Electronic spectroscopy solvents

The preparation of ZnSe materials is an area of interest and study. The coordinating ability of the solvent used in the solvothermal synthesis of zinc selenide was demonstrated to play an important role in the nucleation and growth of nanocrystalline ZnSe.604 Thermolysis of bis [methyl( -hexyl)di-seleno]carbamato]zinc gave highly monodispersed particles characterized by electronic spectroscopy, photoluminescence, X-ray diffraction, and electron microscopy.605... 

In good electron acceptor solvents, such as carbon tetrachloride and chloroform, the photodegradation of carotenoids is significantly increased as compared to other solvents (Christophersen et al. 1991, Mortensen and Skibsted 1999), because of a direct photoinduced electron-transfer reaction from the excited singlet state of the carotenoids to the solvent, as determined by transient absorption spectroscopy (Jeevarajan et al. 1996, Mortensen and Skibsted 1996,1997a,b, El-Agamey et al. 2005), Equation 12.2 ... 

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... 

Karelson, M. M. and Zemer, M. C. Theoretical treatment of solvent effects on electronic spectroscopy, J.Phys. Chem., 96 (1992), 6949-6957... 

A surrounding condensed phase can have enormous impacts on the electronic spectroscopy of a given molecule. Certain dye molecules are sufficiently sensitive to the nature of a surrounding solvent that the color of their solutions can vary across the entire visible spectrum depending on the particular solvent chosen. This solvent effect on spectroscopy is known as solvatochromism. 

Alkanes and cycloalkanes have no low-energy electronic transitions comparable to conjugated systems or molecules with nonbonding electrons. Therefore alkanes and cycloalkanes show no absorption above 200 nm and are good solvents to use for electronic spectroscopy. 

We have discussed recent computational and spectroscopic results on the photoinduced hydrogen transfer and proton transfer chemistry in hydrogen-bonded chromophore-solvent clusters. The interplay of electronic spectroscopy of size-selected clusters and computational studies has led to a remarkably detailed and complete mechanistic picture... 

In general, we find four major categories for the various relaxation processes in solutions. These range from electronic relaxation and solvent relaxation to orientational relaxation, and finally vibrational relaxation [89]. Stationary and time-resolved spectroscopy provides powerful means to explore the electronic and solvent relaxations. Nevertheless, the many different and extremely fast processes... 

Of major concern in measuring CD spectra, is the absorbance ofthe solvent. Whereas in the electronic absorption spectrum (see Electronic Spectroscopy) all components can be measured if the blank is simply set to be the empty cuvette or air in the double beam instrument, in the CD experiment, the nonchiral components are not seen or recorded in the CD spectrum yet they absorb light and can easily reduce the CD spectral data to useless hues. In reality, this only becomes a major problem when solvents and buffers begin to absorb below about 300 mn. Although, we should note that great care should be taken to keep the absorption low (may be less than 0.6) for the CD spectra of porphyrinoids, especially in the Q band region from 580 to 800 mn. However, the problem is insidious in that after the measurement has been made, it is not possible to determine if the background absorbance precluded accurate measurement. 

This equation has been used in several correlations of solvent effects on solute properties such as reaction rates and equilibrium constants of solvolyses, energy of electronic transitions, solvent-induced shifts in UV/visible, IR, and NMR spectroscopy, fluorescence lifetimes, and formation constants of hydrogen-bonded and Lewis acid/base complexes [Kamlet et al., 1986b]. 

It is important to realize that the only approximations that enter into this rate expression is the use of the Fenni golden-rule, which is compatible with the weak coupling nonadiabatic limit, and the Condon approximation which is known to be successful in applications to electronic spectroscopy. The solvent effect on the electronic process, including the slow dielectric response, must arise from the FC factor that contains contributions from all the surrounding intermolecular and intramolecular nuclear degrees of freedom. In fact, if the nuclear component of the solvent polarization was the only important nuclear motion in the system, then on the classical level of treatment used by Marcus Eqs (16.53) and (16.51) with Ea given by (16.49) should be equivalent. This implies that in this case... 

Several solvent polarity scales vere proposed to quantify the polar effects of solvents on physical properties and reactivity parameters in solution, such as rate of solvolyses, energy of electronic transitions, solvent induced shifts in IR, or NMR spectroscopy. Most of the polarity scales vere derived by an empirical approach based on the principles of the linear free energies relationships applied to a chosen reference property and system vhere hydrogen bonding effects are assumed negligible [Reichardt,1965, 1990 Kamlet, Abboud et al., 1981, 1983]. 

The next section devoted to the quantum chemical methods and concepts gives a survey of the computational schemes and theoretical tools adapted to the investigation of electronic spectroscopy and photoreactivity in transition metal complexes. The solvent and other environmental effects are not discussed here and are not taken into account in the selected applications described in the later sections dedicated to the electronic spectroscopy and photoreactivity, respectively. 

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