Solvent-induced oscillations

The Raman effect can be seen, from a classical point of view, as the result of the modulation due to vibrational motions in the electric field-induced oscillating dipole moment. Such a modulation has the frequency of molecular vibrations, whereas the dipole moment oscillations have the frequency of the external electric field. Thus, the dynamic aspects of Raman scattering are to be described in terms of two time scales. One is connected to the vibrational motions of the nuclei, the other to the oscillation of the radiation electric field (which gives rise to oscillations in the solute electronic density). In the presence of a solvent medium, both the mentioned time scales give rise to nonequilibrium effects in the solvent response, being much faster than the time scale of the solvent inertial response. 

The dynamic (nonequilibrium) response of the solvent to the external field-induced oscillation in the solute electronic density (electronic nonequilibrium) has been formulated within the PCM in ref. [9], whereas vibrational nonequilibrium effects (due to the dynamics of the solvent resulting from solute vibrational motions) have been formulated, still within the PCM, in ref. [43],... 

Combining the idea of solvent-induced changes in molecular structure with the concept of a solvent continuum around the solvatochromic molecule, a micro-structural model of solvatochromism has been developed by Dahne et al., which reproduces, qualitatively correctly and quantitatively satisfactorily, the solvatochromic behavior of simple merocyanine dyes [95b], The results obtained with this model for 5-(dimethylamino)penta-2,4-dienal are in good agreement with the solvent-dependent experimental data such as transition energies, oscillator strengths, r-electron densities, and r-bond energies [95b] cf. also [326, 327],... 

There have been limited attempts to correlate the spectrum of the random force responsible for vibrational dephasing with induced spectra. In the fast modulation limitthe dephasing time (t) is determined by the amplitude (< 5vp>)2and relaxation time of the solvent-induced fluctuations in the oscillator frequency... 

Calculations within tire framework of a reaction coordinate degrees of freedom coupled to a batli of oscillators (solvent) suggest tliat coherent oscillations in the electronic-state populations of an electron-transfer reaction in a polar solvent can be induced by subjecting tire system to a sequence of monocliromatic laser pulses on tire picosecond time scale. The ability to tailor electron transfer by such light fields is an ongoing area of interest [511 (figure C3.2.14). 

Bhawe (14) has simulated the periodic operation of a photo-chemically induced free-radical polymerization which has both monomer and solvent transfer steps and a recombination termination reaction. An increase of 50% in the value of Dp was observed over and above the expected value of 2.0. An interesting feature of this work is that when very short period oscillations were employed, virtually time-invariant products were predicted. 

In Equation (2.183) new surface charges, qex, have been introduced these charges can be described as the response of the solvent to the external field (static or oscillating) when the volume representing the molecular cavity has been created in the bulk of the solvent. We note that the effects of qex in the limit of a spherical cavity coincide with that of the cavity field factors historically introduced to take into account the changes induced by the solvent molecules on the average macroscopic field at each local position inside the medium more details on this equivalence will be given in Section 2.7.4. 

Fig. 4.4. (D and E) Primary electron acceptor (An 2) plastoquinone (or quencher Q). (D) Optical spectrum of the light induced plastosemiquinone anion (O), compared to the spectrum of PQ semi-quinone in non-aqueous solvent ( ). The additional spectral shifts at 545 and 685 nm are attributed to electrochromic effects on pheophytin (from Ref. 85). (E) ESR spectrum of A7, j reduced by light (a) or by dithionite (b) in Ch/amydomonas PSIl particles (from Ref. 87). (F) Secondary electron acceptor (An ) plastoquinone. (F) Flash-induced optical changes due to the reduction of the secondary electron acceptor plastoquinone the spectra oscillate in a dampened sequence following subsequent flashes indicating the production of semiquinone (1st and 3rd flash) and quinol species (2nd and 4th) (from Ref. 103).

Sonochemistry (chemical events induced by exposure to ultrasound) occupies an important place in organic chemistry. The chemical effects of high-intensity ultrasound were extensively smdied in aqueous solutions for many years, but is now applied to a variety of organic solvents. The origin of sonochemistry is acoustic cavitation the creation, growth, and implosive collapse of gas vacuoles in solution by the sound field. Acoustic cavitation is the phenomenon by which intense ultrasonic waves induce the formation, oscillation, and implosion of gas... 

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