Studies via Near-Infrared Cavity Ringdown Spectroscopy and Electronic Structure Calculations of the Products of the Photolysis of Dihalomethane/N₂/O₂Mixtures

Abstract

Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O/N mixtures. In particular, photolysis of CHI under conditions previously reported to produce the simplest Criegee intermediate, CHO, gave a complex, structured spectrum between 6800 and 9000 cm, where the lowest triplet-singlet transition (ã-X̃) of CHO might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the ã and X̃ states of CHO and the lowest two doublet states of the iodomethylperoxy radical, CHIO, which also could be produced by the chemistry and whose Ã-X̃ transition likely lies in this spectral region. The conclusion of these calculations is that the ã-X̃ transition of CHO clearly falls outside the observed spectral range and would be extremely weak both because it is spin-forbidden and because of a large geometric change between the ã and X̃ states. Moreover, only a shallow well (with a barrier to dissociation of less than 1900 cm) is predicted on the ã state, which likely precludes the existence of long-lived states. Calculations for the Ã-X̃ transition of CHIO are generally consistent with the observed spectrum in terms of both the electronic origin and vibrational frequencies in the à state. To confirm the carrier assignment to CHIO, calculations beyond the Franck-Condon approximation were carried out to explain the hot band structure of the large-amplitude, low-frequency O-O-C-I torsion mode, ν. Photolysis of other dihalomethanes produced similar spectra which were analyzed and assigned to CHClO and CHBrO. Experimental values for the electronic energies and frequencies for several à state vibrations and the ν vibration of the X̃ state of each are reported. In addition, the observed spectra were used to follow the self-reaction of the CHIO species and its reaction with SO. The rates of these reactions are dramatically faster than those of unsubstituted alkyl peroxy radicals and approach those of the Criegee intermediate.

Products of Criegee intermediate reactions with NO2: experimental measurements and tropospheric implications

“…3,4,7,10 If the adduct signal were to persist in the presence of NO2 and the scrubber, this would demonstrate that the adduct formation is due to side chemistry, such as R2, and not the reaction of interest. Recent work by Huang et al 51 demonstrated that the reaction of CH2IOO with SO2 is rapid, but a factor of 10 slower than CH2OO + SO2. Addition of SO2 therefore will remove both Criegee intermediates and RCR'IOO, but at sufficiently different rates so that the source of the adduct signals can be determined.…”