1) Dynamics of hydrated proton clusters
We studied the vibrarioanl spectrum and ultrafast relaxation dynamics of water clusters containing protons that are dissolved in acetonitrile. These dynamics were measured with femtosecond mid-infrared pump-probe spectroscopy.
We found that water molecules in the core and in the shell of the hydrated proton cluster show quite different vibrational frequencies and vibrational relaxation rates.
For the water molecules in the core we observe an ultrafast energy relaxation that leads to an ultrafast heating of the local aqueous environment of the proton. For the water molecules in the shellwe observe
a relatively slow vibrational relaxation with a T1 time constant ranging from 0.22 to 0.37 ps. (J. Phys. Chem. B 123, 6222-6228 (2019).
We also investigated the structure and dynamics of proton solvation structures in mixed water/dimethyl sulfoxide (DMSO) solvents using two-color mid-infrared femtosecond pump-probe spectroscopy. At a water fraction below 20%, protons are mainly solvated as (DMSO-H)(+) and (DMSO-H)(+) -H2O structures. We find that excitation of the OH-stretch vibration of the proton in (DMSO-H)(+)-H2O structures leads to an ultrafast contraction of the hydrogen bond between (DMSO-H)(+) and H2O. This excited state relaxes rapidly with T1 = 95 +/- 10 fs and leads in part to a strong local heating effect and in part to predissociation of the protonated cluster into (DMSO-H)(+) and water monomers (J. Phys. Chem. B 122,10005-10013 (2018).
2) Dynamics of protons in water nanodroplets
We studied the vibrational and structural dynamics of hydrated protons in reverse micelles using polarization-resolved femtosecond infrared transient absorption spectroscopy. We find that in small nanodroplets with a diameter <4 nm proton hopping is more than 10 times slower than in bulk water. Even in big nanodroplets of ~8 nm diameter, we find that the rate of proton hopping is slowed down by at least 4 times in comparison to bulk water (manuscript in preparation). For the anionic reverse micelles we observe additional transient absorption bands that we assign to the complexes formed by protons and the negatively charged sulfonate group of the AOT surfactant molecules (manuscript in preparation). ACS Cent. Sci. 2020, 6 1150–1158 (2020)
3) Orientation and location of molecules and ions near the surface of water
We observed that the orientation of urea molecules close to a water surface is strongly dependent on the sign of the charge of the surface and closely follows the orientation of the neighboring water molecules. J. Phys. Chem. Lett. 12, 10823–10828 (2021).
We also found that protons are located surprisingly close to the surface of the solution, much closer than their negative counterions, like for instance Cl- or Br-. The different locations of the protons and their counterions create a strong electric field close to the surface that strongly orients water molecules.
4) Vibrational response of surface water
We found that the surface vibrational response of the water bending mode consists of both an interfacial dipole and a bulk quadrupolar response. We thus found that the bending mode of water can provide structural information on both the surface and the bulk of aqueous solutions. Phys. Rev. Lett. 127, 116001 (2021)
5) Observation of distinct conformers of the carboxylic acid group
We investigated the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of 60 with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature The relative abundance of the conformers strongly depends on the polarity of the environment and on the possibility of forming an intramolecular hydrogen bond. J. Phys. Chem. Lett. 10, 12, 3217–3222 (2019); J. Phys. Chem. Lett. 11, 9, 3466–3472 (2020); Communications Chemistry 3, 84 (2020)
