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Millimetre-wave spectroscopy uncovers transition phase of chemical reaction

17 February 2020 | News

Scientists at the US Department of Energy’s (DOE) Argonne National Laboratory, in a collaboration with the Massachusetts Institute of Technology (MIT) and several other universities, have demonstrated a way to experimentally detect the extremely short-lived transition state that occurs at the initiation of chemical reactions. This discovery could become instrumental in gaining the ability to predict and externally control the outcomes of chemical processes.

“The transition state is key in all of chemistry because it controls the products of molecular reactions”, said Kirill Prozument, lead author and chemist in Argonne’s Chemical Sciences and Engineering division. The life of this transition phase is as short as a femtosecond. The problem has been that it has not been possible to experimentally observe the structure of this state or even to extract sufficient details about it indirectly from the chemical products created by it.

“The transition state is key in all of chemistry because it controls the products of molecular reactions” commented Kirill Prozument, lead author and chemist in Argonne’s Chemical Sciences and Engineering division. “Physicists cannot directly observe the Big Bang, which happened almost 14 billion years ago, or the transition state that led to the formation of our universe”, explained Prozument. “But they can measure various messengers remaining from the Big Bang, such as the current distribution of matter, and thereby uncover many things about the origin and evolution of our universe. A similar principle holds for chemists studying reactions.”

The team used chirped-pulse Fourier transform millimetre-wave spectroscopy, which allows characterisation of multiple competing transition states on the basis of the vibrationally excited molecules that result in the immediate aftermath of a reaction. This technique is unrivalled in its precision at determining molecular structure and resolving transitions that originate from different vibrational energy levels of the product molecules. Using this technique, the team analysed the reaction between vinyl cyanide and ultraviolet light produced by a laser, which forms various products containing hydrogen, carbon and nitrogen. They were able to measure the vibrational energies associated with the newly formed product molecules and the fractions of molecules in various vibrational levels. The former indicates the amplitudes of which atoms within a molecule move relative to each other. The latter provides information about the geometry of groups of atoms at the transition state as they are giving birth to a product molecule—in this case, the extent of bending excitation in the bond angle between the hydrogen, carbon and nitrogen atoms. Based upon their measurements, the team identified two transition states that govern different pathways by which the molecule hydrogen cyanide (HCN) springs to life from the reaction.

“Our work demonstrates that the experimental technique works in principle”, Prozument says. “The next step will be to apply it to more complex reactions and different molecules.”

Their work has been reported in PNAS.

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