Solvent shift water

Regardless of the initial concentration of zeaxanthin, when ethanol is used as the primary solvent, a water content larger than 50% always promotes the formation of H-aggregates (Ruban et al. 1993, Billsten et al. 2005). Thus, it seems that the proper choice of solvent may shift the concentration window in which the J-aggregates are formed. 

The direct spectroscopic characterization of la in an LFP experiment was achieved by using a different precursor. LFP of p-chlorophenol in aqueous solution produces, at the end of the laser pulse, a transient with absorption maxima at 250, 370, and 384 nm.26 These absorptions are in reasonable agreement with the strong and medium intensity absorptions of la in argon at 10 K (290 and 379 nm), especially if a solvent shift of the highly polar la in water compared to argon is taken into account. The assignment of the transient... 

Applications of TDDFT within the QM/M M Framework -Solvent Shift of the So/St Transition of Acetone in Water... 

As a first test system chosen to probe the capabilities of a QM/ MM approach in reproducing environment-induced shifts in excitation spectra, we have calculated the solvent shift of the first optical excitation (n -> n ) of an acetone molecule in the gas phase and in water (Fig. 1.5) [16]. 

Novak et al. examined the addition of A,A-dimethylaniline to 4-biphenylyni-trenium ion (101, R = H) and found significant yields of meta adduct 103. This product was attributed to an initial addition coupling the para position of the nitre-nium ion to the para position of the aniline to give 102, followed by an acid-catalyzed 1,2-migration of the trap to the meta position (Fig. 13.50). In this case, the solvent was water. In the previous case, the solvent was acetonitrile. Presumably, the difference in the fate of the initial para adduct is governed by the solvent. The 1,2-shift requires a general base, which is abundant in the buffered aqueous solutions used by Novak et al. The homolytic dissociation is more probable in aprotic media such as MeCN. 

The phase transitions for the weakly charged PAA gels which are swollen in mixtures of water with solvents differing in the dielectric constant, (methanol (e = 33.7), ethanol (e = 25.8) and dioxane (e = 2)) were studied in Ref. [29]. It was shown that the decrease in the dielectric constant of the solvent shifts the point of the phase transition to the region of lower concentration of a precipitant. The amplitude of the collapse also decreases when the dielectric constant is lowered [29]. 

For the particular case of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines and by means of absorption UV-VIS spectroscopy, Stoyanov and collaborators122 have shown that polar solvents shift the thiol/thione tautomerism towards the thione form, while in dilute solutions of nonpolar solvents the thiol form predominates. However, one of the most significant contributions of this work122 is the observation of self-association. It also favors the thione forms and is followed by quantitative transformation of the thiol form into the corresponding symmetrical disulfides (see Scheme 6). More importantly thione-disulfide process is reversible in water, which can be of some relevance in biological systems. 

We studied two of the test molecules used in the previous section (formaldehyde and phenoxide) in water. As far as the formaldehyde dynamics is concerned we will analyze the energy conservation as well as the oscillations of the potential energy. As for the phenoxide we will examine the solvent shift in the normal mode frequencies. 

As shown in Figure 2.5, continuum solvent models (PCM) reproduce satisfactorily solvent effects on the aN parameter only for aprotic solvents (bulk effects), whereas there is a noticeable underestimation of solvent shifts for protic solvents (methanol and water). In these media also specific solute-solvent interactions have to be taken into account. 

Table 4.11 Solvent shift and its components (in kcal mo 1) for the vertical absorption and emission transitions of the acrolein in water solution...

The importance of the entropy of activation in these reactions is clear when it is realized that if the rate constant were solely a function of AH, the rate constant would increase initially as 2 increases. Instead, the fall in rate constant is determined by the decrease in AS. Thus, as the co-solvent enhances water-structure, so the partial molar entropy of the apolar initial state increases, the solute entering the structure-enhanced solvent, possibly as a guest in a preformed cavity. In any event, the change in this entropy term is probably more important than the endothermic shift in SmH, so... 

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