Water molecules dielectric constant

A practical way of describing the environment is to consider the DNA molecule to be placed inside a cavity. Physically, the cavity is due to the sugar-phosphate backbone and the hydrophobicity of the DNA bases. Its characteristic size R is determined by the radius of the helix k 10 A. The space outside the cavity is filled with water, with dielectric constant 78 at room temperature, in which counterions are dissolved with equilibrium densities (far from the DNA molecule) Na= ci- Neglecting the polarizability of the backbone and the bases, we assume the cavity to be empty. It is readily shown that, because of the large dielectric constant of water, the interaction with the ions may be neglected [64]. 

AG ° (i) = standard free energy change for the transfer of i from water into the mixed solvent N = Avogadro s number e = electronic charge Dw = dielectric constant of water Ds = dielectric constant of the mixed solvent Th2o = radius of the water molecule Ew° = standard electrode potential in water EB° = standard electrode potential in the mixed solvent M = cation X = anion... 

Water as an electrolytic solvent electric d.pole moment of molecules dielectric constant. 

Water molecules are constantly in motion, even in ice. In fact, the translational and rotational mobility of water directly determines its availability. Water mobility can be measured by a number of physical methods, including NMR, dielectric relaxation, ESR, and thermal analysis (Chinachoti, 1993). The mobility of water molecules in biological systems may play an important role in a biochemical reaction s equilibrium and kinetics, formation and preservation of chemical gradients and osmotic pressure, and macromolecular conformation. In food systems, the mobility of water may influence the engineering processes — such as freezing, drying, and concentrating chemical and microbial activities, and textural attributes (Ruan and Chen, 1998). 

The dipolar nature of the water molecule (see Fig. 3.1) is an important property of water that influences its interactions with cations. Because of its dipolar character, charge is unevenly distributed at the surface of each water molecule, and the protons of one molecule attract the oxygens of adjacent water molecules. This attractive force is called hydrogen bonding and relates to e, the dielectric constant of water. The dielectric constant is a measure of a. solvent s ability to dissolve ionic solids and... 

The standard continuum model takes the dielectric constant of the fluid to be uniform and invariant. Since for fluids comprised of dipolar molecules (such as water) the dielectric constant is a strong function of density, this assumption of uniform dielectric constant is equivalent to an assumption of uniform solvent density - generally a poor a.ssumption for SCFs in their compressible regimes. Note, however, that Johnston and Rossky and co-workers have extensively examined the usefulness of such standard continuum models in the SC regime, mapping our regions of the phase diagram for which this approach remains effective. ... 

By using an effective, distance-dependent dielectric constant, the ability of bulk water to reduce electrostatic interactions can be mimicked without the presence of explicit solvent molecules. One disadvantage of aU vacuum simulations, corrected for shielding effects or not, is the fact that they cannot account for the ability of water molecules to form hydrogen bonds with charged and polar surface residues of a protein. As a result, adjacent polar side chains interact with each other and not with the solvent, thus introducing additional errors. 

Use a constant dielectric of 1.0 with TIP3P water molecules m a periodic box. Because of ihe paramelerizatioii of TIP3P molecules, using a distart ce-dependen t dielectric or a value other th an 1.0 gives un Tialiiral results. 

In this model of electrostatic interactions, two atoms (i and j) have point charges q and qj. The magnitude of the electrostatic energy (Veel) varies inversely with the distance between the atoms, Ry. The effective dielectric constant is 8. For in vacuo simulations or simulations with explicit water molecules, the denominator equals eRij. In some force fields, a distance-dependent dielectric, where the denominator is eRy Rjj, represents solvent implicitly. 

A distance-dependent dielectric constant is commonly used to mimic the effect of solvent in molecular mechanics calculations, in the absence of explicit water molecules. 

Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Progressive chlorination of a hydrocarbon molecule yields a succession of Hquids and/or soHds of increasing nonflammability, density, and viscosity, as well as improved solubiUty for a large number of inorganic and organic materials. Other physical properties such as specific heat, dielectric constant, and water solubihty decrease with increasing chlorine content. 

The dielectric constant is also affected by stmctural changes on strong heating. Also the value is very rank dependent, exhibiting a minimum at about 88 wt % C and rising rapidly for carbon contents over 90 wt % (4,6,45). Polar functional groups are primarily responsible for the dielectric of lower ranks. For higher ranks the dielectric constant arises from the increase in electrical conductivity. Information on the freedom of motion of the different water molecules in the particles can be obtained from dielectric constant studies (45). 

Consider an alchemical transformation of a particle in water, where the particle s charge is changed from 0 to i) (e.g., neon sodium q = ). Let the transformation be performed first with the particle in a spherical water droplet of radius R (formed of explicit water molecules), and let the droplet then be transferred into bulk continuum water. From dielectric continuum theory, the transfer free energy is just the Born free energy to transfer a spherical ion of charge q and radius R into a continuum with the dielectric constant e of water ... 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

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