Infrared identification of the Criegee intermediates syn- and anti-CH₃CHOO, and their distinct conformation-dependent reactivity.

Hui-Yu Lin,Henryk A Witek,Yoshifumi Nishimura,Joel M Bowman,Yu-Hsuan Huang,Xiaohong Wang,Yuan-Pern Lee

doi:10.1038/ncomms8012

Abstract

The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere. So far, the mid-infrared spectrum of only the simplest Criegee intermediate CH2OO has been reported. Methyl substitution of CH2OO produces two conformers of CH3CHOO and consequently complicates the infrared spectrum. Here we report the transient infrared spectrum of syn- and anti-CH3CHOO, produced from CH3CHI + O2 in a flow reactor, using a step-scan Fourier-transform spectrometer. Guided and supported by high-level full-dimensional quantum calculations, rotational contours of the four observed bands are simulated successfully and provide definitive identification of both conformers. Furthermore, anti-CH3CHOO shows a reactivity greater than syn-CH3CHOO towards NO/NO2; at the later period of reaction, the spectrum can be simulated with only syn-CH3CHOO. Without NO/NO2, anti-CH3CHOO also decays much faster than syn-CH3CHOO. The direct infrared detection of syn- and anti-CH3CHOO should prove useful for field measurements and laboratory investigations of the Criegee mechanism.

Highlights

The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere
Following the reaction scheme of CH2I þ O2-I þ CH2OO initially developed by Welz et al.3, CH2OO has been detected with ultraviolet depletion4, ultraviolet absorption5,6, infrared absorption7 and microwave spectroscopy8,9
We have demonstrated that coupling a step-scan Fouriertransform infrared (FTIR) spectrometer with a multipass absorption cell enables the recording of temporally resolved infrared absorption spectra of the simplest Criegee intermediate, CH2OO; five distinct absorption bands of CH2OO were clearly identified to provide a simple and direct detection method

Summary

Introduction

The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere. The Criegee intermediates are carbonyl oxides that are produced during the reactions of ozone (O3) with unsaturated hydrocarbons; they play a central role in controlling the atmospheric budget of OH, organic acids and secondary organic aerosols, especially under low-light conditions. These Criegee intermediates have eluded detection in the gaseous phase until recently because of their great reactivity and small concentration. Liu et al. employed infrared activation of cold CH3CHOO to produce OH, which was detected with laser-induced fluorescence, and assigned several absorption features in the region 5,600–6,100 cm À 1 to be the CH-overtone and combination bands of syn-CH3CHOO; no bands of anti-CH3CHOO were identified. We have demonstrated that coupling a step-scan Fouriertransform infrared (FTIR) spectrometer with a multipass absorption cell enables the recording of temporally resolved infrared absorption spectra of the simplest Criegee intermediate, CH2OO (ref. 7); five distinct absorption bands of CH2OO were clearly identified to provide a simple and direct detection method

Methods

Results

Conclusion