Hydrophobicity with atomic resolution: Steady-state and ultrafast X-ray absorption and molecular dynamics studies

Abstract

Static and time-resolved X-ray absorption spectroscopy (XAS) is used to probe the solvent shell structure around iodide and iodine. In particular, we characterize the changes ob served upon electron abstraction of aqueous iodide, which reflects the transition from hydrophilic to hydrophobic solvation after impulsive electron abstraction from iodide. The static spectrum of aqueous iodide, which is analyzed using quantum mechani cal/molecular mechanics (QM/MM) molecular dynamics (MD) simulations, indicates that the hydrogens of the closest water molecules point toward the iodide, as expected for hydrophilic solvation. In addition, these simulations demonstrate a small anisotropy in the solvent shell. Following electron abstraction, most of the water molecules move away from iodine, while one comes closer to form a complex with it that survives 3-4 ps. This lifetime is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform a hydrogen bond network in the hydrophobic solvation shell.

Enhancing the analysis of disorder in X-ray absorption spectra: application of deep neural networks to T-jump-X-ray probe experiments

“…7,8 In T-jump pump/X-ray probe experiments, T-jump spectroscopy and XS are coupled together to deliver direct structural insight into thermally-activated chemical reactions; such experiments have, for example, been used to study the response of the structure of water following the transfer of heat via NIR excitation. 9,10 The use of X-ray absorption near-edge structure (XANES) [11][12][13][14][15][16][17] and/or extended X-ray absorption fine structure (EXAFS) [18][19][20][21][22][23] measurements to characterise solvent and/or solvation structures is well established. Consequently, T-jump pump/X-ray probe experiments have the potential to deliver information on the fundamental structural dynamics of thermally-activated processes occurring in solution, such as ligand exchange, which are common in chemical and biological systems.…”

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…233,234 They could thus monitor the change from hydrophilic solvation (around iodide) to hydrophobic solvation (around iodine), which was supported by quantum and classical molecular dynamics simulations and DFT calculations. [234][235][236] For more complex systems, advanced Hybrid DFT/classical or full ab-initio molecular dynamics simulations of the solvent effect on the excited state dynamics of systems such as [ combined XES, XRS and XAS to observe the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution in the case of [Fe II (bpy)3]. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes, discussed in § 4.2, are long completed as well as the solvent heating due to the initial energy dissipation from the excited HS molecule.…”

Nonequilibrium quantum solvation with a time-dependent Onsager cavity

“…The formation of a new shell of water molecules around iodine is more involved and occurs within 3-4 ps. A single water molecule is pulled towards the bulk of the solvent 15 , which has been confirmed by quantum chemical and molecular dynamics simulations. Alternative approaches formulate equations of motion for the solvent polarization charges and couple them to a real-time time-dependent density functional theory 16 or a timedependent configuration interaction description 17 .…”

“…The time-dependent rearrangement of the molecular cavity has also been revealed for aqueous iodide by a joint experimental and theoretical analysis 15 . Static and timeresolved X-ray absorption spectroscopy have disclosed a transition from hydrophilic to hydrophobic solvation after an electron has been removed from the iodide.…”