Skip to main content

Ultrafast Structural Dynamics of Elementary Water-Mediated Proton Transport Processes

Periodic Reporting for period 1 - XRayProton (Ultrafast Structural Dynamics of Elementary Water-Mediated Proton Transport Processes)

Reporting period: 2018-09-01 to 2020-02-29

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.

Experiments on the electronic structure of hydrated protons have led to superb results due to the well planned and well organized July and November 2018 beamtimes at the Berlin BESSYII user facility, for which the persistence and endurance by Dr. Maria Ekimova should be credited who co-led students Carlo Kleine and Jan Ludwig. After a testing round in the July 2018 beamtime, we have successfully measured the oxygen K-edge spectra of hydrated proton complexes with varying hydration degrees. In a manuscript (submitted to Science 10 April 2020) we present measured and calculated oxygen K-edge X-ray absorption spectra and ab initio molecular-dynamics simulations of water-proton complexes in acetonitrile solution. We use oxygen K-edge spectroscopy as an orbital-specific marker for the covalent bonding between protons and neigbouring water molecules. Hydrated proton complexes underlie a hierarchy of orbital interactions. Oxygen K-edge spectral features, clearly distinct from those of weakly hydrogen-bonded water, single out H7O3+ as the key inner proton complex consisting of a superstrong hydrogen-bonded central H5O2+ Zundel-motif, with an additional strongly bound water molecule.

The dynamics of electronic structure of the 8-aminopyrene-1,3,6-trisulfonate (APTS) photoacid in aqueous solution has been studied with picosecond nitrogen K-edge spectroscopy as a local probe of the proton donating/accepting amine group. For this Dr. Sebastian Eckert augmented our existing flatjet endstation with additional data acquisition and controlling electronics and software. The joint effort by Dr. Sebastian Eckert, Dr. Marc-Oliver Winghart, Dr. Maria Ekimova and students Carlo Kleine and Jan Ludwig made the May/June BESSYII beamtime successful. A manuscript on these time-resolved nitrogen K-edge results obtained on APTS along the four stages of its Förster cycle is in preparation, with additional characterization of APTS with UV/IR pump-probe measurements (performed by Dr. Marc-Oliver Winghart at the MBI).

In a February 2020 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 recorded by Dr. Maria Ekimova in 2015-2017 (as published in 2016 and 2019), first results on the protonation of the quinolone nitrogen atom in 7HQ has been measured. Dr. Marc-Oliver Winghart commenced additional UV/IR pump-probe measurements on 7HQ, with which we can plan a future beamtime at BESSYII.

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 and the photophysics of nitrogen containing charge transfer compounds (University of Hamburg) had resulted in a joint October 2018 beamtime at FLASH. A publication on these results is in its final stages before submission.

We continue to further explore at the MBI table-top laser-based extreme high-order harmonic generation (HHG) to be used as soft-x-ray light pulses for solution phase x-ray absorption spectroscopy. In particular Carlo Kleine is actively involved (with support by Dr. Marc-Oliver Winghart and Dr. Sebastian Eckert) 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, Here a new spectrometer has been designed and implemented with optimized x-ray optics using reflective zone plates. A manuscript on this improved HHG set-up for solution phase soft-x-ray spectroscopy is in preparation, to be submitted in the late spring of 2020.
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). For the latter we have established a collaboration with the teams from Hamburg (Nils Huse/ Matin Beye, Günter Brenner; DESY-FLASH), and with an Early Science Activity coordinated by Kelly Gaffney (SLAC; LCLSII).

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