Solvation effects, importance

Hydrogen-bonded clusters are an important class of molecular clusters, among which small water clusters have received a considerable amount of attention [148, 149]. Solvated cluster ions have also been produced and studied [150, 151]. These solvated clusters provide ideal model systems to obtain microscopic infonnation about solvation effect and its influence on chemical reactions. 

Two important contributions to the study of solvation effects were made by Bom (in 192( and Onsager (in 1936). Bom derived the electrostatic component of the free energ) c solvation for placing a charge within a spherical solvent cavity [Bom 1920], and Onsagi extended this to a dipole in a spherical cavity (Figure 11.21) [Onsager 1936]. In the Bor... 

An understanding of a wide variety of phenomena concerning conformational stabilities and molecule-molecule association (protein-protein, protein-ligand, and protein-nucleic acid) requires consideration of solvation effects. In particular, a quantitative assessment of the relative contribution of hydrophobic and electrostatic interactions in macromolecular recognition is a problem of central importance in biology. 

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. 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

The comparisons made by Parchment et al. [271] illustrate the importance of combining electronic polarization effects with corrections for specific solvation effects. The latter are accounted for parametrically by the explicit simulation, but that procedure cannot explicitly account for the greater polarizability of tautomer 8. The various SCRF models do indicate 8 to be more polarizable than any of the other tautomers, but polarization alone is not sufficient to shift the equilibrium to that experimentally observed. Were these two effects to be combined in a single theoretical model, a more accurate prediction of the experimental equilibrium would be expected. 

On the other hand, Equation (91) may be easily used in conextion with an orbital theory with the electron density and the electrostatic potential obtained from a standard SCRF wavefunction. The third term may be also evaluated from finite difference approximation formula. The charm of Eq (91) comes from the fact that it introduces for the first time, the natural reactivity indices of DFT in the expression of the solvation energy. This feature should be of great importance for the study of solvation effects in... 

In this contribution, we describe and illustrate the latest generalizations and developments[1]-[3] of a theory of recent formulation[4]-[6] for the study of chemical reactions in solution. This theory combines the powerful interpretive framework of Valence Bond (VB) theory [7] — so well known to chemists — with a dielectric continuum description of the solvent. The latter includes the quantization of the solvent electronic polarization[5, 6] and also accounts for nonequilibrium solvation effects. Compared to earlier, related efforts[4]-[6], [8]-[10], the theory [l]-[3] includes the boundary conditions on the solute cavity in a fashion related to that of Tomasi[ll] for equilibrium problems, and can be applied to reaction systems which require more than two VB states for their description, namely bimolecular Sjy2 reactions ],[8](b),[12],[13] X + RY XR + Y, acid ionizations[8](a),[14] HA +B —> A + HB+, and Menschutkin reactions[7](b), among other reactions. Compared to the various reaction field theories in use[ll],[15]-[21] (some of which are discussed in the present volume), the theory is distinguished by its quantization of the solvent electronic polarization (which in general leads to deviations from a Self-consistent limiting behavior), the inclusion of nonequilibrium solvation — so important for chemical reactions, and the VB perspective. Further historical perspective and discussion of connections to other work may be found in Ref.[l],... 

The existence of critical solvation numbers for a given process to happen is an important concept. Quantum chemical calculations using ancillary solvent molecules usually produce drastic changes on the electronic nature of saddle points of index one (SPi-1) when comparisons are made with those that have been determined in absence of such solvent molecules. Such results can not be used to show the lack of invariance of a given quantum transition structure without further ado. Solvent cluster calculations must be carefully matched with experimental information on such species, they cannot be used to represent solvation effects in condensed phases. 

Electron and charge transfer reactions play an important role in many chemical and biochemical processes. Dynamic solvation effects, among other factors, can largely contribute to determine the reaction rate of these processes and can be studied either by quantum mechanical or simulation methods. 

In a donor solvent the iodide ions is much more strongly solvated than the neutral donor and hence the donor properties of the iodide ion are lowered in solution. This event has been described as the thermodynamic solvatation effect. It becomes increasingly important with an increase of the ratio of the free enthalpy of solvation to the free enthalpy of the ligand exchange reaction. 

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. 

To date, only a few solution calculations for carbohydrates have been attempted (one such study of mannitol and sorbitol is described in the chapter by Grigera in this volume), but the results of these early studies bear out the expectation that solvation effects in carbohydrate systems can be both significant and difficult to predict. In the case of pyranoid rings, molecular solvation is further complicated by the close juxtaposition of these groups in essentially fixed relative orientations (assuming no conformational changes in the ring). Under such circumstances, molecular stereochemistry could play important physical roles, as is... 

The nature of the medium may also have a strong influence on the complexation process via specific or non specific solvation effects on both the complexed and uncomplexed states. The solvent plays a very important role both on enthalpy and entropy of complexation. Stability and selectivity result from a subtle balance between solvation (of both L and S) and complexation (i.e. "solvation of S by L). 

Bernhardsson and coworkers have recently used CASPT2 calculations (electron-correlation correction to the CAS wave function) to model carbonyl oxides in solution. Solvation effects in acetonitrile solvent also suggest that the zwitterionic form would be favored with an elongation of the 0—0 bond length and a decrease in the C—O bond. Ab initio calculations have been recently reported for monofluorocarbonyl oxide , diflu-orocarbonyl oxide , methylcarbonyl oxide and cyclopropenone carbonyl oxide. In the recent literature the idea that carbonyl oxide can be an important source of OH radicals has also been presented. ... 

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