Electrostatic Interactions with a Continuum

Besides affecting equilibria and kinetics on single energy surfaces, differential solvation effects on distinct electronic states can cause significant changes in UV-Vis absorption spectra. Such so-called solvatochromic effects are discussed in more detail in Chapter 14. 

The various effects of solvation discussed above may in principle be modeled in different ways. For the remainder of this chapter, we will focus on the utility of continuum solvation models in this regard. Having identified the importance and utility of the free energy of solvation, we will pay special attention to prediction of this quantity as a measure of quantitative accuracy. 

When a solute is immersed in a solvent, its charge distribution interacts with that of the solvent. In a continuum model, rather than representing the charge distribution of the solvent 

The charge density p of the solute may be expressed either as some continuous function of r or as discrete point charges, depending on the theoretical model used to represent the solute. The polarization energy, Gp, discussed above, is simply the difference in the work of charging the system in the gas phase and in solution. Thus, in order to compute the polarization free energy, all that is needed is the electrostatic potential in solution and in the gas phase (the latter may be regarded as a dielectric medium characterized by a dielectric constant of 1). 

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Miertus S, Scrocco E and Tomasi J 1981 Electrostatic interactions of a solute with a continuum. A direct utilization of ab initio molecular potentials for the provision of solvent effects Ohem. Rhys. 55 117-25... 

Paschual-Ahuir J L, E Silla, J Tomasi and R Bonaccorsi 1987. Electrostatic Interaction of a Solute with a Continuum. Improved Description of the Cavity and of the Surface Cavity Bound Charge Distribution. Journal of Computational Chemistry 8 778-787. 

The continuum model, in which solvent is regarded as a continuum dielectric, has been used to study solvent effects for a long time [2,3]. Because the electrostatic interaction in a polar system dominates over other forces such as van der Waals interactions, solvation energies can be approximated by a reaction field due to polarization of the dielectric continuum as solvent. Other contributions such as dispersion interactions, which must be explicitly considered for nonpolar solvent systems, have usually been treated with empirical quantity such as macroscopic surface tension of solvent. 

S. Miertus, E. Scrocco and J. Tomasi, Electrostatic Interaction of a Solute with a Continuum. A Direct Utilization of ab initio Molecular Potentials for the Prevision of Solvent Effects, Chetn. Phys. 55,117 (1981). 

The molecule is often represented as a polarizable point dipole. A few attempts have been performed with finite size models, such as dielectric spheres [64], To the best of our knowledge, the first model that joined a quantum mechanical description of the molecule with a continuum description of the metal was that by Hilton and Oxtoby [72], They considered an hydrogen atom in front of a perfect conductor plate, and they calculated the static polarizability aeff to demonstrate that the effect of the image potential on aeff could not justify SERS enhancement. In recent years, PCM has been extended to systems composed of a molecule, a metal specimen and possibly a solvent or a matrix embedding the metal-molecule system in a molecularly shaped cavity [62,73-78], In particular, the molecule was treated at the Hartree-Fock, DFT or ZINDO level, while for the metal different models have been explored for SERS and luminescence calculations, metal aggregates composed of several spherical particles, characterized by the experimental frequency-dependent dielectric constant. For luminescence, the effects of the surface roughness and the nonlocal response of the metal (at the Lindhard level) for planar metal surfaces have been also explored. The calculation of static and dynamic electrostatic interactions between the molecule, the complex shaped metal body and the solvent or matrix was done by using a BEM coupled, in some versions of the model, with an IEF approach. 

Electrostatic Interaction of a Solute with a Continuum. Improved Description of the Cavity and of the Surface Caviry Bound Charge Distribution. 

Electrostatics. The most difficult aspect of molecular mechanics is electrostatics (35-38). In most force fields, the electronic distribution surrounding each atom is treated as a monopole with a simple coulombic term for the interaction. The effect of the surrounding medium is generally treated with a continuum model by use of a dielectric constant. More... 

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