Environmental effects, computational studies

The development of DFT computations of electronic g-tensors has mainly focused on improving the accuracy and applicability for isolated systems, while only little attention has been devoted to account for environmental effects. Most studies of solvent or matrix effects on electronic g-tensors have adopted the supermolecular approach, in which the solvent molecules are explicitly introduced into the model used in the calculations. Recently, we developed an electronic g-tensor formalism in which solvent effects are accounted for by the polarizable continuum model [154]. We applied this approach to investigate solvent effects on electronic g-tensors of di-r-butyl nitric oxide (N-I) and diphenyl nitric oxide (N-II). Calculations were... 

The empirical valence bond (EVB) approach introduced by Warshel and co-workers is an effective way to incorporate environmental effects on breaking and making of chemical bonds in solution. It is based on parame-terizations of empirical interactions between reactant states, product states, and, where appropriate, a number of intermediate states. The interaction parameters, corresponding to off-diagonal matrix elements of the classical Hamiltonian, are calibrated by ab initio potential energy surfaces in solu-fion and relevant experimental data. This procedure significantly reduces the computational expenses of molecular level calculations in comparison to direct ab initio calculations. The EVB approach thus provides a powerful avenue for studying chemical reactions and proton transfer events in complex media, with a multitude of applications in catalysis, biochemistry, and PEMs. 

Then, as case study, we consider the glycine and glycyl radicals (Fig. 6.2) in solution. As mentioned above, the calculation of magnetic tensors needs to take into account the several factors such as the geometries, environmental effects, and dynamical effects (vibrational averaging from intramolecular vibrations and/or solvent librations). We use an integrated computational approach where the molecular... 

Computational Studies of Environmental Effects and Their Interplay With Experiment... 

Nowadays, computational techniques have become useful interpretative and predictive tools to investigate environmental effects on properties and processes in supramo-lecular systems of increasing complexity. The purpose of this chapter is to show the capabilities of such techniques, focussing particularly on the simulation of spectroscopic properties, since they allow a direct comparison between calculated and experimental data. Moreover, the computation of the spectroscopic response permits an analysis of the relationship between the nuclear and electronic structure of the molecular probes and the interactions with the environment These ideas are illustrated with case studies involving different spectroscopic techniques and various molecular and environmental systems. 

Chelate cooperative model, 60—61 Chelate cooperativity, 42—43, 51—55, 78 binding of divalent asymmetric ligand, 51 f binding of divalent ligand, 53 factor, 54, 54f—55f Chelate effect, 51—55 Chlorophyll a/b (Chi a/b), 232 Chromophore, 221—222 Cluster, 204—205 COCOC chain, 30-31 Coherences, 230—231 Computational studies of environmental effects... 

Complex supramolecular systems may also be studied by modem computational methods, which have become useful interpretative and predictive tools. Benedetta Menucci, Stefano Caprasecca and Giro Guido review environmental effects on properties and processes involving molecular probes in solution or in biomacromolecular systems. The capabilities of such techniques are demonstrated with a particular focus on simulations of spectroscopic properties, which allow for direct comparison between calculated and... 

Among the several possible environmental effects, in this paper we focus our attention almost exclusively on the influence of the solvent on e hfs, which is the most thoroughly studied by the experiments [8,22]. The solvent can influence the magnetic properties of a nitroxide both polarizing the electron density and modifying the equilibrium geometry. In the past, experimentalists and theoreticians have considered mainly the former aspect. However, since these factors can affect in opposite direction the hfs of nitroxides, it is very important to perform geometry optimizations of nitroxides in solution. Moreover, it is necessary to take into account both the effect of the bulk properties of the solvent (e.g. the dielectric constant) and the influence of specific interactions between the solvent molecules and the nitroxide. As a consequence, the computational recipes must be sufficiently flexible to treat both aspecific and specific solute-solvent interactions. 

Organometallic chemistry is possibly one of the fields in which the application of computational quantum chemistry methods has been most successful [1-3]. Indeed, it is now common practice to complement synthetic or spectroscopic studies with the computational characterization of putative species present in reaction mixtures [4-6]. Detailed mechanistic studies, employing state-of-the-art electronic structure methods (notably, density functional theory, DFT), may also be performed to rationalize experimental outcomes [7, 8]. Many of these studies are based on reduced model systems, in which the reactive moieties are represented explicitly and environmental effects are included by means of mean field theories [9-11]. The approach has turned out to be successful in many instances and may be routinely performed on general-purpose hardware. 

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