Ion–molecule complex dynamics as well as water dynamics in concentrated lithium chloride (LiCl) solutions are examined using ultrafast two-dimensional infrared (2D IR) spectroscopy with the CN stretching mode of methyl thiocyanate (MeSCN) as the vibrational probe. In pure water, MeSCN has a narrow symmetric absorption line shape. 2D IR spectral diffusion measurements of the CN stretch give the identical time dependence of water dynamics, as previously observed using the OD stretch of HOD in H2O. In concentrated LiCl solutions, the IR absorption spectrum of MeSCN displays two distinct peaks, one corresponding to water H-bonded to the N lone pair of MeSCN (W) and the other corresponding to Li+ associated with the N (L). These two species are in equilibrium, and switching of the CN bonding partner from Li+ to H2O and vice versa was observed and explicated with 2D IR chemical exchange spectroscopy. The MeSCN·Li+ complex dissociation time constant, τLW, and the MeSCN·H2O dissociation time constant, τWL, were determined. The observed τLW chemical exchange dissociation time constant changes from 60 to 40 ps as the LiCl concentration decreases from ∼10.7 to ∼7.7 M, mainly due to the increase of the water concentration as the LiCl concentration is reduced. The observed time constants are independent of the model for the chemical reaction. With the assumption of a simple chemical equation, MeSCN·Li+ + H2O ⇄ MeSCN·H2O + Li+, the equilibrium equation rate constants were obtained from the observed chemical exchange time constants. It was determined that the equilibrium rate constants barely change even though the viscosity changes by a factor of 2 and the ionic strength changes by a factor of 1.4. Extrapolation to dilute LiCl solution estimates the τLW to be ∼30 ps. The orientational relaxation (anisotropy decay) of both the W and L complexes was measured using polarization selective 2D IR experiments. The lithium-bonded species undergoes orientational relaxation ∼3 times slower than the water-bonded species in each LiCl solution studied. The difference demonstrates the distinct interactions with the medium experienced by the neutral and charged species in the concentrated salt solutions.

Functionalized self-assembled monolayers (SAMs) are the focus of ongoing investigations because they can be chemically tuned to control their structure and dynamics for a wide variety of applications, including electrochemistry, catalysis, and as models of biological interfaces. Here we combine reflection 2D infrared vibrational echo spectroscopy (R-2D IR) and molecular dynamics simulations to determine the relationship between the structures of functionalized alkanethiol SAMs on gold surfaces and their underlying molecular motions on timescales of tens to hundreds of picoseconds. We find that at higher head group density, the monolayers have more disorder in the alkyl chain packing and faster dynamics. The dynamics of alkanethiol SAMs on gold are much slower than the dynamics of alkylsiloxane SAMs on silica. Using the simulations, we assess how the different molecular motions of the alkyl chain monolayers give rise to the dynamics observed in the experiments.self-assembled monolayer | dynamics | 2D IR spectroscopy | MD simulation S elf-assembled monolayers (SAMs) on planar metal surfaces enable the tailoring of interfacial properties by functionalization of the alkyl chains. SAMs formed by alkanethiol chains on gold surfaces are of particular interest due to the ordered packing of the chains, chemical stability, and facile methods of preparation, as well as the diverse array of chemical functionalization that can be added (1). The properties of SAMs on gold have led to applications including electrochemical devices (2), surface patterning (3), model biological surfaces (4), and heterogeneous catalysis (5). In many of these applications, the interfacial properties of the monolayer are determined largely by the particular head group linked at the terminal site of the alkyl chain.The structure of SAMs on gold has been well characterized by scanning probe microscopy (6), helium diffraction (7), X-ray photoelectron spectroscopy (8), sum-frequency generation spectroscopy (SFG) (9), and linear infrared spectroscopy (10). However, to determine how the physical and chemical properties of SAMs are related to their microscopic dynamics and structure and the influence of head groups present in most applications, fast time-resolved experimental techniques, with sufficient selectivity and sensitivity, are required to measure the structural dynamics of a monolayer of molecules on the appropriate picosecond (ps) timescale.Two-dimensional infrared vibrational echo spectroscopy (2D IR) provides the necessary observables by measuring spectral diffusion, i.e., the time-dependent evolution of the probe vibrational frequency in response to structural fluctuations of the chemical environment (11-13). To use 2D IR to investigate monolayer dynamics requires selectivity for the interfacial region. One method to achieve this is to combine 2D IR spectroscopy with SFG (14, 15), which requires a vibrational mode that has both a large IR transition dipole and a large Raman cross-section. However, for a monolayer functionalized with the vibra...

The dynamic nature of hydrogen bonding between a molecular anion, selenocyanate (SeCN(-)), and water in aqueous solution (D2O) is addressed using FT-IR spectroscopy, two-dimensional infrared (2D IR) vibrational echo spectroscopy, and polarization selective IR pump-probe (PSPP) experiments performed on the CN stretching mode. The CN absorption spectrum is asymmetric with a wing on the low frequency (red) side of the line in contrast to the spectrum in the absence of hydrogen bonding. It is shown that the red wing is the result of an increase in the CN stretch transition dipole moment due to the effect of hydrogen bonding (non-Condon effect). This non-Condon effect is similar in nature to observations on pure water and other nonionic systems where hydrogen bonding enhances the extinction coefficient. The 2D IR measurements of spectral diffusion (solvent structural evolution) yield a time constant of 1.5 ps, which is within error the same as that of the OH stretch of HOD in D2O (1.4 ps). The orientational relaxation of SeCN(-) measured by PSPP experiments is long (4.04 ps) compared to the spectral diffusion time. The population decay at or near the absorption line center is a single-exponential decay of 37.4 ± 0.3 ps, the vibrational lifetime. However, on the red side of the line the decay is biexponential with a low amplitude, fast component; on the blue side of the line there is a low amplitude, fast growth followed by the lifetime decay. Both of the fast components have 1.5 ps time constants, which is the spectral diffusion time. The fast components of the population decays are the results of the non-Condon effect that causes the red side of the line to be over pumped by the pump pulse. Spectral diffusion then produces the fast decay component on the red side of the line and the growth on the blue side of the line as the excess initial population on the red side produces a net population flow from red to blue.

Polymeric hydrogels have wide applications including electrophoresis, biocompatible materials, water superadsorbents, and contact lenses. The properties of hydrogels involve the poorly characterized molecular dynamics of water and solutes trapped within the three-dimensional cross-linked polymer networks. Here we apply ultrafast two-dimensional infrared (2D IR) vibrational echo and polarization-selective pump-probe (PSPP) spectroscopies to investigate the ultrafast molecular dynamics of water and a small molecular anion solute, selenocyanate (SeCN), in polyacrylamide hydrogels. For all mass concentrations of polymer studied (5% and above), the hydrogen-bonding network reorganization (spectral diffusion) dynamics and reorientation dynamics reported by both water and SeCN solvated by water are significantly slower than in bulk water. As the polymer mass concentration increases, molecular dynamics in the hydrogels slow further. The magnitudes of the slowing, measured with both water and SeCN, are similar. However, the entire hydrogen-bonding network of water molecules appears to slow down as a single ensemble, without a difference between the core water population and the interface water population at the polymer-water surface. In contrast, the dissolved SeCN do exhibit two-component dynamics, where the major component is assigned to the anions fully solvated in the confined water nanopools. The slower component has a small amplitude which is correlated with the polymer mass concentration and is assigned to adsorbed anions strongly interacting with the polymer fiber networks.

Proton transfer in water is ubiquitous and a critical elementary event that, via proton hopping between water molecules, enables protons to diffuse much faster than other ions. The problem of the anomalous nature of proton transport in water was first identified by Grotthuss over 200 years ago. In spite of a vast amount of modern research effort, there are still many unanswered questions about proton transport in water. An experimental determination of the proton hopping time has remained elusive due to its ultrafast nature and the lack of direct experimental observables. Here, we use two-dimensional infrared spectroscopy to extract the chemical exchange rates between hydronium and water in acid solutions using a vibrational probe, methyl thiocyanate. Ab initio molecular dynamics (AIMD) simulations demonstrate that the chemical exchange is dominated by proton hopping. The observed experimental and simulated acid concentration dependence then allow us to extrapolate the measured single step proton hopping time to the dilute limit, which, within error, gives the same value as inferred from measurements of the proton mobility and NMR line width analysis. In addition to obtaining the proton hopping time in the dilute limit from direct measurements and AIMD simulations, the results indicate that proton hopping in dilute acid solutions is induced by the concerted multi-water molecule hydrogen bond rearrangement that occurs in pure water. This proposition on the dynamics that drive proton hopping is confirmed by a combination of experimental results from the literature.