Article

Direct observation of the gas-phase Criegee intermediate (CH2OO).

Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, Livermore, California 94551-0969, USA.
Journal of the American Chemical Society (Impact Factor: 10.68). 10/2008; 130(36):11883-5. DOI:10.1021/ja804165q
Source: PubMed

ABSTRACT Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called "Criegee intermediates," in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species.

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• Article: The reaction of Criegee intermediates with NO, RO(2), and SO(2), and their fate in the atmosphere.
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ABSTRACT: The reaction of Criegee intermediates (CI) with NO and RO(2) radicals is studied for the first time by theoretical methodologies; additionally, the reaction of CI with SO(2) molecules is re-examined. The reaction of CI with NO was found to be slow, with a distinct energy barrier. Their reaction with RO(2) radicals proceeds by the formation of a pre-reactive complex followed by addition of the RO(2) radical on the CI carbon over a submerged barrier, leading to a larger peroxy radical and opening the possibility for oligomer formation in agreement with experiment. The impact of singlet biradicals on the reaction of CI with SO(2) is examined, finding a different reaction mechanism compared to earlier work. For larger CI, the reaction with SO(2) at atmospheric pressures mainly yields thermalized sulfur-bearing secondary ozonides. The fate of the CI in the atmosphere is examined in detail, based on observed concentration of a multitude of coreactants in the atmosphere, and estimated rate coefficients available from literature data. The impact of SCI on tropospheric chemistry is discussed.
Physical Chemistry Chemical Physics 10/2012; 14(42):14682-95. · 3.83 Impact Factor
• Article: Direct measurement of Criegee intermediate (CH2OO) reactions with acetone, acetaldehyde, and hexafluoroacetone.
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ABSTRACT: Criegee biradicals, i.e., carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF(3)COCF(3)), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH(2)OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH(2)I radicals, which react with O(2) to yield CH(2)OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH(2)OO and the formation of reaction products are monitored. The rate coefficient for CH(2)OO + hexafluoroacetone is k(1) = (3.0 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1), supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k(2) = (9.5 ± 0.7) × 10(-13) cm(3) molecule(-1) s(-1), and with acetone, k(3) = (2.3 ± 0.3) × 10(-13) cm(3) molecule(-1) s(-1), are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH(2)OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck-Condon-factor calculations. No secondary ozonide was observable in the reaction of CH(2)OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).
Physical Chemistry Chemical Physics 04/2012; 14(30):10391-400. · 3.83 Impact Factor
• Article: Spectroscopy of the Simplest Criegee Intermediate CH(2) OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra.
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ABSTRACT: CH(2) OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas-phase studies on CH(2) OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH(2) OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH(2) OO. In order to encourage more direct studies on CH(2) OO and other Criegee intermediates, the electronic and photoelectron spectra of CH(2) OO have been simulated using high level electronic structure calculations and Franck-Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH(2) OO were calculated with TDDFT, EOM-CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low-lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH(2) OO corresponds to the ${{\rm{\tilde B}}}$(1) A' ← ${{\rm{\tilde X}}}$(1) A' absorption. It is a broad band in the region 250-450 nm showing extensive structure in vibrational modes involving O-O stretching and C-O-O bending. Evidence is presented to show that the electronic absorption spectrum of CH(2) OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH(2) OO was prepared in this earlier work from the reaction of CH(2) I with O(2) and that the assignment of the observed spectrum solely to CH(2) IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH(2) OO consists of two overlapping vibrationally structured bands corresponding to one-electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.
Chemistry 08/2012; 18(39):12411-23. · 5.93 Impact Factor