Solvent effects solvation time scales

Spectroscopy provides a window to explain solvent effects. The solvent effects on spectroscopic properties, that is, electronic excitation, leading to absorption spectra in the nltraviolet and/or visible range, of solutes in solution are due to differences in the solvation of the gronnd and excited states of the solute. Such differences take place when there is an appreciable difference in the charge distribution in the two states, often accompanied by a profonnd change in the dipole moments. The excited state, in contrast with the transition state discussed above, is not in equilibrium with the surrounding solvent, since the time-scale for electronic excitation is too short for the readjustment of the positions of the atoms of the solute (the Franck-Condon principle) or of the orientation and position of the solvent shell around it. 

In spectroscopy we may distinguish two types of process, adiabatic and vertical. Adiabatic excitation energies are by definition thermodynamic ones, and they are usually further defined to refer to at 0° K. In practice, at least for electronic spectroscopy, one is more likely to observe vertical processes, because of the Franck-Condon principle. The simplest principle for understandings solvation effects on vertical electronic transitions is the two-response-time model in which the solvent is assumed to have a fast response time associated with electronic polarization and a slow response time associated with translational, librational, and vibrational motions of the nuclei.92 One assumes that electronic excitation is slow compared with electronic response but fast compared with nuclear response. The latter assumption is quite reasonable, but the former is questionable since the time scale of electronic excitation is quite comparable to solvent electronic polarization (consider, e.g., the excitation of a 4.5 eV n — n carbonyl transition in a solvent whose frequency response is centered at 10 eV the corresponding time scales are 10 15 s and 2 x 10 15 s respectively). A theory that takes account of the similarity of these time scales would be very difficult, involving explicit electron correlation between the solute and the macroscopic solvent. One can, however, treat the limit where the solvent electronic response is fast compared to solute electronic transitions this is called the direct reaction field (DRF). 49,93 The accurate answer must lie somewhere between the SCRF and DRF limits 94 nevertheless one can obtain very useful results with a two-time-scale version of the more manageable SCRF limit, as illustrated by a very successful recent treatment... 

At the beginning of this decade, Zewail and coworkers reported a fundamental work of solvation effect on a proton transfer reaction [195]. a-naphthol and n-ammonia molecules were studied in real-time for the reaction dynamics on the number of solvent molecules involved in the proton transfer reaction from alcohol towards the ammonia base. Nanosecond dynamics was observed for n=l and 2, while no evidence for proton transfer was found. For n=3 and 4, proton transfer reaction was measured at pisosecond time scale. The nanosecond dynamics appears to be related to the global cluster behavior. The idea of a critical solvation number required to onset proton transfer... 

Experiments on 1 -CO using benzene in place of CTAB were also done to examine the effects solvent and environment on the photodissociation. None were found. The photointermediates arrived at the same time, had the same peak wavelengths, extinction coefficients and band shape. In so far as the dynamics observed in these experiments are independent of CO pressure and since there is no detectable geminate CO recombination, it is reasonable to expect effects on the photodissociation due to solvation to be minimal as diffusion has not yet occurred on the time scale studied. 

In other polar solvents such as alcohols and acetonitrile (CH3CN) the ejected electron can be trapped as a solvated electron , shared between several solvent molecules, or as a negative ion by attachment to a solvent molecule. Many aromatic molecules such as naphthalene, anthracene, etc., undergo such photoionizations with low quantum yields. The ions eventually recombine on a time-scale of microseconds, and there is no overall chemical effect (Figure 4.6). 

To date, there have been only a handful of time-resolved studies in dense fluid media (33,34,69-72). Of these, the bulk have focused on understanding a particular chemical reaction by adjusting the solvent environment (69-71). Only over the past two years have there been experiments directed toward studying the peculiar effects of supercritical fluids on these solvation processes (33,34,72). The initial work (33,34) showed that 1) time-resolved fluorescence can be used to improve our understanding of solvation in supercritical fluids and 2) the local solvent composition, about a solute molecule, could change significantly on a subnanosecond time scale. 

Other methods of including nonequilibrium solvation are reviewed elsewhere [86], and the reader is also referred to selected relevant and more recent original papers [66,88-100], Particularly relevant to the present volume are methods that introduce extra degrees of freedom by using the solvent reaction field not only at the current value of R but also at nearby values [65,66], Many of the approaches introduce finite-time effects and additional degrees of solvent freedom by introducing different time scales for electronic and atomic polarization [88-97,99,100],... 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

The details of the mechanism of decay of states in alkanes retain their interest. The effect of deuterium on fluorescence lifetimes has been discussed in terms of the theory of radiationless transitions. Analysis of fluorescence line shapes and Raman excitation profiles of tetradesmethyl-p-carotene in isopentane has been carried out at 190 and 230K . Solvation occurs over a time scale of about 100 fs whilst vibrational relaxation has a time scale of about 250 fs. The kinetics of the interaction of alcohols with the excited state of triethylamine shows involvement of a charge transfer exciplex . Ionizing radiation is a means of exciting saturated hydrocarbons and the complexity of three component systems containing saturated hydrocarbons, aromatic solvent, and fluorescent solute has been examined. ... 

The simulations of BPTI in vacuum and in a van der Waals solvent, described above (this chapter, Sect. A), were analyzed to determine the effect of solvation on the time scales of the atomic motions.199 Given the displacement autocorrelation function, C(t), for a fluctuation, the relaxation time is defined... 

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