Different chemical reactivities of para- and ortho-water toward trapped diazenylium ions
Unraveling the structure-function relationship through chemical-dynamics studies
The paper in Nature Communications is here: https://go.nature.com/2snbFgp
The world around us is dominated by chemical reactions, slow and fast, as is, for example, outlined by Addy Pross in his book “What is life?” Pross describes the chemical principles driving biological complexity and dynamics. At the same time, we have to realize that the tremendously successful chemistry-textbook knowledge relies on static structures and simple headed arrows. But, the world is not static! In order to further develop a deep understanding of the dynamic processes in chemical reactions, scientists have invented wonderful new techniques and look for novel models, a new basis for depicting chemistry.
This brought forward the quest for the recording of so-called “molecular movies”, sequences of pictures containing the atoms during a chemical reaction, with picometer spatial and femtosecond temporal resolution. Femtochemistry,1 spectroscopic measurements of the temporal evolution of molecular systems provided the first glimpse. Ultrafast electron diffraction and x-ray diffraction using free-electron lasers are now providing structures with femtosecond temporal and atomic spatial resolution.2–4 However, these experiments are performed as “pump-probe” experiments, where photons trigger chemical reactions in single molecules.
How do we understand the dynamics of “real” chemical reactions, i.e., the processes occurring when two molecules collide? Ultrafast-imaging approaches of these rare events pose a problem: One would need to image the system at high resolution on the order of 1015 Hz for seconds or minutes and then replay the relevant part once the reaction proceeded — currently a formidable task in the case of chemical reactions, while possible for kHz-rate cameras and the 90 min of a football game. Furthermore, the imaging would have to be performed non-destructively, which is quite the opposite in current approaches.5
Reactive-scattering experiments come to the rescue. Their quantum-state selective analysis of chemical reaction dynamics enable to develop an accurate description of the underlying dynamics and, for small molecular systems, to benchmark highly developed quantum-chemistry approaches that allow to extract the underlying dynamics in detail. In our work, we set out to perform such experiments for ion-molecule reactions, which are challenging to study, partly due to the difficulties of preparing dense enough reaction targets. Previous experiments had unraveled the mechanism behind SN2 reactions6 or conformer-specific reactivities.7 In the current work, we set out to disentangle reactions of water, with a special emphasis on the two different kinds of water, para and ortho water. In Hamburg we had demonstrated the separation and purification of both of these species8 and the Willitsch group in Basel had, based on their ion-trapping expertise,9 developed a new and improved setup for reactivity studies of trapped-ions with species-selected neutral molecules. The general aim of these experiments is to accurately disentangle the fine details of reactive intermolecular interactions through highly-controlled reactants and reaction conditions.10,11
In the current study, a species-selective water stream was coupled to an ion trap, in which N2+ ions were captured, which quickly reacted with omnipresent H2 to form N2H+, which was then sympathetically cooled by cotrapped lasercooled Ca+ ions.12 Carefully performing the reaction experiment and analyzing the intricate details, Kilaj and coworkers could unravel the differences in the reaction dynamics of the two species, corresponding to two different rovibrational states of water, through capture rate calculations that quantitatively agree with the experimental findings.
The distinct reactivities of para- and ortho-water can be explained as follows: when these molecules approach the trapped N2H+ ions, they are re-oriented in the electric field of the ion, resulting in a strong charge-dipole interaction and a correspondingly strong attraction between the two reaction partners. However, this reorientation of water molecules is larger for para- than for ortho-water. The quantitative agreement of this simple rotational-dynamics model provides a direct visual explanation – in terms of pendular states – for the underlying chemical processes. Further details will be available from a future fully-state-controlled experiment with state-selected neutrals12 and ions.13
Finally, unraveling the actual dynamics of chemical reactions, one can envision to merge the state-selected reactive-collision approach utilized here12 with the utilization of purified pre-reactive complexes10 in time-resolved imaging experiments, which could be triggered by infrared photons mimicking the thermal excitation in natural environments.
Jochen Küpper, Hamburg
Ardita Kilaj, Hong Gao, Daniel Rösch, Uxia Rivero, Stefan Willitsch, Basel
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12 A. Kilaj, H. Gao, D. Rösch, U. Rivero, J. Küpper, and S. Willitsch, “Observation of different reactivities of para- and ortho-water towards trapped diazenylium ions,” Nat. Commun. 9, 2096 (2018).↩
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