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Ultrafast Structural Dynamics of Elementary Water-Mediated Proton Transport Processes

Periodic Reporting for period 3 - XRayProton (Ultrafast Structural Dynamics of Elementary Water-Mediated Proton Transport Processes)

Reporting period: 2021-09-01 to 2023-02-28

The aim of the project is to develop experimental methods to determine the electronic structure and structural changes of acids and bases and of the mediating water molecules exchanging protons in aqueous media. The spectroscopic method of choice is soft-x-ray spectroscopy as a means to locally probe transitions from inner-shell levels to unoccupied molecular orbitals, providing direct key insight into the chemical bonding of hydrogen bonds that form the reaction pathways of proton transport. Deciphering the dynamical aspects of the electronic structure will enable to determine the interplay of the proton with the electronic degrees of freedom of acids, bases and intermediate water molecules with which acid-base chemistry can be understood in full microscopic detail.

The main goals of this project are of fundamental nature, i.e. understanding the key underlying microscopic mechanisms and associated dynamics of electronic structure dictating the outcome of aqueous acid-base chemistry, that may alter chemistry textbooks on this basic chemical reaction. Yet, results of this project will ultimately result in a better understanding proton transport in real-world applications, e.g. hydrogen fuel cells for energy storage or transmembrane proton channel proteins for energy transport and signal transduction in biology.

The aim is to further develop steady-state and time-resolved soft-x-ray spectroscopy of acids and bases in water-poor and water-rich solutions to elucidate the electronic structural evolution of proton transfer pathways. For this novel liquid flatjet technology needs to be further developed as a means of sample delivery in end stations used at soft-x-ray sources at large scale facilities as well as table-top laser-based high-order harmonic systems. Questions to be solved are electronic structural changes upon hydrogen bond formation, the nature of hydrated proton species, and the impact of conversion from acid to conjugate base (or base to conjugate acid) in aromatic alcohols, carboxylic and amine compounds, and ultimately the electronic structure of the water units in hydrated proton complexes. To determine the impact of the fluctuations of the surrounding solvent molecules on the electronic structure of acids, bases and of hydrated proton complexes, the development, implementation and application of femtosecond soft-x-ray spectroscopy is a major activity in this project.
Measurements on the electronic structure of acid-base encounter pairs had been put on hold after earlier achievements on (protonated) amines had been published in 2017-2018.

In the years 2018-2020 the electronic structure of hydrated proton complexes in acetonitrile has been studied. Crucial was the development of a stable acetonitrile flatjet, used in the July and November 2018 beamtime measurements at the Berlin BESSYII user facility. Strong orbital interactions and electric field effects by the positively charged proton have been probed with oxygen K-edge spectroscopy. In the May/June 2019 BESSYII beamtime charge distribution changes of a photoacid molecule, 8-aminopyrene-1,3,6-trisulfonate (APTS) along the four stages of its Förster cycle, have been explored with picosecond N K-edge spectroscopy, with additional characterization of APTS with UV/IR pump-probe measurements. For both projects publications have been published in 2022 in the renowned journal Angewandte Chemie International Edition. The publication on the O K-edge spectroscopy of the hydrated proton complex has led to a News item in Chemistry World (by Kira Welter, dated 31.10.2022).

Combining our expertise on flatjet technology for endstation sample delivery with the novel split-and-delay geometry for femtosecond nitrogen K-edge spectroscopy at DESY-FLASH (Hamburg) and the photophysics of nitrogen containing charge transfer compounds (University of Hamburg) had resulted in a joint October 2018 beamtime at FLASH, and one publication in Structural Dynamics.

A collaboration with Yale University on ultrafast X-ray absorption spectroscopy probing the vibronic wavepacket dynamics of photodissociation of ICN has resulted in one publication in Angewandte Chemie International Edition. This theoretical work makes clear how spectral signatures on the carbon K-edge provide direct insight into the dynamics of the frontier orbitals during the I-CN bond cleavage processes. A follow-up study in this collaboration has resulted in a complete quantum dynamics calculation of the photoinduced enol* --> keto* tautomerization reaction of 2-(2′-hydroxyphenyl)benzothiazole (HBT). A full quantum treatment of the electronic and nuclear dynamics of upon electronic excitation reveals how spectral signatures of local excitations from core to frontier orbitals display the distinctly different stages of charge relocation for the H atom, donating, and accepting sites. These results show the interplay among charge relocation, hydrogen bond modulation motions, and proton motion, and hence the tautomerization involves a proton-coupled electron transfer reaction that is is not fully concerted, but rather sequential, with charge relocations occurring on distinctly different times for the oxygen-donating atom, the inner proton, and the nitrogen-accepting atom. Our findings indicate that ultraviolet/X-ray pump-probe spectroscopy provides a unique way to probe ultrafast electronic structure rearrangements in photoinduced chemical reactions essential to understanding the mechanism of PCET. This has been reported in a publication in The Journal of Physical Chemistry Letters.

In a February 2020 BESSYII beamtime we have performed first time-resolved nitrogen K-edge spectroscopic measurements on 7-hydroxyquinoline (7HQ). Building on previous femtosecond UV/IR pump-probe measurements on protonation dynamics of 7HQ in water/methanol (as published in 2016 and 2019), first results on the electronic structural changes upon protonation of the quinolone nitrogen atom in 7HQ have been measured. Further experiments had been planned for a June 2020 beamtime at the same large scale facility, however, due to the COVID-19 pandemic, this was cancelled. This beamtime had been rescheduled to January 2021, when it was cancelled two times.

The COVID-19 pandemic has slowed down the XRayProton activities at large scale facilities for about one year now. To prevent a severe standstill, activities focussed more on experiments using the laser laboratories at the Max Born Institute. These include the determination of proton transport dynamics of 7HQ interacting with added bases using femtosecond UV-pump-IR-probe spectroscopy. This enables a benchmarking of photoacid-base reaction pairs with much detail, as reported in two publications, one on the 7HQ – formate and the other on the 7HQ – imidazole system.

Meanwhile research activities at large scale facilities have resumed operation in the course of 2021. Experiments on photoacid – base reaction dynamics have been studied in 2021 – 2022 during four beamtimes at BESSYII and one remote access beamtime at LCLS in December 2021. Manuscripts reporting the results obtained are in preparation.

A second alternate route to guarantee a continuation of the XRayProton project is the further development of ultrafast UV-pump-soft-X-ray-probe spectroscopy using table-top laser systems exploiting extreme high-order harmonic generation (HHG). In previous years this has resulted in a successful demonstration of steady-state C and N K-edge spectroscopy of small molecules and ions in aqueous solution. In further pursuing this methodology in terms of improvement in HHG efficiency, stability and delivery at the flatjet sample target, and improving the photon flux at the CCD detector, a new spectrometer has been designed and implemented with optimized x-ray optics using reflective zone plates, as described in a publication in Structural Dynamics.

First femtosecond pump-probe spectroscopic experiments using extreme HHG pulses with energies extending into the N K-edge spectral region have been accomplished. Strong field ionization of molecular nitrogen N2 with a pump pulse with a carrier wavelength of 800 nm and a duration of 50 fs results in the generation of molecular nitrogen cation N2+. Ultrafast probing at the N K-edge reveals that N2+ is prepared in three possible electronic states (the electronic ground X2g+, first excited A2u, and second excited B2u+ states) within the pump pulse duration. With this probing method we can quantitatively determine the time-dependent electronic state population distribution dynamics of N2+. Our results show a remarkably low population of the A2u state, and nearly equal populations X2g+ and B2u+ states. Interestingly, our results, published in the high profile journal Physical Review Letters, contribute to the field of air-lasing, subject to propagation of intense femtosecond filaments through air, that has witnessed a lively debate since two decades.
A crucial step for the upcoming years will be to develop solution-phase femtosecond soft-x-ray spectroscopy probing the elementary steps in proton transfer. For this I will aim to continue further development of both methodologies, table-top laser-based systems exploiting extreme high-order harmonic soft-x-ray pulses at the MBI, and large scale facilities (free electron lasers).

Ultimately the interplay between the proton and the electronic degrees of freedom of the photoacid/photobase and mediating water will be resolved. I aim to answer the question of what the nature of the primary hydrated proton complex is, formed directly after the proton dissociation event of an acid to nearby water. For this the number of directly involved water molecules in hydrating a proton, and the degree of involvement of these particular water molecules will be determined.
X-Ray Absorption Spectra of Gaseous Water, Water Monomer and Hydrated Proton in Acetonitrile