Direct Probing of Vibrational Interactions in UiO-66 Polycrystalline Membranes with Femtosecond Two-Dimensional Infrared Spectroscopy

[Image: see text] UiO-66 is a benchmark metal–organic framework that holds great promise for the design of new functional materials. In this work, we perform two-dimensional infrared measurements on polycrystalline membranes of UiO-66 grown on c-sapphire substrates. We study the symmetric and antisymmetric stretch vibrations of the carboxylate groups of the terephthalate linker ions and find that these vibrations show a rapid energy exchange and a collective vibrational relaxation with a time constant of 1.3 ps. We also find that the symmetric vibration of the carboxylate group is strongly coupled to a vibration of the aromatic ring of the terephthalate ion. We observe that the antisymmetric carboxylate vibrations of different terephthalate linkers show rapid resonant (Förster) energy transfer with a time constant of ∼1 ps

Environmental issues in the structure and ultrafast kinetics of acids and hydrated protons

Protons (H+) are the most abundant cations in chemical processes. Proton itself is a superacid and in solution it always interacts either with the conjugate base or with the solvent molecules forming complex solvation structures. In this thesis we study the structure of hydrated protons in mixed solvents and in nanoconfined water. We also study the kinetics of proton transfer in bulk and nanoconfined water. The molecular events of proton transfer occur on picosecond time scale. Thus, to study structure and dynamics of various protonated species we use femtosecond pump-probe and two-dimensional infrared spectroscopy. In these methods we excite the vibrations of the solvated protons with an intense femtosecond pump pulse and measure the time dependent changes in the infrared absorption spectrum using a delayed weak probe pulse. From these spectral changes we can draw conclusions about the dynamics of energy redistribution within the proton solvation structures. In addition, from the dependence of the absorption change on the relative polarization of pump and probe pulses (parallel or perpendicular) we obtain the information about the relative orientation of the transition dipole moments of the vibrations and the molecular groups corresponding to them. This information helps us to determine geometry of molecular structures which participate in proton solvation and proton transfer

Peptide Side-COOH Groups Have Two Distinct Conformations under Biorelevant Conditions

The carboxyl (COOH) side chain groups of amino acids, such as aspartic acid, play an important role in biochemical processes, including enzymatic proton transport. In many theoretical studies, it was found that the (bio)chemical reactivity of the carboxyl group strongly depends on the conformation of this group. Interestingly, up to now there has been no experimental investigation of the geometry and the stability of different COOH conformers under biorelevant conditions. Here, we investigate the conformational isomerism of the side chain COOH group of N-acetyl aspartic acid amide using polarization-resolved two-dimensional infrared spectroscopy. We find that the carboxyl group shows two distinct near-planar conformers (syn and anti) when dissolved in water at room temperature. Both conformers are significantly populated in aqueous solution (75 +/- 10% and 25 +/- 10% for syn and anti, respectively). Molecular dynamics simulations show that the anti conformer interacts more strongly with water molecules than the syn conformer, explaining why this conformer is significantly present in aqueous solution