Organic molecules solvation effects

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

Thermochemistry. Chen et al.168 combined the Kohn-Sham formalism with finite difference calculations of the reaction field potential. The effect of mobile ions into on the reaction field potential Poisson-Boltzman equation. The authors used the DFT(B88/P86)/SCRF method to study solvation energies, dipole moments of solvated molecules, and absolute pKa values for a variety of small organic molecules. The list of molecules studied with this approach was subsequently extended182. A simplified version, where the reaction field was calculated only at the end of the SCF cycle, was applied to study redox potentials of several iron-sulphur clusters181. 

The level of accuracy that can be achieved by these different methods may be viewed as somewhat remarkable, given the approximations that are involved. For relatively small organic molecules, for instance, the calculated AGsoivation is now usually within less than 1 kcal/mole of the experimental value, often considerably less. Appropriate parametrization is of key importance. Applications to biological systems pose greater problems, due to the size and complexity of the molecules,66 156 159 161 and require the use of semiempirical rather than ab initio quantum-mechanical methods. In terms of computational expense, continuum models have the advantage over discrete molecular ones, but the latter are better able to describe solvent structure and handle first-solvation-shell effects. 

Note Added in Proof After we sent the manuscript to the publishers we became aware of CNDO studies on alkali ion solvation performed by Gupta and Rao 270> and Balasubramanian et al.271 >, which might be of some importance for readers interested in cation solvation by water and various amides. Another CNDO model investigation on the structure of hydrated ions was published very recently by Cremaschi and Simonetta 272> They studied CH5 and CH5 surrounded by a first shell of water molecules in order to discuss solvation effects on structure and stability of these organic intermediates or transition states respectively. 

What is the exact influence of water and organic molecules on the enzyme structure Could its effects on properties such as selectivity, affinity, binding constants, and catalytic constants be predictable by controlling the hydration/solvation state ... 

It would have been difficult to find these kinds of relations with inorganic materials proper. The organic molecule is unique in that its physical properties may be changed in a continuous way by small modifications in its structure or by complexation with different metal ions. Another advantage is the fact that solid state effects are of smaller importance, and catalytic properties of the solid phase may be compared with physical properties in solution. In particular an extended jr-electron system works as a catalytic entity in itself, irrespective of whether it is surrounded by other molecules of its kind (solid phase) or solvating molecules (solutions). 

C. Kolle and K. Jug, Solvation effects in SINDOl Application to organic molecules, J. Comput. Chem., 18 (1997) 1-8. 

The mechanistic subtypes presented throughout this book include those related to the acid-base properties of organic molecules. These are protonations, deprotonations, and proton transfers. Mechanistic types based on solvation effects include solvolysis reactions, SN1, and El processes. Additional mechanisms utilizing ionic interactions include SN2, SN2, E2, 1,2-additions, 1,4-additions, and addition-elimination processes. Finally, those mechanistic types dependent upon the presence of cationic species include alkyl shifts and hydride shifts. 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

Another problem is explaining the mechanism of solvation of an organic molecule at a molecular level. It is well known that even for the same compound, different polymorphic crystals have different solubility characteristics. In addition the rate of solvation varies according to the face of the crystals this is also observed in crystal growth. These phenomena could be influenced by kinetic factors, which have a significant effect on the efficiency of resolution. Thus treatment of the kinetic factors of solvation is another problem that must be explored. 

Electrosorption is a replacement reaction. We have already discussed the role of the solvent in the interphase, in the context of its effect on the double-layer capacitance. It is most important for our present discussion to know that the electrode is always solvated and that the solvent molecules are held to the surface both by electrostatic and by chemical bonds. Adsorption of a molecule on such a surface requires the removal of the appropriate number of solvent molecules, to make place for the new occupant, so to speak. This is electrosorption. In this chapter we shall restrict our discussion to the electrosorption of neutral organic molecules from aqueous solutions, without charge transfer. Using the notation RH for an unspecified organic molecule, we can then represent electrosorption in general by the reaction... 

Solvation effects arising from the organization of solvent molecules near an interlace or between the chains of adsorbed macroraolecules. 

Visible-light induced photoreduction of CO2 proceeds on CdS nanocrystallites when they are stabilized by specific solvation of organic solvent molecules like DMF. Formation of sulfur vacancies as CO2 adsorptive sites and sustained quantization effect of CdS nanocrystallites are requisites for their effective photocatalysis. Adsorptive activation of both CO2 molecules and appropriate organic molecules leads to photofixation of CO2 to the organic molecules on CdS nanocrystallites (CdS-DMF). 

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