Solvent induced absorption band

By carefully controlling the temperature we were able to obtain fairly good matching of thermally- and photo-induced absorption bands. Thus, it seems probable that low temperatures, polar solvents and irradiation with ultraviolet light all lead to the same absorbing species. 

While excited-state properties of monomeric carotenoids in organic solvents have been the subject of numerous experimental and theoretical studies (Polfvka and Sundstrom 2004), considerably less is known about excited states of carotenoid aggregates. Most of the knowledge gathered so far stems from studies of aggregation-induced spectral shifts of absorption bands of carotenoid aggregates that are explained in terms of excitonic interaction between the molecules in the aggregate. 

The above observations provide a clear demonstration that cosolvents in selected ranges of concentration create reversible perturbations of protein similar to those induced by other modifiers. The reversibility of the cosolvent effect is a prerequisite to cosolvent use and will depend on the concentration of cosolvent, which in turn will vary markedly with the type of solvent used. For instance, polyols can be used at concentrations up to 8 Af while methanol at 3 M causes the appearance of a new absorption band (410 nm) and, after further increases in concentration, an irreversible conversion of cytochrome P-450 into P-420. Other aliphatic alcohols cause denaturation at much lower concentrations. 

The CTTS band can also be found in the absorption spectrum of some polyatomic anions together with transitions to the excited states described above43 44 In the case of SCN, an intense absorption band with 2max = 225 nm (s = 3.5 x 103 M 1 cm-1) has been assigned to a charge transfer to solvent transition. The wavelength-dependent photochemistry of SCN induces, however, the formation of solvated electrons according to Equation 6.89 and the detachment of S (Equation 6.91) in a parallel process. 

In a study of chiral dipeptide [2]rotaxanes it was found that the presence of an intrinsically achiral benzylic amide macrocycle near to the chiral center could induce an asymmetric response in the aromatic ring absorption bands [62], This induced circular dichroism (ICD) effect was stronger in apolar solvents (Fig. 9), where intercomponent interactions are maximized, showing a direct relationship to the tightness with which the macrocycle binds the chiral thread. Computer simulations showed that chirality is transmitted from the amino acid asymmetric center on the thread via the achiral macrocycle to the aromatic rings of the achiral C-terminal stopper. 

The addition of small amounts of inert salts such as tetrabutylammo-nium perchlorate (TBAP) or hexafluorophosphate (TBAH) to solutions of charge-transfer salts induces large changes in the intensity of the charge-transfer absorption bands. The magnitude of the salt effect is most pronounced in nonpolar solvents (THF, CH2C12). The monotonic decrease in the CT absorbance with increasing amounts of added TBAP is characteristic of the facile competition for the contact ion pair (42), namely,... 

It has in fact been anticipated for many years that the CT free energy surfaces may deviate from parabolas. A part of this interest is provoked by experimental evidence from kinetics and spectroscopy. Eirst, the dependence of the activation free energy, Ff , for the forward (/ = 1 ) and backward i = 2) reactions on the equilibrium free energy gap AFq (ET energy gap law) is rarely a symmetric parabola as is suggested by the Marcus equation,Eq. [9]. Second, optical spectra are asymmetric in most cases and in some cases do not show the mirror symmetry between absorption and emission.In both types of experiments, however, the observed effect is an ill-defined mixture of the intramolecular vibrational excitations of the solute and thermal fluctuations of the solvent. The band shape analysis of optical lines does not currently allow an unambiguous separation of these two effects, and there is insufficient information about the solvent-induced free energy profiles of ET. 

The Franck-Condon factors of polarizable chromophores in Eq. [153] can be used to generate the complete vibrational/solvent optical envelopes according to Eqs. [132] and [134]. The solvent-induced line shapes as given by Eq. [153] are close to Gaussian functions in the vicinity of the band maximum and switch to a Lorentzian form on their wings. A finite parameter ai leads to asymmetric bands with differing absorption and emission widths. The functions in Eq. [153] can thus be used either for a band shape analysis of polarizable optical chromophores or as probe functions for a general band shape analysis of asymmetric optical lines. 

Sometimes, external solvent polarization interactions can lift internal symmetry restrictions in the solute molecule and can induce new bands not observable in the gas-phase spectrum. A well-known example is the vibrationally forbidden 0 0 vibrational component of the long-wavelength n absorption of benzene (at 262 nm in n-hexane), which appears when benzene is dissolved in organic solvents, but not in the gas phase [320]. 

Overall, the band shifts experimentally observed for all kinds of absorptions are the net results of three, partly counteracting contributions electrostatic (dipole/dipole dipole/induced dipole blue shift), dispersion ( red shift), and specific hydrogen-bonding blue shift). Which of these solute/solvent interactions are dominant for the solute under study depends on the solvents used. For example, the results obtained for pyridazine, as shown in Fig. 6-5, clearly implicate hydrogen-bonding as the principle cause of the observed hypsochromic band shift that occurs when the HBD solvent ethanol is added to solutions of pyridazine in nonpolar -hexane [98]. The intensity of n n absorption bands is usually very low because they correspond to symmetry-forbidden transitions, which are made weakly allowed by vibronic interactions cf. Fig. 6-5). 

The solvent-induced shifts of absorption and emission bands can be used to calculate dipole moments of electronically excited molecules [32, 33, 47, 303], Excited-state dipole moments have also been obtained by the measurement of fluorescence polarization caused by external electric fields [32, 33],... 

The application of eight different reaction-field models to the solvent-induced shift of the carbonyl IR absorption band of 2-butanone, determined in 27 non-HBD solvents, has shown that in this group of solvents the dipolarity and polarizability are the dominating solvent properties responsible for the observed band shift (Avc o = —16 cm for -hexane —> sulfolane) [499]. Among the various reaction-field functions tested, a two-parameter equation with the Kirkwood-Bauer-Magat function /(fir) and the cross function f si,n ) was the most successful one. 

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