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1. Water Dynamics and Structure of Highly Concentrated LiCl Solutions Investigated Using Ultrafast Infrared Spectroscopy

   https://doi.org/10.1021/jacs.2c00616  

   Roget, Sean A. ; Carter-Fenk, Kimberly A. ; Fayer, Michael D. ( March 2022 , Journal of the American Chemical Society)

   NA (Ed.)

   Using ultrafast polarization-selective pump-probe spectroscopy (PSPP) of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to 4 waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions were obtained through analysis of the frequency-dependent anisotropy decays. It was found that the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding when the salt concentration was increased. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. Finally, the structural evolution of the ionic medium was observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to pure water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion, 13 ps, is virtually identical to the time for complete orientation randomization of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network. Using ultrafast polarization-selective pump-probe spectroscopy (PSPP) of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to 4 waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions were obtained through analysis of the frequency-dependent anisotropy decays. It was found that the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding when the salt concentration was increased. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. Finally, the structural evolution of the ionic medium was observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to pure water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion, 13 ps, is virtually identical to the time for complete orientation randomization of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network. 

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2. Origins of the diffuse shared proton vibrational signatures in proton-coupled electron transfer model dyad complexes

   https://doi.org/10.1063/5.0122777  

   Chen, Liangyi ; Ma, Zifan ; Fournier, Joseph A. ( October 2022 , The Journal of Chemical Physics)

   Phenol-benzimidazole and phenol-pyridine dyad complexes have served as popular model systems for the study of proton-coupled electron transfer (PCET) kinetics in solution-phase experiments. Interpretation of measured PCET rates in terms of key structural parameters, such as the H-bond donor–acceptor distance, however, remains challenging. Herein, we report vibrational spectra in the electronic ground state for a series of phenol-benzimidazole and phenol-pyridine complexes isolated and cryogenically cooled in an ion trap. The four models studied each display highly red-shifted and broadened OH stretching transitions that arise from strong H-bonding interactions between the phenol OH group and the basic N site on benzimidazole/pyridine rings. The OH stretch transition in each model displays relatively strong absorption onsets near 2500 cm −1 with broad shoulders that extend asymmetrically to higher frequencies over hundreds of wavenumbers. In contrast, the deuterated isotopologues yield much weaker OD stretch transitions that appear symmetrically broadened. The spectral breadth and shape of the OD stretch transitions are ascribed to variations in OD stretch frequencies that arise from zero-point distributions in the proton donor–acceptor low-frequency soft mode vibration. The asymmetric structure of the OH stretch transitions is attributed to a set of combination bands between the OH stretch and a series of low-frequency H-bond soft modes. The spectra and modeling highlight the importance of OH stretch-soft mode couplings, which are thought to play important roles in PCET and proton transfer dynamics. 

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3. Structural aspects of the topological model of the hydrogen bond in water on auto-dissociation via proton transfer

   https://doi.org/10.1039/c8cp02592d  

   Lentz, Jesse ; Garofalini, Stephen H. ( January 2018 , Physical Chemistry Chemical Physics)

   Molecular dynamics (MD) simulations were used to investigate the structure and lifetimes of hydrogen bonds and auto dissociation via proton transfer in bulk water using a reactive and dissociative all-atom potential that has previously been shown to match a variety of water properties and proton transfer. Using the topological model, each molecule's donated and accepted hydrogen bonds were labeled relative to the other hydrogen bonds on neighboring waters, providing a description of the effect of these details on the structure, dynamics and autoionization of water molecules. In agreement with prior data, asymmetric bonding at the sub-100 femtosecond timescale is observed, as well as the existence of linear, bifurcated, and dangling hydrogen bonds. The lifetime of the H-bond, 2.1 ps, is consistent with experimental data, with short time librations on the order of femtoseconds. The angular correlation functions, the presence of a second shell water entering the first shell, and OH vibrational stretch frequencies were all consistent with experiment or ab initio calculations. The simulations show short-lived (femtoseconds) dissociation of a small fraction of water molecules followed by rapid recombination. The role of the other H-bonds to the acceptor and on the donor plays an important part in proton transfer between the molecules in auto dissociation and is consistent with the role of a strong electric field caused by local (first and second shell) waters on initiating dissociation. The number of H-bonds to the donor water is 4.3 per molecule in the simulations, consistent with previous data regarding the number of hydrogen bonds required to generate this strong local electric field that enhances dissociation. The continuous lifetime autocorrelation function of the H-bond for those molecules that experience dissociation is considerably longer than that for all molecules that show no proton transfer. 

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4. Ultrafast Processes Occurring in Radiolysis of Highly Concentrated Solutions of Nucleosides/Tides

   https://doi.org/10.3390/ijms20194963  

   MA, Jun ; Denisov, Sergey A. ; Adhikary, Amitava ; Mostafavi, Mehran ( October 2019 , International Journal of Molecular Sciences)

   null (Ed.)

   Among the radicals (hydroxyl radical (•OH), hydrogen atom (H•), and solvated electron (esol−)) that are generated via water radiolysis, •OH has been shown to be the main transient species responsible for radiation damage to DNA via the indirect effect. Reactions of these radicals with DNA-model systems (bases, nucleosides, nucleotides, polynucleotides of defined sequences, single stranded (ss) and double stranded (ds) highly polymeric DNA, nucleohistones) were extensively investigated. The timescale of the reactions of these radicals with DNA-models range from nanoseconds (ns) to microseconds (µs) at ambient temperature and are controlled by diffusion or activation. However, those studies carried out in dilute solutions that model radiation damage to DNA via indirect action do not turn out to be valid in dense biological medium, where solute and water molecules are in close contact (e.g., in cellular environment). In that case, the initial species formed from water radiolysis are two radicals that are ultrashort-lived and charged: the water cation radical (H2O•+) and prethermalized electron. These species are captured by target biomolecules (e.g., DNA, proteins, etc.) in competition with their inherent pathways of proton transfer and relaxation occurring in less than 1 picosecond. In addition, the direct-type effects of radiation, i.e., ionization of macromolecule plus excitations proximate to ionizations, become important. The holes (i.e., unpaired spin or cation radical sites) created by ionization undergo fast spin transfer across DNA subunits. The exploration of the above-mentioned ultrafast processes is crucial to elucidate our understanding of the mechanisms that are involved in causing DNA damage via direct-type effects of radiation. Only recently, investigations of these ultrafast processes have been attempted by studying concentrated solutions of nucleosides/tides under ambient conditions. Recent advancements of laser-driven picosecond electron accelerators have provided an opportunity to address some long-term puzzling questions in the context of direct-type and indirect effects of DNA damage. In this review, we have presented key findings that are important to elucidate mechanisms of complex processes including excess electron-mediated bond breakage and hole transfer, occurring at the single nucleoside/tide level. 

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5. Ruthenium Electrodeposition from Water-in-Salt Electrolytes and the Influence of Tetrabutylammonium

   https://doi.org/10.1149/1945-7111/ab847e  

   Sides, William D. ; Huang, Qiang ( April 2020 , Journal of The Electrochemical Society)

   The use of water-in-salt electrolytes is evaluated for the electrodeposition of metallic ruthenium. The mechanisms of proton reduction inhibition by concentrated LiCl and dilute tetrabutylammonium is evaluated. Concentrated LiCl is found to disrupt the hydrogen bonding network within the solution bulk, whereas TBA is found to adsorb onto the electrode surface, blocking proton access. Ruthenium exists as a different complexed species in water-in-salt electrolytes vs dilute aqueous electrolytes, leading to a −300 mV shift in the deposition onset potential. Greater current efficiencies of Ru deposition can be obtained when depositing at proton overpotentials by the use of water-in-salt electrolytes, and TBA can offer further improvements. The grain structure and resistivities of Ru thin films are studied.

    

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