Other Specific Solute-Solvent Interactions

Table II shows the average end-to-end distance over 20 ps for mannitol and sorbitol in vacuuo and in solution of an argon-like (L-J) solvent and SPC/E water. The average lengths all indicate sickle shapes, except for mannitol in water which is fully extended. This points to a specific solute-solvent interaction between mannitol and water, not just an unspecific solvent effect that is not present in solvent other than water. The model non-aqueous solvent is very artificial, but it should represent the main features of the class of non-polar, spherically symmetric solvents.

Specific solute-solvent interactions, such as hydrogen bonds, undergo a significant change in the comse of SD. This was first observed by Fonseca and Ladanyi in the case of SD in methanoF and has since then been seen in a munber of other simulation studies. " ... 

Another problem that has been tackled by multivariate statistical methods is the characterization of the solvation capability of organic solvents based on empirical parameters of solvent polarity (see Chapter 7). Since such empirical parameters of solvent polarity are derived from carefully selected, strongly solvent-dependent reference processes, they are molecular-microscopic parameters. The polarity of solvents thus defined cannot be described by macroscopic, bulk solvent characteristics such as relative permittivities, refractive indices, etc., or functions thereof. For the quantitative correlation of solvent-dependent processes with solvent polarities, a large variety of empirical parameters of solvent polarity have been introduced (see Chapter 7). While some solvent polarity parameters are defined to describe an individual, more specific solute/solvent interaetion, others do not separate specific solute/solvent interactions and are referred to as general solvent polarity scales. Consequently, single- and multi-parameter correlation equations have been developed for the description of all kinds of solvent effects, and the question arises as to how many empirical parameters are really necessary for the correlation analysis of solvent-dependent processes such as chemical equilibria, reaction rates, or absorption spectra. 

In conclusion, it can be stated, according to a general rule proposed by Palit in 1947 [270], that solvents which impede the active centre of a reactant through hydrogen bonding or by some other means, will suppress the reactivity of that reactant. Conversely, solvents capable of promoting a favourable electron shift, necessary to the reaction, by specific solute/solvent interactions, will enhance the reaction rate. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

The extreme solvent sensitivity of the exciplex fluorescence is very interesting. Fullerene-amine exciplex emissions observed in saturated hydrocarbon solvents are absent in solvents such as benzene and toluene (27,84,88,101), which has been explained in terms of solvent polarizability effects [101]. However, there has also been an explanation [84] that the formation of exciplexes in a solvent such as benzene is hindered by specific solute-solvent interactions that result in complexation between the fullerene and solvent molecules. The two explanations are fundamentally different. In the former, the exciplex state is effectively quenched through a radiationless decay pathway facilitated by a stronger dielectric field of the solvent. However, the latter assumes that the ground state fiillerene-solvent complexation prevents the formation of fullerene-donor exciplexes. In order to understand whether the extreme solvent sensitivity is solvent specific (limited to benzene, toluene, and other aromatic solvents) or solvent property specific (solvent polarity and polarizability), fluorescence spectra of C70-DEA were measured systematically in mixtures of hexane and a polar solvent (acetone, THF, or ethanol) with volume fraction up to 10% [101]. The results are consistent with the explanation of solvent polarity and polarizability effects. 

The use of a single parameter (e.g., E(A) or E(F) or any other photophysical parameter) to describe solvent polarity is based on the assnmption that it is necessary to take only one mechanism of solute-solvent interaction into acconnt. The inadequacy of the dielectric model of solvent to represent the solvent effect on the varions properties of solutes aud proliferation of empirical polarity scales point to the existence of specific solute-solvent interaction. According to Equation 7.1 any solvent-dependent property (A) of a solute 5" in a solvent T can be represented as... 

Spectroscopic measurements may, in certain cases, yield direct information on these interactions. On the other hand, thermodynamic values, obtained by measuring certain bulk properties of the system, require the aid of statistical mechanical methods to be related to specific interactions between the solute and the solvent. However, the thermodynamic aspects of the solute-solvent interactions reflect the preference of the solute for one solvent over another and, thereby, determine distribution of the solute in a solvent extraction system. 

In conclusion, it can be said that the electrostatic theory of solvent effects is a most useful tool for explaining and predicting many reaction patterns in solution. However, in spite of some improvements, it still does not take into account a whole series of other solute/solvent interactions such as the mutual polarization of ions or dipoles, the specific solvation etc., and the fact that the microscopic relative permittivity around the reactants may be different to the macroscopic relative permittivity of the bulk solvent. The deviations between observations and theory, and the fact that the relative permittivity cannot be considered as the only parameter responsible for the changes in reaction rates in solution, has led to the creation of different semiempirical correlation equations, which correlate the kinetic parameters to empirical parameters of solvent polarity (see Chapter 7). 

Specific effects on spectroscopy and photophysics induced by complexation of the D-A chromophores with various solvent molecules have been examined for all the compounds under consideration. The idea of the beam work is to generate n solute-solvent complexes and to determine thereby the relation between the solute-solvent interactions and the excited-state CT process. Kajimoto et al. [81a,c, 89], Phillips and co-workers [82], Peng et al. [83], Bernstein and co-workers [84] and others [85, 88, 90-92] have shown that solute-solvent complexes of CDMA were readily produced by varying the partial pressure of the compounds and the stagnation pressure of the carrier gas. Cyclohexane, chloroform, carbon tetrachloride, methyl fluoride, trifluoromethane, dichloromethane, acetone, acetonitrile, metha-... 

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