Time resolved solvation studies

Simon, J.D., 1988, Acc. Chem. Res., 21 128-134 (bn time-resolved solvation studies of large polar molecules and comments on dielectric friction approaches). 

The nitrenium ion +NH2 has been the subject of a detailed, comprehensive calculation. Calculations on (48) with 15 different X substituents reveal a large substituent sensitivity, and also that aqueous solvation preferentially stabilizes the singlet state. This substiment sensitivity agrees with the results of a time-resolved IR study of the diphenylnitrenium ion (49), which shows that resonance contributors such as (50) and (51) are very important to the overall structure. Substituted 4-biphenyl nitrenium ions... 

The time-resolved solvation of s-tetrazine in propylene carbonate is studied by ultrafast transient hole burning. In agreement with mode-coupling theory, the temperature dependence of the average relaxation dme follows a power law in which the critical temperature and exponent are the same as in other relaxation experiments. Our recent theory for solvation by mechanical relaxation provides a unified and quantitative explanation of both the subpicosecond phonon-induced relaxation and the slower structural relaxation. 

Time resolved studies on dye molecules can help to elucidate the solvation dynamics and can give information on the time constants of diffusion of the ionic components of an RTIL [69-75], Time resolved fluorescence studies show the diffusional motion of the dissolved solutes [76], Luminescence quenching of fluorescent transition metal dyes by oxygen has been used in case of so-called core-shell soft-sphere ionic liquids [77] to monitor the oxygen permeability of these ILs [78],... 

Deeper insight into this mechanism was afforded by femtosecond time-resolved infrared studies that enabled observation of intermediates and the calculation of relative energy barriers. Thus, upon UV irradiation 430 loses CO (<100fs) to afford a 16-electron monocarbonyl complex that is rapidly solvated (barrier-less process) to afford Tp Rh(CO)(RH), which vibrationally cools in 20 ps. Thermal mono-dechelation of the Tp ligand (zlG=4.2kcalmol ) proceeds... 

The time taken for reorganization of the solvent molecules around an instantly created dipole is termed as solvation time or solvent relaxation time (r, ) [72]. As the ILs are polar, time-resolved fluorescence studies on dipolar fluorescent molecules provide valuable information on the timescales of reorganization of the constituents of the ILs around a photoexcited molecule. The timescale of solvation depends on the viscosity, temperature, and molecular structure of the surrounding solvent [72]. As ILs are highly viscous, the solvation in ILs is a much slow process compared with that in less viscous conventional solvents. The dynamics of solvation is commonly studied by monitoring the time-dependent fluorescence Stokes shift of a dipolar molecule following its excitation by a short pulse (Scheme 7.2). This phenomenon is called dynamic fluorescence Stokes shift [73], and the solvation dynamics in several ILs has been studied by this method using various fluorescent probes. 

Despite the significant computational and experimental progress made in recent years, much more research is needed at both fronts to gain a molecular-level understanding of structure, thermodynamics, and dynamics at liquid interfaces. In particular, molecular-level, time-resolved experimental studies of solvation, relaxation, and reactions at liquid interfaces are needed. 

Simon J D 1988 Time-resolved studies of solvation in polar media Acc. Chem. Res. 21 128-34... 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

In excerpt I5D, Walker begins with a statement of the topic (solvation at hydrophobic and hydrophilic solid-liquid interfaces) and then moves directly to the signihcance of the work. He emphasizes the need for information on interfacial phenomena and points out possible applications of his work for other areas of science (molecular recognitions, electron transfer, and macromolecular self-assembly). He goes on to describe his experimental methods, focusing on three aspects of his approach (in order of difficulty) equilibrium measurements, time-resolved studies, and distance-dependent measurements of solvation strength. 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

The time-resolved spectroscopy is a sensitive tool to study the solute-solvent interactions. The technique has been used to characterize the solvating environment in the solvent. By measuring the time-dependent changes of the fluorescence signals in solvents, the solvation, rotation, photoisomerization, or excimer formation processes of a probe molecule can be examined. In conventional molecular solutions, many solute-solvent complexes. 

Femtosecond dynamics of the solvated electron in water studied by time-resolved Raman spectroscopy... 

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

Time-resolved fluorescence from sub-picosecond to the nanosecond time-scale of dye molecules like coumarins has been widely used to study solvation dynamics in liquids [1], As the dye is photoexcited, its dipole moment abruptly changes. Then by monitoring the time-dependent fluorescence energy one can have access to the solvent dynamical response to the charge reorganization in the dye. The microscopic interpretation of these experiments has greatly benefited from Molecular Dynamics (MD) studies [2], Recently, few experimental [3-5] and theoretical [6,7] works have been performed on solvation dynamics in liquid mixtures. A number of new phenomena can arise in mixtures which are not present in pure solvents, like association, mutual diffusion and preferential solvation [6]. We present here a... 

In this study we use the dye Coumarin 343 (C343) adsorbed on the surfaces of ZrC>2 nano particles in aqueous solution to study the solvation dynamics close to these surfaces. Zr02 is, in many respects, very similar to Ti02 and serves as a suitable model substance since, due to its higher band gap energy, electron injection from adsorbed dyes does not occur. To measure the time resolved Stokes shift, we used femtosecond frequency-resolved upconversion. 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

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