Vacuum Ultraviolet Spectroscopy

Open shell organometallic bismuth are promising agents for catalytic applications, but difficult to characterize due to their high reactivity. The simplest methylbismuth (Bi-CH₃), a biradical species, was in-situ synthesized and spectroscopically characterized for the first time. Electronic and thermochemical properties could be obtained, which will guide future synthetic applications.

Light alkanes are abundantly available and cheap resources that are often burned at oil wells because of the missing infrastructure for valorization. Novel technologies are needed for their selective functionalization to use natural gas as an energy vector in the transition between the oil and the renewables era. Catalytic oxyhalogenation may unlock the transformation of cheap and abundant alkanes into commodities. When chlorine-based reactions are compared with bromine, improved selectivities above an iron catalyst arise from surface-confinement of the reaction mechanism in the case of chlorine as halogen.

The resonantly stabilized xylyl radicals (C₈H₉•) distinctively influence the combustion chemistry and, therefore, ultimately determine the performance of combustion engines. At that, the three different isomers (methyl group in ortho, para or meta position) exhibit notable differences at elevated temperatures. We have tracked down these dynamics on a femtosecond timescale by monitoring the response to preparation of a well-defined electronic and vibrational state.

Methylperoxy radicals are crucial oxidation intermediates and could be synthesized photolytically in an exothermic reaction. Despite their vanishingly small concentration, their temperature could be measured after a few ten thousand collisions inside the reactor, which opens up the possibility of time-resolved operando temperature measurements. Also the reaction energy to yield methyl cations and oxygen could be determined with sub-kJ mol^–1 precision, which firmly anchors the methylperoxy energetics to that of well-known stable species and opens up the possibility of highly accurate radical thermochemistry measurements.

Studying organic molecular magnets is a challenge, because the high-spin diradical character of these compounds dramatically increases the reactivity and reduces the lifetime. Researchers from PSI, ETH Zurich, Wollongong and Melbourne, Australia succeeded in taming the meta-xylylene diradical and were able to study its electronic and thermochemical properties.

Catalytic oxybromination may turn the cheap and abundant feedstock methane into the platform compounds bromomethane and dibromomethane. Yet researchers have been puzzled by the catalysis mechanism, which was speculated to involve free radical intermediates. Operando photoelectron photoion coincidence helped distinguish surface and gas-phase reaction steps and elucidated the crucial halogen-mediated C–H bond activation step, which is driven by elusive bromine and methyl radicals.

Lignin is a major constituent of plants, and may be used as a precursor for fuels and fine chemicals. Catalytic fast pyrolysis of lignin is one of the most promising approaches. By using vacuum ultraviolet synchrotron radiation and threshold photoelectron spectroscopy we could identify elusive intermediates, which are responsible for the formation of phenol and benzene and could thus tackle this reaction mechanism. Mechanistic understanding could enable targeted improvement of production methods in the future, beyond the currently used "cook-and-look" approach.

Online combustion analysis relies heavily on spectral data to detect reactive intermediates isomer-selectively to establish e.g. kinetic flame models. Due to the difficulty to generate these species cleanly, spectral data are rather scarce. Here we report on the selective generation of three picolyl radical isomers (C₅H₄N-CH₂*) by deamination of aminomethylpyridines. Picolyl radicals are relevant in biofuel combustion, and could now be characterized by threshold photoelectron spectroscopy using synchrotron radiation. Vibrationally resolved bands and distinct ionization energies allow for isomer-specific detection of these elusive species in complex environments and permit us to explore new avenues in soot- and NOx formation kinetics.

The false coincidence background has so far limited the analytical application of PEPICO, photoelectron photoion coincidence. A new photoioin rastering technique has been developed to separate the wheat from the chaff and identify true coincidences based on the ion hit time and position. This expands the dynamic range of the experiment by at least two orders of magnitude, allowing for novel applications to look for reactive intermediates and short lived species in reaction environments.