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

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.

Microscopic origin of pressure-induced phase-transitions in urea: a detailed investigation through first principles calculations

The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4[combining macron]21m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ-X direction. Another phase transition from the P212121 → P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U-R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N-HO bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source

We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds O=C(NH₂)₂, CaCl₂, and NaNO₃ using flatjet technology. We monitor the absorption of the molecular samples between the carbon (∼280 eV) and nitrogen (∼400 eV) K-edges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way toward table-top femtosecond, solution-phase soft X-ray absorption spectroscopy in the water window.