Studies via Near-Infrared Cavity Ringdown Spectroscopy and Electronic Structure Calculations of the Products of the Photolysis of Dihalomethane/N2/O2 Mixtures

Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O-2/N-2 mixtures. In particular, photolysis of CH2I2 under conditions previously reported to produce the simplest Criegee intermediate, CH2O2, gave a complex, structured spectrum between 6800 and 9000 cm(-1), where the lowest triplet-singlet transition ((a) over tilde-(X) over tilde) of CH2O2 might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the a and (X) over tilde states of CH2O2 and the lowest two doublet states of the iodomethylperoxy radical, CH2IO2, which also could be produced by the chemistry and whose A X transition likely lies in this spectral region. The conclusion of these calculations is that the (a) over tilde-(X) over tilde transition of CH2O2 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 (a) over tilde and (X) over tilde states. Moreover, only a shallow well (with a barrier to dissociation of less than 1900 cm(-1)) is predicted on the (a) over tilde state, which likely precludes the existence of long-lived states. Calculations for the (A) over tilde-(X) over tilde transition of CH2IO2 are generally consistent with the observed spectrum in terms of both the electronic origin and vibrational frequencies in the (A) over tilde state. To confirm the carrier assignment to CH2IO2, 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, nu(12). Photolysis of other dihalomethanes produced similar spectra which were analyzed and assigned to CH2ClO2 and CH2BrO2. Experimental values for the electronic energies and frequencies for several (A) over tilde state vibrations and the nu(12) vibration of the (X) over tilde state of each are reported. In addition, the observed spectra were used to follow the self-reaction of the CH2IO2 species and its reaction with SO2. The rates of these reactions are dramatically faster than those of unsubstituted alkyl peroxy radicals and approach those of the Criegee intermediate.