Neal D. Kline

Ã-X absorption of propargyl peroxy radical (H-C≡C-CH2OO·): a cavity ring-down spectroscopic and computational study.

The Ã-X electronic absorption spectrum of propargyl peroxy radical has been recorded at room temperature by cavity ring-down spectroscopy. Electronic structure calculations predict two isomeric forms, acetylenic and allenic, with two stable conformers for each. The acetylenic trans conformer, with a band origin at 7631.8 ± 0.1 cm(-1), is definitively assigned on the basis of ab initio calculations and rotational simulations, and possible assignments for the acetylenic gauche and allenic trans forms are given. A fourth form, allenic cis, is not observed. Simulations based on calculated torsional potentials predict that the allenic trans form will have a long, poorly resolved progression in the OOCC torsional vibration, consistent with experimental observations.

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 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 Ã-X̃ 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 Ã-X̃ 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.

Detection and Characterization of Reactive Chemical Intermediates Using Cavity Ringdown Spectroscopy

Cavity ringdown spectroscopy is a powerful technique for detecting reactive chemical intermediates in a variety of circumstances. The characterization of the ethyl peroxy radical in a variety of ways using different ringdown techniques is used as an example to illustrate the diverse capabilities. Several results are discussed including the room temperature, moderate resolution \(\widetilde{A}\)–\(\widetilde{X}\) spectrum and the jet-cooled, rotationally resolved \(\widetilde{A}\)–\(\widetilde{X}\) spectrum of ethyl peroxy. The concept of dual wavelength cavity ringdown spectroscopy is explored and its utility is demonstrated by a measurement of the \(\widetilde{A}\)–\(\widetilde{X}\) absorption cross section of ethyl peroxy. The self-reaction kinetics of ethyl peroxy are also studied by means of cavity ringdown spectroscopy with a continuous source. The capability of CRDS to measure dynamical effects is illustrated by work on a closely related radical, hydroxy ethyl peroxy radical.