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

Author

Meng Huang
Neal Kline
Terry A. Miller
Richard Dawes, Missouri University of Science and TechnologyFollow

Abstract

Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O₂/N₂ mixtures. In particular, photolysis of CH₂ I₂ 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 (ã-X ̃) of CH₂O₂ might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the ã and X ̃ states of CH₂O₂ and the lowest two doublet states of the iodomethylperoxy radical, CH₂IO₂, which also could be produced by the chemistry and whose Ẍ transition likely lies in this spectral region. The conclusion of these calculations is that the ã-X ̃ 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 ã and X ̃ states. Moreover, only a shallow well (with a barrier to dissociation of less than 1900 cm-1) is predicted on the ã state, which likely precludes the existence of long-lived states. Calculations for the Ã-X ̃ transition of CH₂IO₂ 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 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, V₁₂. 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 à state vibrations and the V₁₂ vibration of the X ̃ 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.

Recommended Citation

M. Huang et al., "Studies Via Near-Infrared Cavity Ringdown Spectroscopy and Electronic Structure Calculations of the Products of the Photolysis of Dihalomethane/N₂/O₂ Mixtures," The Journal of Physical Chemistry A, vol. 121, no. 1, pp. 98 - 112, American Chemical Society (ACS), Dec 2017.

The definitive version is available at https://doi.org/10.1021/acs.jpca.6b10632

Department(s)

Chemistry

Research Center/Lab(s)

Center for High Performance Computing Research

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2017 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Dec 2017