Dipolar ions defined

The equilibrium constant for a tautomeric interconversion is simply the ratio of the mole fractions of the two forms for example, the ratio of enol to oxo forms of acetone12 in water at 25°C is 6.0 x 10 9, while that for isobutyraldehyde is 1.3 x ICE4. The ratio of 2-hydroxypyridine to 2-pyridone is about 10 3 in water but increases to 0.6 in a hydrocarbon solvent and to 2.5 in the vapor phase.13 14 The ratio of dipolar ion to uncharged pyridoxine (Eq. 2-5) is 4 at 25°C in water.15 The ratios of tautomers B, C, and D to the tautomer A of uracil (Eq. 2-4) are small, but it is difficult to measure them quantitatively.16 These tautomeric ratios are defined for given overall states of protonation (see Eq. 6-82). The constants are independent of pH but will change if the overall state of protonation of the molecule is changed. They may also be altered by... 

The dissociation constants fC — Kn for a multi-protic acid HrA are defined as stepwise or macroscopic constants (also called molecular constants). For some compounds, e.g. alanine, the pfCa values are far apart (pfC and pK2 are 2.4 and 9.8, respectively). The macroscopic constants can be assigned specifically, fCj to the carboxyl group and K2 to the protonated amino group. At the isoelectric pH of 6.1 the alanine exists almost entirely as the dipolar ion. However, for compounds in which the macroscopic pfCa values are closer together, they cannot be assigned to specific groups. We will consider some specific examples in the next section. 

Normally, amino acids exist as dipolar ions. RCH(NH,+ iCOO-. in a neutral state, where both amino and carboxyl groups are ionized. The dipolar form, RCH(NH2)COOH may be considered, but the dipolar form predominates for the usual monoamino monocarboxylic acid and it is estimated that these forms occur 10s to 106 times more frequently than the non-polar forms. Amino acids decompose thermally at what might be considered a relatively high temperature (200-300°C). The compounds are practically insoluble in organic solvents, have low vapor pressure, and do not exhibit a precisely defined melting point. 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

The pseudocontact interaction (perhaps more appropriately called a dipolar interaction) arises from the magnetic dipolar fields experienced by a nucleus near a paramagnetic ion. The effect is entirely analogous to the magnetic anisotropy discussed in Section 4.5. It arises only when the g tensor of the electron is anisotropic that is, for an axially symmetric case, j> g . The g value for an electron is defined as... 

Molecular dipolar polarization was difficult to define from dielectric measurements. A large first dispersion in time for isothermal cures of Resin 5208 is attributed to charge migration in a viscous medium. High values of dielectric constant and dielectric loss factor are attributed to the formation of an ion double-layer and sample conductivity, respectively. Limited frequency data on a smaller second dispersion prevent unequivocal assignment of its relationship to molecular changes. 

At the present time it is unclear whether the spin echo decay spectroscopy approach successfully demonstrated for Na is generally transferable to other alkali ion nuclei such as Li and Cs. The latter two isotopes have moderately small electric quadrupole moments causing first order quadrupolar splittings that are comparable to the radio frequency excitation window. As a result, the 7i-pulse length is ill-defined in many situations and the contribution of dipolar coupling of the observed spins to nuclei in the outer Zeeman levels is difficult to quantify. Nevertheless, some promising initial results on cesium borate... 

Localized orbitals have also been used as a tool to extract the infrared spectrum of a solute in solution [194,195,202] or to decompose the IR spetrum in intramolecular and intermolecular contributions [202]. Model electrostatics of solute molecules was also based on localized orbitals [242, 243], not only at the dipolar level [244]. As an extension we also defined molecular states from localized orbitals to study the electronic states of liquid water [245], or of solvated ions [47]. It is also possible to perform CP-MD propagating the Wannier orbitals, by constraining the Kohn-Sham orbitals to stay in a Wannier gauge [246]. 

The formation of solvent shells around molecules is essential to prevent self-association of the solute species and to allow solution to take place. Solvents other than water which have high dielectric constants, and including some of the dipolar aprotic solvents (defined below), dissolve ionic species by separating and solvating the ions. 

The outer potential is due to the free or excess charge on the surface of phase a and can be measured experimentally. The surface potential is due to the dipolar distribution of charge at the interface due to the unequal adsorption of ions and orientation of molecular dipoles. It cannot be measured experimentally. Since these quantities are defined with respect to the process of bringing a charged species from infinity into the phase, the surface potential is positive when the positive end of the dipolar charge points toward the center of the solution and the negative end toward charge-free infinity. 

A (j) is the potential drop due to the net free charge at the interface is the dipolar potential due to the metal phase, more specifically, to the electron overspill that occurs at the surface of the metal finally, is the dipolar potential due to the solution phase which arises because of the orientation of solvent molecules at the interface due to their proximity to the metal, and because of the unequal distances of closest approach of the cations and anions to the interface. is defined in the opposite direction to because the concept of the dipolar potential originates at the condensed phase vacuum interface where the definition of the potential drop is always from vacuum to the condensed phase. The dipolar potential arises for the same reasons as the surface potential x at the metal vacuum interface. However, it is not the same because of the effect that the proximity of the molecules and ions of the solution phase have on the electron overspill. 

In Eq. 11.7 the first term accounts for the compact layer thickness contribution to the potential jump, the second one for the effective dipolar layer contribution, and the third one for solvation and ions specific adsorption effects (not detailed here) (the other parameters are defined in ). X is an intermediate variable function of a and, and is the solution of the transcendental equation... 

When applied to solvents, this rather iU-defined term covers their overall solvation capabihly (solvation power) for solutes (i.e., in chemical equilibria reactants and products in reaction rates reactants and activated complex in light absorptions ions or molecules in the ground and excited state), which in turn depends on the action of aU possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e., to van der Waals forces). With respect to this definition of polarity, when discussing dipolar molecules, the dipolarity (rather than polarity) of solvents should be considered. Molecules with a permanent dipole moment are dipolar (not polar). Molecules, which are lacking permanent dipole moment should be called apolar (or nonpolar). In literature, the terms polar and apo-lar are used indiscriminately to characterize a solvent by its relative permittivity as well as its permanent dipole moment, even though dipole moments and relative permittivities are not directly related to each other. 

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