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

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• 作者：Meng Huang ; Neal Kline ; Terry A. MillerRichard Dawes
• 刊名：Journal of Physical Chemistry A
• 出版年：2017
• 出版时间：January 12, 2017
• 年：2017
• 卷：121
• 期：1
• 页码：98-112
• 全文大小：640K
• ISSN：1520-5215

文摘

Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O₂/N₂ 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 (ã–X̃) of CH₂O₂ might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the ã and X̃ states of CH₂O₂ and the lowest two doublet states of the iodomethylperoxy radical, CH₂IO₂, which also could be produced by the chemistry and whose ÖX̃ transition likely lies in this spectral region. The conclusion of these calculations is that the ã–X̃ 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 X̃ 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 ÖX̃ 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, ν₁₂. 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 ν₁₂ vibration of the X̃ 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.