Reactive Oxygen Intermediates
Our group is interested in spectroscopic and structural investigations of reactive oxygen species (ROS) using broadband and cavity rotational spectroscopy. ROS are formed in many types of reactions that involve OH, O2, O3, etc., and hence are of considerable interest in fields as diverse as the atmosphere, biochemistry, and combustion. Beyond simple spectroscopic characterization, our effort also focus on understanding at a detailed, molecular level the elementary steps that govern hydrocarbon oxidation from weakly-bound pre-reactive complexes, to stabilized transient intermediates, and ultimately to products, by exploiting the intrinsically high spectral resolution, sensitivity, and wide instantaneous frequency bandwidth of modern rotational spectroscopy.
We have recently demonstrated the ability to detect several important ROS, and in several instances, infer their formation pathway and reactivity.
Criegee intermediates: Spectroscopic characterization, isomers, and heavy atoms counterparts
Carbonyl oxide biradicals, or "Criegee intermediates" (RR'COO), are generated by the ozonolysis of alkenes, and are thought to play a major role in tropospheric chemistry, yielding both OH radicals and secondary organic aerosols, as well as phytotoxic species. The past few years have seen an explosion of experimental activity in this area: the simplest Criegee intermediate CH2OO and several larger ones have now been spectroscopically characterized in the gas-phase by a variety of high resolution techniques.
By using intensive isotopic measurements on all five singly-substituted isotopic species of the simplest Criegee intermediate, CH2OO, and high-level quantum-chemical calculations, our group has determined an accurate semi-empirical structure for this species, making it one of the best characterized reactive species from the perspective of molecular structure. Arguably the most significant finding of this work is that CH2OO is readily produced with high selectivity by simply passing methane and molecular oxygen through an electrical discharge, in contrast to essentially all previous production methods which have relied on halogen chemistry. This finding suggests that atmospheric lightning could be a potential source of Criegee intermediates, a possibility that has apparently not been considered previously, and one that may be relevant for tropospheric chemistry.
Our interest is also focused on isomers and heavy atoms counterparts of Criegee intermediates, and some results have recently been obtained on dihydroxycarbene (HOCOH, an isomer of CH2OO) and its silicon equivalent HOSiOH.
Find more about the project: The simplest Criegee intermediate (H2C=O-O): Isotopic spectroscopy, equilibrium structure, and possible formation from atmospheric lightning. M. C. McCarthy, L. Cheng, K. N. Crabtree, O. Martinez Jr., T. L. Nguyen, C. C. Womack, and J. F. Stanton. The Journal of Physical Chemistry Letters 4, 4133-4139 (2013) Gas-phase structure determination of dihydroxycarbene, one of the smallest stable singlet carbenes. C. C. Womack, K. N. Crabtree, L. McCaslin, O. Martinez Jr., R. W. Field, J. F. Stanton, and M. C. McCarthy. Angewandte Chemie International Edition 53, 4089-4092 (2014)
Identifying and quantifying reactive intermediates
in the ozonolysis of simple alkenes
The ozonolysis of alkenes represents a fundamental set of atmospheric reactions that is thought to proceed via a complex reaction pathway. It is initiated by cycloaddition of ozone across the C=C double bond to form an intermediate (the "primary ozonide") which rapidly rearranges and dissociates to form an aldehyde product and a zwitterionic Criegee intermediate:
In this reaction, the Criegee intermediate is formed with significant internal energy to undergo prompt unimolecular decomposition (on a time scale of 1 ns), yielding amongst other OH radicals, or be stabilized by collisions. Because the lifetime of stabilized Criegee intermediates may be of order 1 s before undergoing secondary unimolecular decomposition, they can participate in bimolecular reactions with other atmospheric species such as H2O, SO2, etc. which, depending on the rates of reaction, may contribute significantly to the atmospheric oxidation capacity. Although this general mechanism is widely accepted in the atmospheric community, direct experimental investigations of Criegee intermediates ozonolysis formation and reactivity have been remarkably limited.
We have recently reported the first, and to date only, observation of the simplest Criegee intermediate, CH2OO, in the gas-phase ozonolysis of ethylene at atmospheric temperature and pressure using a simple reaction flow nozzle and sensitive FTMW detection. Pre-reactive complexes (dioxirane, formic acid) and secondary products (formaldehyde, ethylene ozonide, etc.) have also been detected in this experiment, in amounts that qualitatively support the established reaction pathways of the nascent excited and stabilized Criegee intermediates in the literature.
Find more about the project:
Observation of the simplest Criegee intermediate CH2OO in the gas-phase ozonolysis of ethylene. C. C. Womack, M. A. Martin-Drumel, G. G. Brown, R. W. Field, and M. C. McCarthy. Science Advances 1, e1400105 (2015)