Femtosecond proton transfer in urea solutions probed by X-ray spectroscopy

Abstract

Proton transfer is one of the most fundamental events in aqueous-phase chemistry and an emblematic case of coupled ultrafast electronic and structural dynamics1,2. Disentangling electronic and nuclear dynamics on the femtosecond timescales remains a formidable challenge, especially in the liquid phase, the natural environment of biochemical processes. Here we exploit the unique features of table-top water-window X-ray absorption spectroscopy3–6 to reveal femtosecond proton-transfer dynamics in ionized urea dimers in aqueous solution. Harnessing the element specificity and the site selectivity of X-ray absorption spectroscopy with the aid of ab initio quantum-mechanical and molecular-mechanics calculations, we show how, in addition to the proton transfer, the subsequent rearrangement of the urea dimer and the associated change of the electronic structure can be identified with site selectivity. These results establish the considerable potential of flat-jet, table-top X-ray absorption spectroscopy7,8 in elucidating solution-phase ultrafast dynamics in biomolecular systems.

X-ray induced ultrafast charge transfer in thiophene-based conjugated polymers controlled by core-hole clock spectroscopy

“…43 Taking a step towards potential applications, very recently, proton transfer and the coupled dynamics of the electronic structure in urea solutions have been demonstrated using the element and site selectivity of X-ray absorption spectroscopy in the water window. 44 Availability of higher photon energies for time-resolved experiments at XFEL facilities will allow probing deeper shells of such elements as magnesium or manganese, whose ionic form plays a vital role in the catalytic activity of DNA polymerase. 45,46…”

Probing and Steering Attosecond Electron Motion Using Tailored Ultrafast Laser Fields

“…The advanced ATAS methods shows the capability to probe the fastest electronic and structural dynamics in the broad class of complex molecules 65−67 and even in liquid. 68,69 In addition to the molecular bond breaking, the strong-field laser molecular interaction can also lead to reactions involving bond formation processes, where the relative yields of the reaction channels can be manipulated by tailored femtosecond laser pulses. 70 A typical example is the light-induced formation of trihydrogen cations H 3 + and its isotope D 3 + , which has attracted much attention in the past two decades due to the important role of H 3 + as the initiator for most chemical reactions in interstellar clouds, including those leading to water and organic compounds.…”

“…Recently, time-resolved X-ray ATAS was employed to probe the ultrafast electronic relaxation dynamics below 7 fs measured in C 2 H 4 molecules by extending the attosecond spectrum to the carbon K-edge. The advanced ATAS methods shows the capability to probe the fastest electronic and structural dynamics in the broad class of complex molecules − and even in liquid. , …”

Spectroscopically Visualizing the Evolution of Hydrogen-Bonding Interactions

“…This is a textbook property of proton transfer. Nevertheless, to date, rare experimental evidence has been collected to clearly describe the evolution of these H-bonds because of their transient timescales during this fundamental chemical process …”

“…Nevertheless, to date, rare experimental evidence has been collected to clearly describe the evolution of these Hbonds because of their transient timescales during this fundamental chemical process. 17 In response to this key issue, the primary challenge is the accurate regulation and long-term retention of various different H-bonding configurations during the proton transfer process, as illustrated in Scheme 1. Fortunately, a series of typical acidic materials (e.g., zeolites or heteropolyacids) 9,18 could be undoubtedly regarded as proton donors, whereas basic adsorbate molecules (e.g., acetone 19,20 or trimethylphosphine oxide (TMPO) 21−23 ) can be certainly considered to be proton acceptors.…”