Vibrational Frequency Fluctuation of Ions in Aqueous Solutions Studied by Three-Pulse Infrared Photon Echo Method

Abstract

In liquid water, hydrogen bonds form three-dimensional network structures, which have been modeled in various molecular dynamics simulations. Locally, the hydrogen bonds continuously form and break, and the network structure continuously fluctuates. In aqueous solutions, the water molecules perturb the solute molecules, resulting in fluctuations of the electronic and vibrational states. These thermal fluctuations are fundamental to understanding the activation processes in chemical reactions and the function of biopolymers. In this Account, we review studies of the vibrational frequency fluctuations of solute molecules in aqueous solutions using three-pulse infrared photon echo experiments. For comparison, we also briefly describe dynamic fluorescence Stokes shift experiments for investigating solvation dynamics in water. The Stokes shift technique gives a response function, which describes the energy relaxation in the nonequilibrium state and corresponds to the transition energy fluctuation of the electronic state at thermal equilibrium in linear response theorem. The dielectric response of water in the megahertz to terahertz frequency region is a key physical quantity for understanding both of these frequency fluctuations because of the influence of electrostatic interactions between the solute and solvent. We focus on the temperature dependence of the three experiments to discuss the molecular mechanisms of both the frequency fluctuations in aqueous solutions. We used a biexponential function with sub-picosecond and picosecond time constants to characterize the time-correlation functions of both the vibrational and electronic frequency fluctuations. We focus on the slower component, with time constants of 1-2 ps for both the frequency fluctuations at room temperature. However, the temperature dependence and isotope effect for the time constants differ for these two types of fluctuations. The dielectric interactions generally describe the solvation dynamics of polar solvents, and hydrodynamic theory can describe the slow component for the electronic states. Compared with the slow component of the solvation dynamics, however, the picosecond component for the vibrational frequency fluctuations is less sensitive to temperature. Therefore, the slow component of the vibrational frequency fluctuation is determined by different underlying dynamics, which are important for the solvation dynamics of the electronic state. The time constant for the picosecond component for the vibrational frequency fluctuation does not significantly depend on the solute. We propose that the vibrational frequency fluctuates because of the constant structural changes in the hydrogen-bonding network of water molecules around the solute.

Structural dynamics of nitrosylruthenium isomeric complexes studied with steady-state and transient pump-probe infrared spectroscopies

“…The original coordinates of atoms in the cis-1, cis-2 and trans isomeric complexes were obtained from the crystal structures determined with X-ray diffraction [26][27][28][29][30][31]. Visualization was performed with GaussView 5.0 [33].…”

“…On the other hand, time-resolved nonlinear infrared spectroscopy, as a dynamical tool, can be used to unravel the structural dynamics of condensed-phase molecules on a wide range of time scales [21][22][23][24][25][26][27]. In our recent work, ultrafast two-dimensional infrared (2D IR) spectroscopy has been used to characterized spectral diffusion time, a key parameter for equilibrium structural dynamics that is closely associated with the frequency time-correlation function, of the [Ru(OAc)(2cqn) 2 NO] (H2cqn = 2-chloro-8-quinolinol) isomers in solution phases [28].…”

Vibrational dynamics of azide-derivatized amino acids studied by nonlinear infrared spectroscopy

“…For these two decades, there have been considerable efforts to study vibrational dynamics of solute in water by using ultrafast spectroscopy in the infrared (IR) region. [3][4][5] In these studies, vibrational dynamics such as frequency fluctuations, vibrational energy relaxations, and orientational relaxations have been rigorously investigated.…”

Simultaneously Probing Two Ultrafast Condensed‐Phase Molecular Symmetry Breaking Events by Two‐Dimensional Infrared Spectroscopy

“…In recent years, 2D IR spectroscopy has become a valuable tool for monitoring and characterizing ultrafast molecular structural events and dynamics, as has been successfully demonstrated in a number of condensed-phase systems, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] including very recent studies on the symmetric tricyanomethanide [24] and guanidinium [25] ions. The structural and dynamical sensitivities of the 2D IR method lie in its ability to obtain a series of diagonal and off-diagonal 2D peaks in a compact two-frequency plot, from which one can derive frequency-frequency time correlations from the diagonal signals, and extract vibrational couplings and/or chemical exchange dynamics from the off-diagonal signals.…”