Solute polarizability

Cly-Ala Dipeptide in Aqueous Solution -Polarizable versus Nonpolarizable Force Fields... 

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-... 

In a similar approach, MacRae has included the solute polarizability, which is approximated to a3/2. Equation (7.6) is still valid but Af is replaced by... 

One electrochemical system that can be used to measure the surface tension of the mercuiy/solution interface is shown in Fig. 6.50. The essential parts are (1) a mercuiy/solution polarizable interface, (2) a nonpolarizable interface, (3) an external source of variable potential difference V, and (4) an arrangement to measure the surface tension of the mercuiy in contact with the solution.39... 

Of course, the presence of an electric field means dial a term accounting for the interactions of charged particles with this lield should be included in the solute Hamiltonian. When it is included, the effect is to increase the solute polarity in a fashion proportional to the solute polarizability and the strength of the external lield. Thus, die dipole moment of A increases. The solvent, seeing this increase, itself polarizes and moreover increases its own orientation to oppose A s dipole, and so on. 

Define the molecular cavity and assign gridpoints the appropriate dielectric constant - in classical calculations, the interior is often assigned a dielectric constant between two and four to mimic solute polarizability. 

The OWB equations obtained in this semiclassical scheme analyse the effective polarizabilities in term of solvent effects on the polarizabilities of the isolated molecules. Three main effects arise due to (a) a contribution from the static reaction field which results in a solute polarizability, different from that of the isolated molecules, (b) a coupling between the induced dipole moments and the dielectric medium, represented by the reaction field factors FR n, (c) the boundary of the cavity which modifies the cavity field with respect the macroscopic field in the medium (the Maxwell field) and this effect is represented by the cavity field factors /c,n. 

Aa (Pa) is identified with either the initial or die final ET state, but it may vary with die reaction coordinate (77) due to solute polarizability (see ref. [35]). 

The solvent sensitivity of the diabatic quantities (//DA and AfiDA) is an interesting manifestation of a solvent-driven non-Condon effect, in which the solute polarizability, responding to the RF as the solvent is varied, changes the effective two-state electronic space used to model the ET process [29],... 

Sharp, K., Jean-Charles, J., and Honig, B. (1992) A local dielectric constant model for solvation free energies which accounts for solute polarizability, J. Phys. Chem. 96, 3822-3828. 

The model of polarizable dipolar chromophores suggests that the 3D nuclear reaction field of the solvent serves as a driving force for electronic transitions. Even in the case of an isotropic solute polarizability, two projections of the reaction field should be included the longitudinal (parallel to the difference solute dipole) component and the transverse (perpendicular to the difference dipole) component. The 8 function in Eq. [18] eliminates integration over only one of these two field component. The integral still can be taken analytically resulting in a closed-form solution for the Franck-Condon factor... 

Likewise, it is convenient to define solute polarizabilities in which the molecular polarizabilities are combined with the additional contributions induced by For the linear polarizability one obtains (91),... 

An analogous relation holds for /3 (—w w,0). The solute-solvent interactions are treated here on the level of dipolar reaction fields. Such effects are increasingly taken into account in quantum chemical calculations on linear and non-linear optical properties of molecules (Karelson and Zerner, 1992 Willets and Rice, 1993 Bishop, 1994a Di Bella etai, 1994 Tomasi and Persico, 1994). Therefore, the solute polarizabilities represent a natural level at which theoretical and experimental results should be compared. Expressions (90)-(92) allow us to identify the major terms that are missing when results of NLO solution experiments are compared with theoretical calculations for molecules in the gas phase. It turns out that the contributions induced by the reaction field are substantial and often even larger than those of the isolated molecule (Mikkelsen et al., 1993). 

Hiis polarizability is measured by electric-field-induced second-harmonic generation (EFISHG). Again, local field corrections for the optical fields do not yield the second-order polarizability j8 of the free molecule but rather the solute polarizability /3 which contains a contribution induced by the static... 

K. Sharp, A. Jean-Charles, and B. Honig, /. Phys. Chem., 96, 3822 (1992). A Local Dielectric Constant Model for Solvation Free Energies Which Accounts for Solute Polarizability. 

V is a solute volume term t represents the solute polarizability j8h and a, are measures of hydrogen bond acceptor strength and... 

Fugacity coefficient of the solute in the vapor phase Fugacity coefficient of the solute at the saturation pressure Activity coefficient of the solute in the solution Polarizability of the solute or its chemical class... 

This method is based on the London approximation of dispersion forces, namely in terms of the solute polarizability tensor a, and of the mean excitation energies of solute and solvent molecules, and leads to the compact formula ... 

Orientation factor, proportional to dipole moment of simple molecules (w) Steric parameter (o ) Solute polarizability (P)... 

In this approach, the solute-solvent interactions are modeled using polarizability and the molar volume of the solute. Polarizability, O, is in turn modeled by Ketelaar s method (13). where an atomic contribution scheme is employed. Molar volume is in turn modeled by molecular connectivity indices, %, which are calculated using slightly modified algorithms (9), originally proposed by Kicr and Hall (14.15). These indices encode information on the molecular topology and its hctcroatom content. They have been shown to correlate well with the solutes molar volume, and... 

The most serious limitation remaining after modifying the reaction field method as mentioned above is the neglect of solute polarizability. The reaction field that acts back on the solute will affect its charge distribution as well as the cavity shape as the equipotential surface changes. To solve this problem while still using the polarizable continuum model (PCM) for the solvent, one has to calculate the surface charges on the solute by quantum chemical methods and represent their interaction with the solvent continuum as in classical electrostatics. The Hamiltonian of the system thus is written as the sum of the Hamilton operator for the isolated solute molecule and its interaction with the macroscopic... 

---