Dynamics solvent reorganization

In this section, we switch gears slightly to address another contemporary topic, solvation dynamics coupled into the ESPT reaction. One relevant, important issue of current interest is the ESPT coupled excited-state charge transfer (ESCT) reaction. Seminal theoretical approaches applied by Hynes and coworkers revealed the key features, with descriptions of dynamics and electronic structures of non-adiabatic [119, 120] and adiabatic [121-123] proton transfer reactions. The most recent theoretical advancement has incorporated both solvent reorganization and proton tunneling and made the framework similar to electron transfer reaction, [119-126] such that the proton transfer rate kpt can be categorized into two regimes (a) For nonadiabatic limit [120] ... 

Carbonyl compounds are also suited to the investigation of the role of solvent reorganization in the dynamics of intramolecular dissociative electron transfer as observed in a series of phenacyl derivatives bearing various leaving groups.199... 

The central question in liquid-phase chemistry is How do solvents affect the rate, mechanism and outcome of chemical reactions Understanding solvation dynamics (SD), i.e., the rate of solvent reorganization in response to a perturbation in solute-solvent interachons, is an essential step in answering this central question. SD is most often measured by monitoring the time-evolution in the Stokes shift in the fluorescence of a probe molecule. In this experiment, the solute-solvent interactions are perturbed by solute electronic excitation, Sq Si, which occurs essenhaUy instantaneously on the time scale relevant to nuclear motions. Large solvatochromic shifts are found whenever the Sq Si electroiuc... 

The computational and experimental analysis of time dependent solvatochromic shift in fluorescence spectra of solutes is used by Ladanyi to achieve an accurate description of solvation dynamics, i.e., the rate of solvent reorganization in response to a perturbation in solute-solvent interaction. 

Solvation dynamics refers to the solvent reorganization or relaxation that accompanies the external excitation of a probe solute, usually a fluorescent organic dye or simply an excess solvated electron [55], Experimentally, the process of solvent reorganization can be time monitored by the time evolution of the fluorescence emission in time-dependent ultra-fast Stokes shift spectroscopy. 

Recent theoretical treatments, however, suggest instead that the dynamics of solvent reorganization can play an important and even dominant role in determining vn, at least when the inner-shell barrier is relatively small [43-45]. The effective value of vos can often be determined by the so-called longitudinal (or "constant charge ) solvent relaxation time, rL [43, 44]. This quantity is related to the experimental Debye relaxation time, rD, obtained from dielectric loss measurements using [43]... 

Other spectroscopic methods have also been used to study the statics and dynamics of solvation shells of ions and molecules [351-354], In this respect, solvation dynamics refers to the solvent reorganization e.g. rotation, reorientation, and residence time of solvent molecules in the first solvation shell) in response to an abrupt change in the solute properties, e.g. by photoexcitation of the solute with ultra-short laser-light pulses. Provided that this excitation is accompanied by an electron transfer or a change in the dipole moment, then the dynamics of this process correspond to how quickly the solvent molecules rearrange around the instantaneously created charge or the new dipole. 

Though combination of Eqs. (33) and (34) gives more realistic values of the frequency factor, it shows that this parameter should not be very dependent on the solvent reorganization. This conclusion was challenged in recent years, when the role of the dynamics of solvent reorganization in charge-transfer reactions was taken into account in theoretical work [140-146] and also experimentally by Kapturkiewicz and Behr [147] and later by Weaver and coworkers [1, 3, 148] and Opallo and Kapturkiewicz [2, 149, 150]. 

Good models for such studies are also metallocenes (M = Mn, Fe, Co) and Cr(CgHg)2 °, which were studied by Weaver and Gennett [148] in seven solvents. The authors compared the experimental data with two sets of calculated results. In the calculations of the first set of data, v was identified with the inner-shell vibration frequency V and it was assumed that the reaction is adiabatic (/c = l). In the second set the authors assumed that the frequency of surmounting the free energy barrier is controlled entirely by the dynamics of solvent reorganization. It was found that the second set of calculated data was much closer to the experimental results. 

Mu and Schulz [176] have shown that chloro(tetraphenylporphinato)man-ganese(III) complex, in one electron reduction in six aprotic solvents exhibits a dynamic solvent effect, though its inner reorganization energy is larger than the outer... 

In order to understand the dynamics of the solvent fluctuation, many experimental as well as theoretical efforts have been made intensively in the last decade. One of the most convenient methods to observe solvent reorganization relaxation processes within the excited state molecule is time resolved fluorescence spectroscopy. By using time resolved techniques a time dependent fluorescence peak shift, so ( ed dynamic Stokes shift, has been detected in nanosecond picosecond >, and femtosecond time regions. Another method to observe solvent relaxation processes is time resolved absorption spectroscopy. This method is suitable for the observation of the ground state recovery of the solvent orientational distribution surrounding a solute molecule. 

In the absence of ion pairing and rate limitation by solvent dynamics, the volume of activation for adiabatic outer-sphere electron transfer in couples of the type j (z+i)+/z ju principle, be calculated as in equation 2 from an adaptation of Marcus-Hush theory. In equation 2, the subscripts refer respectively to volume contributions from internal (primarily M-L bond length) and solvent reorganization that are prerequisites for electron transfer, medium (Debye-Huckel) effects, the Coulombic work of bringing the reactants together, and the formation of the precursor complex. 

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