Ultrafast vibrational dynamics of a solute correlates with dynamics of the solvent

Abstract

Two-dimensional infrared (2D-IR) spectroscopy is used to measure the spectral dynamics of the metal carbonyl complex cyclopentadienyl manganese tricarbonyl (CMT) in a series of linear alkyl nitriles. 2D-IR spectroscopy provides direct readout of solvation dynamics through spectral diffusion, probing the decay of frequency correlation induced by fluctuations of the solvent environment. 2D-IR simultaneously monitors intramolecular vibrational energy redistribution (IVR) among excited vibrations, which can also be influenced by the solvent through the spectral density rather than the dynamical friction underlying solvation. Here, we report that the CMT vibrational probe reveals solvent dependences in both the spectral diffusion and the IVR time scales, where each slows with increased alkyl chain length. In order to assess the degree to which solute–solvent interactions can be correlated with bulk solvent properties, we compared our results with low-frequency dynamics obtained from optical Kerr effect (OKE) spectroscopy—performed by others—on the same nitrile solvent series. We find excellent correlation between our spectral diffusion results and the orientational dynamics time scales from OKE. We also find a correlation between our IVR time scales and the amplitudes of the low-frequency spectral densities evaluated at the 90-cm−1 energy difference, corresponding to the gap between the two strong vibrational modes of the carbonyl probe. 2D-IR and OKE provide complementary perspectives on condensed phase dynamics, and these findings provide experimental evidence that at least at the level of dynamical correlations, some aspects of a solute vibrational dynamics can be inferred from properties of the solvent.

Solvent-Dependent Structural Dynamics in the Ultrafast Photodissociation Reaction of Triiodide Observed with Time-Resolved X-ray Solution Scattering

“…Figure 5, top, shows the GP population for acetonitrile (blue crosses), modeled by a biexponential decay convoluted with a Gaussian function (blue solid line). The GP population decay is observed to be well captured by this biexponential fit, with the two time constants being 0.4(1) ps and 38 (6) ps. The population of NG (filled dots) survives for the time scales investigated here.…”

“…Ultrafast photoinduced chemical reactions in solution and condensed-phase environments are the underlying phenomena for many processes, e.g., for photosynthesis and vision in biological systems as well as for photocatalysis and for dye-sensitized solar cells. Despite the importance of such reactions, the influence of the solvent environment on structural dynamics in solution-phase reaction is not yet fully understood. − Solvent–solute interactions are known to affect the potential energy landscapes, a phenomenon which can be experimentally observed e.g., by shifts in absorption spectra and changes in lifetimes of different electronic states as a function of solvent type. − The solvent is therefore also expected to influence the earliest dynamics in solution-phase reactions, fundamentally impacting their course. To understand the solvent effect on the subpicosecond reaction events, the direct observation of reactions on these time scales is necessary.…”

Anisotropic Dynamics of an Interfacial Enzyme Active Site Observed Using Tethered Substrate Analogs and Ultrafast 2D IR Spectroscopy

“…Furthermore, dual-frequency strategies 107 using the C≡N/N3 and imido (~1730 cm -1 ) modes as reporters could provide insight into the rates of intramolecular vibrational energy redistribution (IVR) in each PhENR variant, as well as how it depends on intramolecular coupling 82 and perturbations by bath modes. 25 As research progresses, will explore how the spatial distribution of label dynamics/disorder in PhENR varies as a function of FMN redox state, mutations, protein concentration, 57,58,89,110 pressure and temperature, 61,63,111 and solvent composition. 41,42,44,54,111 We will also explore communication between active site dynamics and fluctuations at distal sites 12 by employing genetically-encoded labels in addition to inhibitors and substrate analogs.…”

“…23 2D IR studies of small molecules and metal complexes in solution have provided insight into effects of viscosity on isomerization 24 as well as possible relationships between short-timescale equilibrium dynamics and energy transfer in reactivity. 25,26 For enzymes and other macromolecules, vibrational probes that absorb in a 'transparent window' of the mid-IR spectrum are attached to amino acid side chains, reactive cofactors, or substrate/TS analogs. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] Frequency fluctuation correlation functions (FFCFs) extracted from spectral diffusion measurements provide detailed descriptions of the timescales and magnitudes of local fluctuations.…”

Tracking Energy Transfer across a Platinum Center

“…Vibrational relaxation and thermalization in molecules occur via an intramolecular vibrational energy redistribution (IVR) process, which has been studied using a variety of experimental methods. − Energy transfer and thermalization in covalent networks − is better understood than the transfer across a metal center in a transition-metal complex for which only a few studies were reported. − The coordination bonds at the metal center are often weak resulting in very inefficient energy transfer across the metal. For example, the lifetime of CO stretching modes in some metal carbonyls reaches 1 ns. − A similar situation is encountered when an organic compound is attached to a metal or semiconductor surface. The surface binding energy is often small resulting in weak thermal conductivity through the interface. − In contrast to surfaces, transition-metal complexes feature high-frequency vibrational modes on each side of the metal atom, which can potentially participate in the exchange of high-frequency quanta across the metal center.…”