June 23, 2017
A Texas Tech University computational and theoretical chemist, along with collaborators around the world, is working to finally answer an old question in organic chemistry.
In a paper published in the journal Nature Communications, a research team of experimentalists at the University of Innsbruck in Austria, computational and theoretical chemists at Harbin University in China, and Texas Tech’s own Bill Hase – a Paul Whitfield Horn Professor and the Robert A. Welch Chair in the Department of Chemistry & Biochemistry – showed their study of the competition between two important reactions of organic chemistry, the E2 elimination reaction and the SN2 substitution reaction.
“We established, at the atomistic level, the manner in which these reaction mechanisms occur,” Hase said. “It is important to understand how reactions occur at the atomistic level because these mechanisms are important in biological reactions and organic synthesis.”
Many chemical reactions are a sequence of very complex processes which are still not fully understood. With laboratory experiments, Roland Wester from the Institute for Ion Physics and Applied Physics at the University of Innsbruck studies such reactions to better understand their dynamics. Wester has built a unique experiment that allows ions and molecules to react and be observed. The angle and velocity at which the ions impinge on a detector is measured.
In a synergistic collaboration with the Wester research group, a research group at Harbin University led by Li Yang and Jiaxu Zhang and the Hase research group at Texas Tech perform chemical dynamics simulations to assist in providing an atomic-level understanding of the experimental measurements. The simulations are performed at Texas Tech and require a high-performance computing infrastructure, benefiting from the university’s High Performance Computing Center (HPCC). The simulations are validated by comparing them with experimental measurements.
In a recent study, a team within the Wester group investigated organic compounds with several methyl groups attached to their central carbon atom and also a chlorine or iodine halogen atom attached to this carbon. In a vacuum chamber, the researchers collided these molecules with fluoride, chloride or iodide anions.
An exciting feature of the experiments, as well as for the simulations, is that it is unpredictable which of two chemical reactions, E2 or SN2, will take place. Either the ion binds to the molecule and the halogen atom bond breaks (SN2 reaction) or the ion strikes a hydrogen atom from the methyl group and thereby flies away with it (E2 reaction). The two reactions are in competition.
With the Wester group’s apparatus, angles at which the reaction products scatter from the reactive collision, and the products’ velocities, are measured. This measurement provides information regarding the relative importance of the E2 and SN2 reactions. Simulations of the reactive collisions give the relative probabilities of the E2 and SN2 reactions and atomic-level animations of their mechanisms. The simulations are confirmed by comparing with the angles and velocities measured in the experiments. For the smaller molecules there is a competition between the SN2 and E2 reactions. In the case of larger molecules, the elimination reaction is preferred and the substitution reaction disappears.
“I think it took about three years to complete this study,” Hase said. “I am happy with our results; it is the first complete atomistic study for competing E2 and SN2 mechanisms.”
Until this work, there were only indirect measurements of the competition between the SN2 and E2 reactions. The Innsbruck research group presented for the first time data from direct observations, and in combination with the simulations, an atomic-level understanding of the competition between the SN2 and E2 reactions has been obtained. The Innsbruck, Harbin and Texas Tech research groups plan to continue their collaborations.
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