Quantum chemistry methods solvent effects

If the activities of the laboratory in this field are said to be at the borders of quantum chemistry and statistical thermodynamics, these two disciplines are declared to be techniques." The problems raised by molecular liquids and solvent effects can be solved, or at least simplified by these techniques. This is firmly stated everywhere the method of calculation of molecular orbitals for the o-bonds was developed in the laboratory (Rinaldi, 1969), for instance, by giving some indications about the configuration of a molecule. The value and direction of a dipolar moment constitutes a properly quantum chemistry method to be applied to the advancing of the essential problems in the laboratory. In the same way, statistical mechanics or statistical thermodynamics constitute methods that were elaborated to render an account of the systems studied by chemists and physicists. In Elements de Mecanique Statistique, these methods are well said to constitute the second step, the first step being taken by quantum chemistry that studies the stuctures and properties of the constitutive particles. [53]... 

To conclude, we hope that readers will share with us the feeling that ASC methods treating solvent effects open the possibility to introduce a large part of the quantum chemistry methods in the realm of solutions, with a computational cost comparable with that needed for isolated molecules. We also think that this report shows that PCM h a structure flexible enough to allow many other modifications, improvements and extensions. To do it a... 

Within the last decade, ab initio and hybrid quantum-chemical methods were in considerable use in tetrazole chemistry, and the level of calculations significantly improved with extended basis sets used for quite complex polyatomic molecules. During this time, theoretical methods were exploited in the study of several fundamental properties of the terazole ring, such as aromaticity and capability to be involved in various kinds of tautomerism, including the effects of substituents and media on these parameters. It was demonstrated that many physical and physicochemical characteristics of tetrazoles could be successfully estimated by these methods not only for the gas phase but also for the condensed state (solvents, crystals). 

The quantum mechanical (QM) (time-independent) problem for the continuum solvation methods refers to the solution of the Schrodinger equation for the effective Hamiltonian of a molecular solute embedded in the solvent reaction field [1-5]. In this section we review the most relevant aspects of such a QM effective problem, comment on the differences with respect to the parallel problem for isolated molecules, and describe the extensions of the QM solvation models to the methods of modern quantum chemistry. Such extensions constitute a field of activity of increasing relevance in many of the quantum chemistry programs [6],... 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

Broo A, Pearl G, Zemer MC (1997) Development of a hybrid quantum chemical and molecular mechanics method with application to solvent effects on the electronic spectra of uracil and uracil derivatives. Journal of Physical Chemistry A 101 2478—2488. 

The development of the theoretical models and computational methods, which can accurately account for the solvent effects on the NLO properties of molecular systems is one of the largest challenges in the contemporary quantum chemistry. The important conceptional and computational difficulty arises from the fact that the nature of the solute-solvent interaction in the condensed phase is extremely complicated. In modern quantum chemistry the solute-solvent interactions are modeled by a number of different approaches that can be divided into three main groups supermolecular approximations, discrete simulations and continuum models. 

The COSMO method is also interesting as the basis of a very successful COSMO-RS method, which extends the treatment to solvents other than water [27,28]. The COSMO method is very popular in quantum chemical computations of solvation effects. For example, 29 papers using COSMO calculations were published in 2001. However, we are not aware of its use together with MM force fields. Compared with the BE method, COSMO introduces one more simplification, that of Eq. (22). On the other hand, the matrix A in Eq. (21) is positively defined [25], which makes solution of the system of linear equations simpler and faster. Also, because both A and B matrices contain only electrostatic potential terms, their computation in quantum chemistry is easier than calculation of the electric field terms in Eq. (12). Another potential benefit is that the long-range electrostatic potential contribution is easier to expand into multipoles than the electric field needed in BE methods, which may benefit linear-scaling approaches. 

Where quantum chemical methods have been used to study problems in medicinal chemistry and drug design, it has usually been combined with a continuum approximation [90,107-112], rather than explicit simulation, for the solvent effect. As noted, molecular simulations with an explicit solvent are traditionally performed using classical force fields. The reason for this is obvious quantum mechanical calculations are too time consuming. The coupling of QM with continuum approximations has therefore become convenient. However, the so-called hybrid quantum mechanical and... 

Many of the available computations on radicals are strictly applicable only to the gas phase they do not account for any medium effects on the molecules being studied. However, in many cases, medium effects cannot be ignored. The solvated electron, for instance, is all medium effect. The principal frameworks for incorporating the molecular environment into quantum chemistry either place the molecule of interest within a small cluster of substrate molecules and compute the entire cluster quantum mechanically, or describe the central molecule quantum mechanically but add to the Hamiltonian a potential that provides a semiclassical description of the effects of the environment. The 1975 study by Newton (28) of the hydrated and ammoniated electron is the classic example of merging these two frameworks Hartree-Fock wavefunctions were used to describe the solvated electron together with all the electrons of the first solvent shell, while more distant solvent molecules were represented by a dielectric continuum. The intervening quarter century has seen considerable refinement in both quantum chemical techniques and dielectric continuum methods relative to Newton s seminal work, but many of his basic conclusions... 

The late Pierre Claverie was one of the most influential French researchers in the field of intermolecular interactions. His work dedicated to the foundation of molecular force fields and their links with quantum chemistry was truly visionary. Since about 1970, Claverie has pointed out the importance of taking polarization effects into account in molecular mechanics, and he proposed the concept of self-encased different levels of computations regarding solvent effects, thereby showing the road to the present development of hybrid quantum mechanics/molecular mechanics (QMVMM) methods. 

An important aspect of computational chemistry is to evaluate the effect of the environment, such as a solvent. Methods for evaluating the solvent effect may broadly be divided into two types those describing the individual solvent molecules and those that treat the solvent as a continuous medium. Combinations are also possible, for example by explicitly considering the first solvation shell and treating the rest by a continuum model. Each of these may be subdivided according to whether they use a classical or quantum mechanical description. By far the most important solvent is water, and since it is also one of the most difficult systems to model, the majority of methods have been focused on water, and we will use this for exemplification in the following. The effects of solvation can be partitioned into two main groups ... 

So far, we have treated the stationary-state quantum mechanics of an isolated molecule. The molecular properties so calculated are appropriate for gas-phase molecules not at high pressure. However, most of chemistry and biochemistry occurs in solution, and the solvent can have a major effect on the position of chemical equilibrium and on reaction rates. (For a survey of solvent effects on rates, equilibria, IR, UV, and NMR spectra, see C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, 1988.) We now examine solvent effects on molecular and thermodynamic properties. (See also Section 17.6 for semiempirical solvation methods.)... 

In the last years the theoretical organic chemistry has been increasingly extended beyond the gas phase realm of quantum mechanics to the study of the course of chemical reactions in solution. The success of these methods will indicate the begin of a new period for modeling chemistry in solution. Here, we mainly restrict our attention to a static solvent treatment. The discussion of the limitation of this approach was recently continued.Such studies assume the solvation to be in equilibrium with the chemical system at each point along a HP. This basic hypothesis may first be questioned from possibly different time scales of solvent relaxation and the chemical process and, secondly, from the motion of a (limited number) of solvent molecules which may form an important part of the motion of the whole system along the HP. But apart from dynamical nonequilibrium solvation effects and other limitations in the application of TST to reaction in solvents (see Chap. 1.4), static approaches will give much information on the intermolecular interactions and may represent a suitable ansatz for the estimation and interpretation of solvent effects in many cases. 

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