Solvating dipoles

It is important to recognize how a QM/MM calculation like that for the nucleic acid base solvated dipole moments is accomplished. We outline here a typical series of steps... 

The hydrolysis of triamido-, diamido-, and monoamido-phosphoric acid has been studied. The triamido-compound reacts by both acid- and base-catalysed mechanisms, and also exhibits spontaneous hydrolysis in the pH range 7—8. The diamido-and monoamido-compounds react by an acid-catalysed pathway, though the dependence of rate upon pH is complicated by zwitterion formation. The reaction of A-(4-chlorophenyl)phosphoramidate anion, [ArNHPOsH]-, in hydroxylamine buffers involves a common intermediate for hydrolysis and hydroxylaminolysis. This is described as a solvated dipole pair, derived from a zwitterion ... 

Much of the work on solvation effects has concentrated on modeling the shift of the centre of an electronic absorption or emission band that occurs on solvation, i.e.. the solvatochromic shift. According to the Franck-Condon principle the centre of such a band corresponds to the vertical excitation energy (from an initial to final electronic state) at a fixed nuclear geometry. Solvation of a chromophore thus implies that while the system in its initial electronic state is in equilibrium with its environment, it is not so in its vertically excited state. On excitation of the solute the electronic polarization of the solvent is assumed to relax instantaneously while the ori-entational/distortional polarization is thought of as remaining frozen, a view which may be somewhat simplistic. Within the reaction-field model application of the above theory to a solvated dipole results in a solvent shift of... 

DIPOLE MOMENT OF COMPONENT I fDEBYES ASSOCIATION OR SOLVATION PARAMETER FOR T E (I.JI PAIR... 

This fomuila does not include the charge-dipole interaction between reactants A and B. The correlation between measured rate constants in different solvents and their dielectric parameters in general is of a similar quality as illustrated for neutral reactants. This is not, however, due to the approximate nature of the Bom model itself which, in spite of its simplicity, leads to remarkably accurate values of ion solvation energies, if the ionic radii can be reliably estimated [15],... 

The quality of the results that can be obtained with point charge or dipole models depends critically on the input solvation shell structure. In view of the computer power available today, taking the most rigorous route... 

Van der Zwan G and Hynes J T 1984 A simple dipole isomerization model for non-equilibrium solvation dynamics in reactions in polar solvents Chem. Phys. 90 21-35... 

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... 

Tiic Langevin dipole method of Warshel and Levitt [Warshel and Levitt 1976] i.itermediate between a continuum and an explicit solvation model. A three-dimension... 

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

The central role of the concept of polarity in chemistry arises from the electrical nature of matter. In the context of solution chemistry, solvent polarity is the ability of a solvent to stabilize (by solvation) charges or dipoles. " We have already seen that the physical quantities e (dielectric constant) and p (dipole moment) are quantitative measures of properties that must be related to the qualitative concept of... 

The neutral reactants possess permanent dipoles, the product is ionic, and the transition state must be intermediate in its charge separation, so an increase in solvent polarity should increase the rate. Except for selective solvation effects of the type cited in the preceding section, this qualitative prediction is correct. 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Let (Xgr and jXex be the dipole moments of the ground and excited states. Then if iXgr > the less polar excited state is surrounded by a solvation shell... 

Continuum models of solvation treat the solute microscopically, and the surrounding solvent macroscopically, according to the above principles. The simplest treatment is the Onsager (1936) model, where aspirin in solution would be modelled according to Figure 15.4. The solute is embedded in a spherical cavity, whose radius can be estimated by calculating the molecular volume. A dipole in the solute molecule induces polarization in the solvent continuum, which in turn interacts with the solute dipole, leading to stabilization. 

Molecules do not consist of rigid arrays of point charges, and on application of an external electrostatic field the electrons and protons will rearrange themselves until the interaction energy is a minimum. In classical electrostatics, where we deal with macroscopic samples, the phenomenon is referred to as the induced polarization. I dealt with this in Chapter 15, when we discussed the Onsager model of solvation. The nuclei and the electrons will tend to move in opposite directions when a field is applied, and so the electric dipole moment will change. Again, in classical electrostatics we study the induced dipole moment per unit volume. 

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