New class of substances detected in atmospheric chemistry

An international research team has now succeeded in detecting hydrotrioxides (ROOOH) under atmospheric conditions for the first time. Until now, there was only speculation that these organic compounds with the unusual OOOH group exist. In laboratory experiments, their formation during the oxidation of important hydrocarbons, such as isoprene and alpha-pinene, has been clearly demonstrated. Important data on this new substance class have been estimated by means of quantum chemical calculations and model calculations. About 10 million tons per year are formed in the Earth’s atmosphere through isoprene oxidation. The lifespan of ROOOHs is estimated to be minutes to hours. Hydrotrioxides represent a hitherto undetected class of substances in the atmosphere whose effects on health and the environment need to be investigated, the researchers led by the Leibniz Institute for Tropospheric Research (TROPOS) write in the current issue of the scientific journal SCIENCE.

The bottom layer of our earth’s atmosphere is a large chemical reactor in which several 100 million tons of hydrocarbons are converted annually, ultimately leading to the formation of carbon dioxide and water. These hydrocarbons are emitted from forests or anthropogenic sources. A wide variety of oxidation processes take place, only a few of which are well understood. A recent focus of atmospheric research is on hydrotrioxides (ROOOH). These are gaseous substances with a group consisting of three consecutive oxygen atoms “O” and a hydrogen atom “H”, which is bonded to an organic moiety (R). Hydroperoxides (ROOH) with two oxygen atoms have been known and proven for a long time. It has previously been speculated in the literature that there could be substances in the atmosphere that carry not only two oxygen atoms (ROOH) but also three oxygen atoms (ROOOH). In organic synthesis, hydrotrioxides are used to form special oxidation products when reacting with alkenes. However, these reactive and thermally unstable hydrotrioxides are produced there in organic solvents at very low temperatures around -80°C and react further. Until now, it was unknown whether this dust class also occurs as a gas in the atmosphere at significantly higher temperatures.

Researchers from the Leibniz Institute for Tropospheric Research (TROPOS), the University of Copenhagen and the California Institute of Technology (Caltech) have now been able to provide direct evidence for the first time that the formation of hydrotrioxides also takes place under atmospheric conditions through the reaction of peroxy radicals ( RO2) with hydroxyl radicals (OH). The laboratory studies were mainly performed at TROPOS in Leipzig in a free-jet flow tube at room temperature and a pressure of 1 bar air – combined with the use of highly sensitive mass spectrometers. Additional experimental information, particularly on the stability of the hydrotrioxides, was provided by the studies at Caltech. Quantum chemical calculations were performed by the University of Copenhagen to describe the reaction mechanisms and the temperature and photostability of hydrotrioxides. Global simulations of TROPOS with the chemistry-climate model ECHAM-HAMMOZ enabled a first estimate of the effects on the Earth’s atmosphere.

“It is really exciting to demonstrate the existence of a universal new class of compounds formed from atmospherically occurring precursors (RO2 and OH radicals),” reports Prof. Henrik G. Kjærgaard from the University of Copenhagen. “It is very surprising that these interesting molecules are so stable with such a high oxygen content. Further research is needed to determine the role of hydrotrioxides for health and the environment,” emphasizes Dr Torsten Berndt of TROPOS. “Our study has shown that direct observation of hydrotrioxides using mass spectrometry is feasible. This means that it is now possible to further investigate these compounds in different systems, including perhaps the quantification of their abundance in the environment.” explains Prof. Paul O. Wennberg of Caltech.

The importance of the first successful detection of this new substance class “hydrotrioxides” will only become clear in the coming years. With the experimental evidence and current knowledge, the research study by Berndt et al. has laid a first foundation that should also arouse the interest of other research groups.

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