New Class of Unusual Reactive Substances Detected in Atmospheric Chemistry

For the first time, an international team of researchers has successfully detected hydrotrioxides (ROOOH) under atmospheric circumstances.

The existence of these chemical molecules with the odd OOOH group had been completely conjectural till now. Their synthesis during the oxidation of key hydrocarbons like isoprene and alpha-pinene has been conclusively confirmed in laboratory tests.

Quantum chemical computations and model calculations were used to derive important data about this unique family of chemicals. Every year, roughly 10 million metric tons of isoprene are produced through oxidation in the Earth's atmosphere. ROOOHs are thought to have a lifespan of minutes to hours.

In the current issue of the prestigious scientific journal Science, researchers led by the Leibniz Institute for Tropospheric Research (TROPOS) write that hydrotrioxides represent a previously unnoticed class of substances in the atmosphere whose effects on health and the environment need to be investigated.

The bottom layer of our planet's atmosphere serves as a massive chemical reactor, converting hundreds of millions of metric tons of hydrocarbons each year, eventually resulting in the production of carbon dioxide and water. Forests or human sources release these hydrocarbons. There are many different types of oxidation processes, but only a few of them are thoroughly known. Hydrotrioxides have been a recent subject of atmospheric study (ROOOH). These are gaseous substances having three successive oxygen atoms "O" and a hydrogen atom "H" that are bound to an organic rest (R). Hydroperoxides (ROOH) containing two oxygen atoms have been recognized and proved for a long time.

It has been suggested in the literature that there may be compounds in the atmosphere that contain not only two but also three oxygen atoms (ROOH) (ROOOH). Hydrotrioxides are utilized in chemical synthesis to create specific oxidation products in the reaction with alkenes.

However, at very low temperatures around -80°C (-112°F), highly reactive and thermally unstable hydrotrioxides are formed in organic solvents and further react. Until recently, it was unknown if this chemical class occurs as a gas in the atmosphere at much higher temperatures.

Until now, all we knew about hydrotrioxides (ROOOH) was that they were organic molecules with the odd OOOH group. Their synthesis via the oxidation of key hydrocarbons like isoprene and alpha-pinene could now be convincingly confirmed in laboratory tests at TROPOS in Leipzig.

Researchers from the Leibniz Institute for Tropospheric Research (TROPOS), the University of Copenhagen, and the California Institute of Technology (Caltech) have now provided direct evidence for the first time that hydrotrioxides are formed under atmospheric conditions when peroxy radicals (RO2) react with hydroxyl radicals (OH). 

The experiments were mostly carried out at TROPOS in Leipzig in a free-jet flow tube at room temperature and 1 bar air pressure, with the use of very sensitive mass spectrometers. Caltech's research offered further experimental data, particularly on the stability of the hydrotrioxides. The University of Copenhagen used quantum chemical simulations to characterize the reaction processes as well as the temperature and photostability of hydrotrioxides. Global simulations using the chemistry-climate model ECHAM-HAMMOZ allowed for a preliminary assessment of the impacts on the Earth's atmosphere.

“It is really exciting to show the existence of a universal new class of compounds formed from atmospherically prevalent precursors (RO2 and OH radicals),” says Prof. Henrik G. Kjaergaard of 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 from 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,” says Caltech's Prof. Paul O. Wennberg.

Only in the coming years will the importance of the first successful identification of this new material class, "hydrotrioxides," become obvious. The research study by Berndt et al., on the other hand, has created the initial footing with the experimental demonstration and current knowledge, which should pique the curiosity of other research organizations.