Quantum mechanics models, solvent

What the authors did was to combine a MM potential for the solvent with an early (MINDO/2) quantum-mechanical model for the solute. Perhaps because of the biological nature of the journal, the method did not become immediately popular with chemists. By 1998, such hybrid methods had become sufficiently well known to justify an American Chemical Society ACS Symposium (Gao and Thompson, 1998). 

The solvent molecules are represented in terms of their wavefunctions in the quantum mechanical model and as dipoles in the classical model. 

In this contribution we have presented some specific aspects of the quantum mechanical modelling of electronic transitions in solvated systems. In particular, attention has been focused on the ASC continuum models as in the last years they have become the most popular approach to include solvent effects in QM studies of absorption and emission phenomena. The main issues concerning these kinds of calculations, namely nonequilibrium effects and state-specific versus linear response formulations, have been presented and discussed within the most recent developments of modern continuum models. 

The accounting of the quantum mechanical models for the mutual solvent-solute polarization in a self-consistent fashion is perhaps their greatest virtue. However, as already alluded to, the costs of ab initio formalisms may not be warranted—either because they cannot attain accuracies beyond the intrinsic limitations of the continuum solvation model or, alternatively, because they are simply not applicable to a prohibitively large system. In such instances, just as in the gas phase, semiempirical quantum mechanical models often provide an attractive alternative to the classical models discussed earlier. 

Computational models that combine a high level quantum mechanical model of the reaction site surrounded by a layer of lower level semi-empirical or molecular mechanical atoms in the surrounding solvent or enzyme environment will one day make electronic structure calculations of transition states in the condensed phase routine, but these are still some distance in the future. At present, if KIEs calculated from electronic structure models do not match the experimental KIEs, it is necessary to use BOVA to find the transition state. 

The main challenge in the present type of quantum mechanical modeling is to estimate the protonation cost The proton needed for the substrate reaction is ultimately provided by the solvent, a part that cannot be included in the model. To be able to work with a limited model, it is assumed that the resting state of the proton is the position of lowest energy in the quantum chemical model. For most models this position turns out to be the carboxyl-ate. This does not mean that the proton actually comes from the carboxylate or that the mechanism requires that the carboxylate is protonated in the reactant. The procedure simply gives a lower limit for the energy required to protonate the base. 

Recently, Ovchinnikov and Benderskii gave a quantum mechanical model of the hydrogen evolution reaction at a metal electrode. In this model, they have emphasized the importance of Gurney-based model rather than that of Marcus, Levich, and Dogonadze. However, they tried to combine the principal features of the two models. They have pointed out that transition along the reaction coordinate, rather than the solvent coordinate, was important to explain the Tafel behavior and the constancy of the transfer coefficient. 

The change in free energy for the transfer of a solute from an aqueous to an organic solvent can be computed using a continuum solvation model (the generalized Born/surface area, GB/SA) or a quantum mechanical model such as SMI, SM2, SMS,. . . , etc., where SM denotes solvation model. These methods are less CPU intensive than the molecular dynamics or Monte Carlo approaches described below. 

The effective matrix elements Hfj describe only the intramolecular terms associated with the chemical bonding but do not take into account long range and intermolecular interactions. For instance, the dipolar interaction between a solute and the molecules of a polar solvent are not accounted by the plain EHT matrix elements. Since semiempirical methods are much faster, the limitations imposed by the use of a continuum dielectric model for the solvent, which do not provide a good approximation for the immediate solvation shells in the vicinity of the solute or near the solid surface, can be overcome by atomistic quantum mechanical models for the solvent. Dynamic solvation effects can also be included through the semiempirical models. The hybrid QM/MM methods are also a valuable alternative to describe the dynamic effects of solvents on the quantum dynamics of the solute. The dipoles can be either intrinsic or induced. In the case of polar solvents, the electronic part of the dipole moment produced by the kth solvent molecule is f k f),... 

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