Gas‐Phase Formation of the Disilavinylidene (H2SiSi) Transient

The hitherto elusive disilavinylidene (H₂SiSi) molecule, which is in equilibrium with the mono‐bridged (Si(H)SiH) and di‐bridged (Si(H₂)Si) isomers, was initially formed in the gas‐phase reaction of ground‐state atomic silicon (Si) with silane (SiH₄) under single‐collision conditions in crossed molecular beam experiments. Combined with state‐of‐the‐art electronic structure and statistical calculations, the reaction was found to involve an initial formation of a van der Waals complex in the entrance channel, a submerged barrier to insertion, intersystem crossing (ISC) from the triplet to the singlet manifold, and hydrogen migrations. These studies provide a rare glimpse of silicon chemistry on the molecular level and shed light on the remarkable non‐adiabatic reaction dynamics of silicon, which are quite distinct from those of isovalent carbon systems, providing important insight that reveals an exotic silicon chemistry to form disilavinylidene.     

Directed Gas‐Phase Synthesis of Triafulvene under Single‐Collision Conditions

The triafulvene molecule (c‐C₄H₄)—the simplest representative of the fulvene family—has been synthesized for the first time in the gas phase through the reaction of the methylidyne radical (CH) with methylacetylene (CH₃CCH) and allene (H₂CCCH₂) under single‐collision conditions. The experimental and computational data suggest triafulvene is formed by the barrierless cycloaddition of the methylidyne radical to the π‐electron density of either C₃H₄ isomer followed by unimolecular decomposition through elimination of atomic hydrogen from the CH₃ or CH₂ groups of the reactants. The dipole moment of triafulvene of 1.90 D suggests that this molecule could represent a critical tracer of microwave‐inactive allene in cold molecular clouds, thus defining constraints on the largely elusive hydrocarbon chemistry in low‐temperature interstellar environments, such as that of the Taurus Molecular Cloud 1 (TMC‐1).     

Gas‐Phase Synthesis of the Elusive Cyclooctatetraenyl Radical (C8H7) via Triplet Aromatic Cyclooctatetraene (C8H8) and Non‐Aromatic Cyclooctatriene (C8H8) Intermediates

The 1,2,4,7‐cyclooctatetraenyl radical (C₈H₇) has been synthesized for the very first time via the bimolecular gas‐phase reaction of ground‐state carbon atoms with 1,3,5‐cycloheptatriene (C₇H₈) on the triplet surface under single‐collision conditions. The barrier‐less route to the cyclic 1,2,4,7‐cyclooctatetraenyl radical accesses exotic reaction intermediates on the triplet surface, which cannot be synthesized via classical organic chemistry methods: the triplet non‐aromatic 2,4,6‐cyclooctatriene (C₈H₈) and the triplet aromatic 1,3,5,7‐cyclooctatetraene (C₈H₈). Our approach provides a clean gas‐phase synthesis of this hitherto elusive cyclic radical species 1,2,4,7‐cyclooctatetraenyl via a single‐collision event and opens up a versatile, unconventional path to access this previously largely obscure class of cyclooctatetraenyl radicals, which have been impossible to access through classical synthetic methods.     

Femtochemie. Elementare Schritte chemischer Reaktionen

Femtochemistry is about the investigation and control of ultrafast elementary molecular dynamics, which are the basis of every chemical reaction. The processes finally resulting in breaking of chemical bonds or molecular structure changes take place on a time scale of only femto to picoseconds. Solely femtosecond laser pulses are fast enough to resolve these fast processes. Different techniques were developed, which make use of a combination of femtosecond pulses having a relative temporal delay, in order to get access to the dynamics even in complex molecules. The knowledge of the elementary processes allows for a better understanding of the reaction mechanisms and their dependence on environmental conditions. The interaction with the molecules even before the final reaction path is entered, opens up new exciting possibilities for the control of chemical processes. A specific manipulation of the molecular dynamics using adapted pulse shapes appears to be realistic also for complex reactions and systems. The evolutionary optimization strategies, which exploit the experimental results as feedback, make selective chemistry come true even without knowledge of all system parameters.