More Like this

1. Vibrational relaxation by methylated xanthines in solution: Insights from 2D IR spectroscopy and calculations

   https://doi.org/10.1063/5.0135412  

   Hanes, Alex T. ; Grieco, Christopher ; Lalisse, Remy F. ; Hadad, Christopher M. ; Kohler, Bern ( January 2023 , The Journal of Chemical Physics)

   Two-dimensional infrared (2D IR) spectroscopy, infrared pump–infrared probe spectroscopy, and density functional theory calculations were used to study vibrational relaxation by ring and carbonyl stretching modes in a series of methylated xanthine derivatives in acetonitrile and deuterium oxide (heavy water). Isotropic signals from the excited symmetric and asymmetric carbonyl stretch modes decay biexponentially in both solvents. Coherent energy transfer between the symmetric and asymmetric carbonyl stretching modes gives rise to a quantum beat in the time-dependent anisotropy signals. The damping time of the coherent oscillation agrees with the fast decay component of the carbonyl bleach recovery signals, indicating that this time constant reflects intramolecular vibrational redistribution (IVR) to other solute modes. Despite their similar frequencies, the excited ring modes decay monoexponentially with a time constant that matches the slow decay component of the carbonyl modes. The slow decay times, which are faster in heavy water than in acetonitrile, approximately match the ones observed in previous UV pump–IR probe measurements on the same compounds. The slow component is assigned to intermolecular energy transfer to solvent bath modes from low-frequency solute modes, which are populated by IVR and are anharmonically coupled to the carbonyl and ring stretch modes. 2D IR measurements indicate that the carbonyl stretching modes are weakly coupled to the delocalized ring modes, resulting in slow exchange that cannot explain the common solvent-dependence. IVR is suggested to occur at different rates for the carbonyl vs ring modes due to differences in mode-specific couplings and not to differences in the density of accessible states.

    

   more » « less
2. Ruthenium hydrides encapsulated in sol–gel glasses exhibit new ultrafast vibrational dynamics

   https://doi.org/10.1063/5.0082752  

   Pyles, Cynthia G. ; Patrow, Joel G. ; Cheng, Yukun ; Tonks, Ian A. ; Massari, Aaron M. ( March 2022 , The Journal of Chemical Physics)

   Vibrational dynamics were measured by IR pump–probe spectroscopy and two-dimensional IR spectroscopy for triruthenium dodecacarbonyl and the undecacarbonyl hydride that forms when it is encapsulated in an alumina sol–gel glass. For comparison, a triruthenium undecacarbonyl hydride salt was also synthesized and studied in neat solution to identify the potential influence of the confined solvent environment on the dynamics experienced by carbon monoxide ligands. The vibrational lifetime was found to be significantly decreased for both hydride species relative to the dodecacarbonyl compound. Conversely, spectral diffusion of the CO vibrations was measured to be faster for the parent compound. The most significant dynamic changes occurred upon transformation from the starting compound to the hydride, while only minor differences were observed between the dynamics of the freely dissolved and sol–gel encapsulated hydrides. The results suggest that the structural change to the hydride has the largest impact on the dynamics and that its improved catalytic properties likely do not originate from confined solvent effects.

    

   more » « less
3. 2D spectroscopies from condensed phase dynamics: Accessing third-order response properties from equilibrium multi-time correlation functions

   https://doi.org/10.1063/5.0107087  

   Jung, Kenneth A. ; Markland, Thomas E. ( September 2022 , The Journal of Chemical Physics)

   The third-order response lies at the heart of simulating and interpreting nonlinear spectroscopies ranging from two-dimensional infrared (2D-IR) to 2D electronic (2D-ES), and 2D sum frequency generation (2D-SFG). The extra time and frequency dimensions in these spectroscopic techniques provide access to rich information on the electronic and vibrational states present, the coupling between them, and the resulting rates at which they exchange energy that are obscured in linear spectroscopy, particularly for condensed phase systems that usually contain many overlapping features. While the exact quantum expression for the third-order response is well established, it is incompatible with the methods that are practical for calculating the atomistic dynamics of large condensed phase systems. These methods, which include both classical mechanics and quantum dynamics methods that retain quantum statistical properties while obeying the symmetries of classical dynamics, such as LSC-IVR, centroid molecular dynamics, and Ring Polymer Molecular Dynamics (RPMD), naturally provide short-time approximations to the multi-time symmetrized Kubo transformed correlation function. Here, we show how the third-order response can be formulated in terms of equilibrium symmetrized Kubo transformed correlation functions. We demonstrate the utility and accuracy of our approach by showing how it can be used to obtain the third-order response of a series of model systems using both classical dynamics and RPMD. In particular, we show that this approach captures features such as anharmonically induced vertical splittings and peak shifts while providing a physically transparent framework for understanding multidimensional spectroscopies. 

   more » « less
4. Inhibition of vibrational energy flow within an aromatic scaffold via heavy atom effect

   https://doi.org/10.1063/5.0153760  

   Hassani, Majid ; Mallon, Christopher J. ; Monzy, Judith N. ; Schmitz, Andrew J. ; Brewer, Scott H. ; Fenlon, Edward E. ; Tucker, Matthew J. ( June 2023 , The Journal of Chemical Physics)

   The regulation of intramolecular vibrational energy redistribution (IVR) to influence energy flow within molecular scaffolds provides a way to steer fundamental processes of chemistry, such as chemical reactivity in proteins and design of molecular diodes. Using two-dimensional infrared (2D IR) spectroscopy, changes in the intensity of vibrational cross-peaks are often used to evaluate different energy transfer pathways present in small molecules. Previous 2D IR studies of para-azidobenzonitrile (PAB) demonstrated that several possible energy pathways from the N3 to the cyano-vibrational reporters were modulated by Fermi resonance, followed by energy relaxation into the solvent [Schmitz et al., J. Phys. Chem. A 123, 10571 (2019)]. In this work, the mechanisms of IVR were hindered via the introduction of a heavy atom, selenium, into the molecular scaffold. This effectively eliminated the energy transfer pathway and resulted in the dissipation of the energy into the bath and direct dipole–dipole coupling between the two vibrational reporters. Several structural variations of the aforementioned molecular scaffold were employed to assess how each interrupted the energy transfer pathways, and the evolution of 2D IR cross-peaks was measured to assess the changes in the energy flow. By eliminating the energy transfer pathways through isolation of specific vibrational transitions, through-space vibrational coupling between an azido (N3) and a selenocyanato (SeCN) probe is facilitated and observed for the first time. Thus, the rectification of this molecular circuitry is accomplished through the inhibition of energy flow using heavy atoms to suppress the anharmonic coupling and, instead, favor a vibrational coupling pathway. 

   more » « less
5. 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. 

   more » « less