The carbene is seen! Unstable intermediate finally found with mass spectrometry

The Science

Pyruvic acid is a type of alpha-keto carboxylic acid. These are organic compounds that are involved in the Krebs citric acid cycle, the main source of energy for cells. Pyruvic acid is one of the few organic molecules destroyed in the Earth’s inner atmosphere by solar radiation rather than by reactions with highly reactive free radicals. To date, scientists have only identified the final products from the breakdown of pyruvic acid. This makes it hard to understand how light breaks down this acid. In this research, scientists degraded pyruvic acid using a near ultraviolet photon and detected the resulting degradation products. They obtained the first experimental evidence that the primary degradation products are carbon dioxide and the carbene methylhydroxycarbene (MHC). They also obtained the first evidence of a particular chemical reaction between MHC and pyruvic acid.

The Impact

Identifying MHC as a product of the breakdown by light of pyruvic acid opens the doors for further studies that will improve scientific understanding of the chemistry of the atmosphere. For example, MHC generated by sunlight in the troposphere may react with oxygen and water vapor. The research is also important to synthetic chemistry overall. MHC is a carbene, a family of highly reactive molecules containing carbon atoms that use only two of the four bonds they are capable of forming with other atoms. This means carbenes are highly reactive and thus usually have very short lives before they form new bonds. Furthermore, because much of the energy of the photon is transferred to the carbene molecule when pyruvic acid breaks down, MHC contains enough energy to promote further reactions that would otherwise not occur. The new research will also help scientists study the possibility that MHC was involved in the formation of simple sugars during the early history of the universe.

Summary

Scientists have previously proposed that MHC is an intermediate in the photodegradation of pyruvic acid in the atmosphere. However, it is difficult to generate carbenes in sufficiently high quantity to carefully study and understand their reactions. In this study, researchers successfully generated MHC by UV irradiation of gaseous pyruvic acid diluted with helium in a tube at low pressure. Researchers used Multiplexed Photoionization Mass Spectrometry to enable the first definitive detection of MHC in the gas phase. MHC and carbon dioxide accounted for 97 to 100 percent of the degradation products. The study also detected vinyl alcohol and acetaldehyde, likely formed by the isomerization of MHC.

The measured photodissociation quantum yield and measured product branching fractions suggest a mechanism in which hydrogen transfer on the S1 excited state occurs at least partially by quantum mechanical tunnelling, in competition with intersystem crossing to the T1 state. Much of the photon’s energy (62 percent) was transferred to the chemical bonds of MHC, which contributed to its reactivity and hence its instability. Using mass spectrometry, an unknown chemical compound was detected with the molecular formula C4H8O2. The researchers altered the concentration of the pyruvic acid and found that the rate of formation of this unknown compound changed in direct proportion, indicating that it is the product of a reaction between pyruvic acid and MHC.

Funding

Funding for this research came from the Department of Energy Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.