies. The vibrational envelope of the first photoelectron (PE) band was then computed and used to derive the photoionization curve by integrating the vibrational envelope. Unfortunately, the experimental photoionization curves for CH 2 OO and the SOZs showed no www.chemeurj.org