Solute-solvent interactions, long-range

Self-consistent reaction field (SCRF) models are the most efficient way to include condensed-phase effects into quantum mechanical calculations [8-11]. This is accomplished by using SCRF approach for the electrostatic component. By design, it considers only one physical effect accompanying the insertion of a solute in a solvent, namely, the bulk polarization of the solvent by the mean field of the solute. This approach efficiently takes into account the long range solute-solvent electrostatic interaction and effect of solvent polarization. However, by design, this model cannot describe local solute-solvent interactions. 

We have presented and compared different solvation models (continuum, discrete, continuum + discrete) to study solvent effects on molecular properties. In particular, the nitrogen nuclear shielding, which is known to be very sensitive to even small modifications of electronic and/or nuclear charge distributions, has been analyzed. Such alternation/combination of different models has been required to study the complex nature of solute-solvent interactions when both long-range polar and shorter-range specific H-bond effects are active. 

The models just discussed apply to chains in an ideal or 0 solvent, in which excluded volume interactions are absent. In practice, solution properties are more often determined in good solvents in which long-range excluded volume interactions lead to an increase in . These long-range interactions increase with M, so that equation lb is no longer valid. Excluded volume interactions and chain stiffness can be difficult to distinguish without measurements in a 0 solvent. 

The zero mode is the self-diffusion of the center of mass whose diffusion coefficient is given by the Stokes-Einstein relation D = k TIN. The time Tj will be proportional to the time required for a chain to diffuse an end-to-end distance, that is, R )/D = t N b lk T. This means that for time scales longer than Tj the motion of the chain will be purely diffusive. On timescales shorter than Tj, it will exhibit viscoelastic modes. However, the dynamics of a single chain in a dilute solution is more complex due to long-range forces hydrodynamic interactions between distant monomers through the solvent are present and, in good solvents, excluded volume interactions also have to be taken into account. The correction of the Rouse model for hydrodynamic interaction was done by Zimm [79]. Erom a mathematical point of view, the problem becomes harder and requires approximations to arrive at some useful results. In this case, the translational diffusion coefficient obtained is... 

Concerning the best way of treating the solvent molecules, there is still plenty to do before this question can be answered. The supermolecule approach is quite appropriate for describing specific or short-range solvation effects. On the other hand, long-range effects require a large number of solvent molecules and the use of sophisticated ab initio methods to properly describe the solvent-solvent and solute-solvent interactions becomes prohibitive. Hybrid methods (QM/MM), where part of the solvent structure is represented by a MM force field, have not been extensively tested. In... 

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