SCRF methods

If the species is charged then an appropriate Born term must also be added. The react field model can be incorporated into quantum mechanics, where it is commonly refer to as the self-consistent reaction field (SCRF) method, by considering the reaction field to a perturbation of the Hamiltonian for an isolated molecule. The modified Hamiltoniar the system is then given by ... 

A drawback of the SCRF method is its use of a spherical cavity molecules are rarely exac spherical in shape. However, a spherical representation can be a reasonable first apprc mation to the shape of many molecules. It is also possible to use an ellipsoidal cavity t may be a more appropriate shape for some molecules. For both the spherical and ellipsoi cavities analytical expressions for the first and second derivatives of the energy can derived, so enabling geometry optimisations to be performed efficiently. For these cavil it is necessary to define their size. In the case of a spherical cavity a value for the rad can be calculated from the molecular volume ... 

The self-consistent reaction held (SCRF) method is an adaptation of the Poisson method for ah initio calculations. There are quite a number of variations on this method. One point of difference is the shape of the solvent cavity. Various models use spherical cavities, spheres for each atom, or an isosurface... 

The most popular of the SCRF methods is the polarized continuum method (PCM) developed by Tomasi and coworkers. This technique uses a numerical integration over the solute charge density. There are several variations, each of which uses a nonspherical cavity. The generally good results and ability to describe the arbitrary solute make this a widely used method. Flowever, it is sensitive to the choice of a basis set. Some software implementations of this method may fail for more complex molecules. 

One femily of models for systems in non-aqueous solution are referred to as Self-Consistent Reaction Field (SCRF) methods. These methods all model the solvent as a continuum of uniform dielectric constant e the reaction field. The solute is placed into a cavity within the solvent. SCRF approachs differ in how they define the cavity and the reaction field. Several are illustrated below. 

The graph on the right plots the predicted energy difference by SCRF method and solvent environment, and the graph on the left plots the predicted solvent effect for the various methods and solvents. 

Use the RHF/6-31+G(d) level of theory with Onsager SCRF method. 

Optimize the two equilibrium structures in solution, using the Onsager SCRF method and the RHF/6-31G(d) model chemistry. You ll of course need to determine the appropriate cavity radius first. 

Perform frequency calculations on all three optimized structures, using the same SCRF method and model chemistry. 

SCRF keyword 239 input for 239 options 239 SCRF methods... 

Solvation effects on the molecular vibrations of 128 were studied by SCRF methods and by supermolecular approaches of 128 with one water molecule [97JPC(B) 10923, 98JPC(A)6010]. Correlations between the N—H (uracil) and O—H (water) bond elongations and the corresponding frequency shifts of the stretching vibrations are reported as... 

Thus, the structure of a-lactones, as reflected in their observed TRIR spectra, is dependent both on substituents at the a-carbon and on solvent polarity, with electron-donating substituents and polar solvents favoring a zwitterionic ring-opened structure. B3LYP calculations using SCRF methods to account for solvent polarity are consistent with these experimental conclusions. 

Although the Kohn-Sham equations form the quantum mechanical core of the DFT/SCRF methods, the final energetical results obtained by these methods also depend on other features of a particular DFT/SCRF implementation. It is important, therefore, to stress that the DFT/SCRF represents a whole family of methods. Each particular implementation may differ in the following details concerning ... 

Conformational Equilibria. The solvent effect on the conformational equilibria represents a typical problem studied using the DFT/SCRF methods. The presence of the environment may affect the free energy of a given conformer, its equilibrium conformation or even destabilize a particular conformation. The DFT/SCRF calculations have been applied to study such effects using various KS methods as well as different techniques for calculating [Pg.112]

Thermochemistry. Chen et al.168 combined the Kohn-Sham formalism with finite difference calculations of the reaction field potential. The effect of mobile ions into on the reaction field potential Poisson-Boltzman equation. The authors used the DFT(B88/P86)/SCRF method to study solvation energies, dipole moments of solvated molecules, and absolute pKa values for a variety of small organic molecules. The list of molecules studied with this approach was subsequently extended182. A simplified version, where the reaction field was calculated only at the end of the SCF cycle, was applied to study redox potentials of several iron-sulphur clusters181. 

F. J. Luque, M. Bachs, C. Alemin, and M. Orozco, Extension of the MST/SCRF method to organic solvents. Ab initio and semiempirical parametrization for neutral solutes in CC14, J. Comput. Chem. 17 806 (1996). 

The most common approach to solvation studies using an implicit solvent is to add a self-consistent reaction field (SCRF) term to an ab initio (or semi-empirical) calculation. One of the problems with SCRF methods is the number of different possible approaches. Orozco and Luque28 and Colominas et al27 found that 6-31G ab initio calculations with the polarizable continuum model (PCM) method of Miertius, Scrocco, and Tomasi (referred to in these papers as the MST method)45 gave results in reasonable agreement with the MD-FEP results, but the AM1-AMSOL method differed by a number of kJ/mol, and sometimes gave qualitatively wrong results. 

In most work reported so far, the solute is treated by the Hartree-Fock method (i.e., Ho is expressed as a Fock operator), in which each electron moves in the self-consistent field (SCF) of the others. The term SCRF, which should refer to the treatment of the reaction field, is used by some workers to refer to a combination of the SCRF nonlinear Schrodinger equation (34) and SCF method to solve it, but in the future, as correlated treatments of the solute becomes more common, it will be necessary to more clearly distinguish the SCRF and SCF approximations. The SCRF method, with or without the additional SCF approximation, was first proposed by Rinaldi and Rivail [87, 88], Yomosa [89, 90], and Tapia and Goscinski [91], A highly recommended review of the foundations of the field was given by Tapia [71],... 

When the SCRF method is employed in conjunction with Hartree-Fock theory for the solute, then the Fock operator is given by... 

Next we discuss two aspects of the physical interpretation of the SCRF method that are well worth emphasizing (i) the time scales and (ii) the assumption of linear response. 

It should be clear from the presentation in the previous section that the SCRF method is a model that by design focuses on only one physical effect accompanying the insertion of a solute in a solvent, namely the bulk polarization... 

Fe3+X6...Fe2+X6, which is the reactant of the outer-sphere electron transfer reaction mentioned above when X = Y. Clearly the ground state involves a symmetric linear combination of a state with the electron on the right (as written) and one with the electron on the left. Thus we could create the localized states by using the SCRF method to calculate the symmetric and antisymmetric stationary states and taking plus and minus linear combinations. This is reasonable but does not take account of the fact that the orbitals for non-transferred electrons should be optimized for the case where the transferred electron is localized (in contrast to which, the SCRF orbitals are all optimized for the delocalized adiabatic structure). The role of solvent-induced charge localization has also been studied for ionic dissociation reactions [109],... 

THE SELF CONSISTENT REACTION FIELD, SCRF, METHOD... 

Values of Geiectrostatic Predicted by Three Quantum-Mechanical SCRF Methods, in kcal/mole." The... 

The similar accuracies of different well-parameterized continuum models implies that they will also perform similarly for the computation of partition coefficients, and that has proven to be the case in most studies to date (see, for example, Bordner, Cavasotto, and Abagyan 2002 and Curutchet et al. 2003b). In Table 11.4 the previously presented SMx results for the chloroform/water partitioning of die methylated canonical nucleic acid bases are compared to results from die MST-ST/HF/6-31G method, and also to purely electrostatic results obtained using a multipole expansion SCRF method. As the latter does not include any accounting for non-electrostatic effects, its performance is significantly degraded compared to the other two. 

Computational studies [89] on the model reactions shown in Scheme 22 at the RHF/6-31G, MP2/6-31G, B3LYP/6-31G in vacuo and including solvent effects via SCRF methods showed that both processes take place via stepwise mechanisms similar to those found for the reaction between ketenes and imines. In both cases the nucleophilic attack of the iminic moiety was the rate-limiting step, the electrocyclic ring closure of intermediates (93, 96) being much faster. However, the first... 

SCRF methods [92-94] (Scheme 27). It was found that the first step of the reaction takes place with a concomitant N-O displacement of the MH3 moiety, thus yielding neutral azadienic intermediates (109), whose electrocyclic conrotatory reaction yields cycloadducts (110). The N-substituted-p-lactams (111) are formed via another O-N pseudopericyclic displacement of the MH3 unit. 

SCRF methods use cavities for the solute which interact with the solvent as a field the induced dipoles lead to an energy stabilisation that is summed up as the solvation energy. MC simulations utilise a combination of force fields for the solvent molecules that statistically (large sampling) interact with solutes usually, differences for solutes in the same solvent... 

---