Dipolar solute Dipole moment

That electrostatic forces could be crucial to vibrational energy relaxation was amply demonstrated by the liquid water simulations of Whitnell et al. (34). They noted that since the electrostatic portion of the force between their solvent and a dipolar solute was linear in the solute dipole moment, Equations (12) and (13) implied that the electrostatic part of the friction ought to scale as the dipole moment squared. When they then found that their entire relaxation rate scaled with the square of the solute dipole moment, it certainly seemed to be convincing evidence that electrostatics forces were indeed the primary ingredients generating ultrafast relaxation. Subsequent theoretical work on relaxation rates in such manifestly protic solvents as water and alcohols has largely served to reinforce this message (37,38,60,61). 

For a pure dipolar liquid the prefactors m (101) reduce to the Onsager-Lorentz factor (87). We use a double bar ( ) for the effective dipole moment in the energy expression (100). Note that the dipole moment (101) is different from the effective solute dipole moment (102) which results from (84), (88) and (90)... 

Dipolar solute in a nonpolar solvent. In this case, the forces contributing to solvation are dipole-induced dipole and dispersion forces. If the solute dipole moment increases... 

Dipolar solute in a polar solvent. Since the ground-state solvation results largely from dipole-dipole forces in this case, there is an oriented solvent cage around the dipolar solute molecules, resulting in a net stabilization of their ground state. If the solute dipole moment increases during the electronic transition the Franck-Condon excited... 

On the other hand, electric dipolar moments of the solute molecules can be obtained with standard methods in ab initio molecular orbital calculations, whereas the induced dipole moments in solution are determined from differences between the values obtained in solution and in the gas phase. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

When a nonpolar solute is in solution in any solvent, either nonpolar or polar, then mainly dispersive forces operate between them, and any solvent effects are very small and bathochromic (Reichardt, 1988), increasing with the polarizability of the solvent. If the solute is dipolar in a nonpolar solvent, then both hypso- and bathochromic shifts, increasing with solvent polarizability, are possible, depending on the dipole moments of the ground and excited states. The situation becomes more complicated for a dipolar solute in a dipolar solvent. 

Polypeptides are electrically polar, carrying permanent dipoles at the planar CO-NH groups of the backbone chain and generally at some atomic groups of the side-chains. Because of the vector nature of dipoles, we must speak of the mean-square dipole moment, averaged over all possible conformations of the backbone chain and all accessible orientations of the side-chains when the dipolar nature of a polypeptide in solution is considered. The of a polypeptide thus may depend on what conformation the molecule assumes in a given solvent. 

Whether this treatment is valid for a dipolar solute in a mixture of polar solvents is doubtful, but it would not appear to be valid for ions without intrinsic dipole moments. Nevertheless, Equation 53, as a ratio of wavelength shifts, has been applied to the analysis of CTTS spectra (19, 20, 21). 

Figure 2.5 Ordering of solvent molecules around (a) a dipolar solute molecule and (b) a solute positive ion. The orientation will be most pronounced in the innermost shell of solvent molecules and will become increasingly random as distance from the solute particle increases. The strength of the interaction will depend on the molecular sizes and shapes and on the magnitudes of the dipole moments of both solutes and solvent particles.

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

In spite of claims to the contrary, to date no completely satisfactory method exists to calculate the polarity / polarizability parameter, n, as it applies to the equilibrium of solute between water and octanol. The excess molar refractivity of the solute (compared to an alkane of equal size) can be estimated separately from polarizability/dipolarity (Abraham, 1994) and seems an attractive approach to this problem, but it needs further verification. The dipole moment of the entire molecule has been used as a polarity parameter (Bodor, 1992), but there are good reasons to believe it has marginal value at best. The square of the dipole moment also has been used (Leahy, 1992), and it, at least, has some theoretical basis (Kirkwood, 1934). 

Here inv stands for an invariant in respect to transformation consistent with the symmetry of the system. For quantum mechanical operators, this means unitary transformations. The parameter Ae in Eq. [107] quantifies the extent of mixing between two adiabatic gas-phase states induced by the interaction with the solvent. For a dipolar solute, it is determined through the adiabatic differential and the transition dipole moments... 

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