Reaction Dyamics of Two-Centre Hydrocarbons

Understanding and predicting the detailed outcome of a chemical reaction requires studies of the intrinsic dynamics of the collision process of the isolated atomic and molecular reactants. Using a crossed-beam spectrometer for measuring differential scattering cross-sections of mass-selected reactants we propose to study ion-molecule reactions of small hydrocarbon molecules with two carbon atoms. These are the smallest molecules for which several types of reactions appear, all of which are of great interest for applications in molecular synthesis, in hydrocarbon plasmas and flames, and in in carbon-rich planetary atmospheres and interstellar clouds. We will focus on three major reaction types, the formation of different structural isomers by nucleophilic substitution, the elimination reaction and its competition with substitution, and the growth of carbon chains. For this we will use the unique capabilities of crossed-beam imaging to measure differential scattering cross sections for several selected collision energies, which yields information on how the reactive collisions occur. From the extracted reaction mechanisms and the measured product internal energies an analysis scheme will be developed for the differentiation of the open product channels. This may be employed also when a mass-spectrometric differentiation of the reaction products is not possible. With these methods, we will analyse isomer formation in reactions of the cyanide anion, which may form a bond either at the carbon or at the nitrogen side. In reactions of atomic halide and of hydroxyl anions colliding with ethyl halide molecules, we will investigate the competition between elimination and substitution. And we will study carbon chain-formation in reactive collisions of acetylene with de- protonated acetylene and de-protonated diacetylene. With these experiments we will demonstrate a new method to extract branching ratios for isomer formation and pathway competition in gas-phase reactive collisions and we will obtain qualitatively new insight in the reaction dynamics of several important ion-molecule reactions.

In the course of this project we have improved the understanding of the outcome of chemical reactions. To achieve that we have performed studies of the atomistic dynamics of the collision process of isolated atomic and molecular species. We have used a special crossed-beam spectrometer for measuring the scattering of mass-selected reactants. This unique spectrometer provides product angle- and velocity-resolved data for several selected collision energies.Specifically, we have studied ion-molecule reactions of small hydrocarbon molecules. The chosen molecules can proceed via different and competing reactions, which are of great interest for applications in molecular synthesis, in hydrocarbon plasmas, and in carbon-rich planetary atmospheres and interstellar clouds. We have focused on three distinct types of reactions: the formation of different structural isomers by nucleophilic substitution, the elimination reaction and its competition with substitution, and the growth of carbon chain molecules. For all three systems we have performed a range of experimental measurements and have compared them, where available, with theoretical calculations. By analysing the measured product velocities we have extracted information on the branching of reactions into different product isomers, which are composed of the same atoms, but with different geometrical structures. In reactions of several small negative ions, fluorine, chlorine, and cyanide anions, with alkyl halide molecules we have investigated the competition between elimination and substitution reactions. These studies relied on the obtained scattering data, since conventional mass spectrometry can not distinguish such channels as they lead to the same product mass. Finally, we have studied carbon chain-formation in reactive collisions of different molecular carbon anions with acetylene.With these experiments we have demonstrated a new method to extract branching ratios for isomer formation and pathway competition in gas-phase reactive collisions. This has allowed us to obtain new and very detailed insight in the reaction dynamics of several important ion-molecule reactions. This work has received a lot of attention at international scientific conferences. It has also stimulated several theory groups to perform new and detailed calculations to continue to improve our atomistic understanding of these reactions.