Electron delocalization computational methods

Dithiolene complexes with the maleonitriledithiolate (mnt) ligand form highly delocalized systems and are widespread in studies of conducting and magnetic materials. The electronic properties have been extensively studied with various computational methods including Hiickel and extended Hiickel approaches to identify the nature of the orbitals involved in intramolecular and intermolecular interactions. These structural properties allow the complexes to interact in the solid state via short stacking S, S and short interstack S---S contacts.10 4-1048... 

During the last few years, both neutral and cationic 1,3,2-diazaphospholes and NHP have been studied extensively by computational methods. The best part of these studies focused on a discussion of n-electron delocalization and their implication on chemical reactivities and stabilities, the explanation of the unique ionic polarization of exocyclic P-X bonds noted for some species, and the evaluation of structural and spectroscopic properties with the aim of helping in the interpretation of experimental data. 

Generalized valence bond interaction energies were computed for mono/poly-nitrogenous five- and six-membered heterocycles.203 Results that diverged from those obtained by other methods were obtained only for poly-nitrogenous systems such as pyridazine, benzotriazole, and tetrazole, which may confirm Bird s earlier finding123 204 that electron delocalization is not a stand-alone and direct measure of aromaticity for nitrogenous heterocyclic compounds. 

The reason for this behavior in the case of the double-bonded structures ( A -type phosphorus) is the easy dimerization of the P=C bond. Tricoordinate planar (or nearly planar) phosphorus ( B - and C -type bonding) can be stabilized by repyramidalization when the cyclic electron delocalization is disturbed or lost (e.g., in a chemical reaction). The fine balance between these energetic effects cannot easily be predicted by using analogies or other simple models. Such predictions, however, can be made by using the sophisticated methods of computational chemistry, leaving the field of the chemistry of the aromatic phosphorus compounds an interesting research area with unexpected results in the future. 

Cortes, F. Tenorio, J. Collera, O. Cuevas, G. Electronic delocalization contribution to the anomeric effect evaluated by computational methods./. Org. Chem. 2001, 66, 2918-2924. 

The quality of these methods depends on the force fields parameters, the way the QM and MM parts are linked, and how QM and MM parts affect each other. The main advantages of QM/MM are to present a limited increase of required computer power as a function of the size of the system. The MM part can be constituted by up to thousands of atoms. A drawback is that it is not easy to define a priori what should be the size of the QM and MM parts. Ramanchandran et al." observed in their periodic study that during transition state electron delocalization from the Br0nsted site to others zeolite framework oxygen atoms was an important phenomenon. Then, large QM part is required which makes more costly calculations. Furthermore, another drawback of QM/MM is the complexity of the tuning which can lead to misleading results. ... 

Pyridazine and its derivatives have been extensively studied by a variety of computational methods to correlate the predicted values of their physical properties with the measured ones. As for other azines also for pyridazine several methods were used to calculate its thermodynamic stability, molecular geometry, electron density distribution, ionization potentials, and dipole moments (89KGS1587 91RRC399) and the significance of different criteria for assessing the 77-electron delocalization were also reviewed (92H1631). 

This article is divided into two sections. In the first section, we overview the recent computational electrodynamics studies that have been performed on metallic (silver or gold) particles with an emphasis on problems of more interest to chemistry, such as the detection of molecules through adsorption-induced shifts of the plasmon resonance wavelength. In the second section, we turn our attention to a subject of more direct interest to theoretical chemistry, namely the calculation of SERS intensities using electronic stmcture methods. The challenge to the electronic structure community here is how to treat the interaction of an electronically localized system like a molecule with an electronically delocalized structure like a metal particle that is tens of nanometer in dimension. There have been attempts at dealing with this problem that we will describe, but this is a field that is still in a relatively primitive state, so our review will also consider new developments in the field that are likely to be important in the future. 

If the reader has actually made it up to this point, he or she will have the impression that the whole universe of solid-state materials, i.e., insulators, semiconductors, metals, and intermetallic compounds can nowadays be studied by electronic-structure theory, and predictive conclusions are really in our own hands. Indeed, the numerical limitations of most classical approaches - in particular, the ionic model of everything - have been overcome. While the computational methods of today include very different quantum-chemical methods, their varying levels of accuracy and speed are due to differences in their atomic potentials and the choice of the basis sets that are involved. The latter may either be totally delocalized (plane waves) or localized (atomic-like), adapted to the valence electrons only (pseudopotentials) or to all the electrons. In order to understand structures and compositions of solid-state materials, the results of electronic-structure theory are typically investigated in terms of some quantum-chemical analysis. 

It will become evident in later sections that the nature of the weak noncovalent interactions in a cluster dictate which computational methods will produce accurate results. In particular, it is far more difficult to compute reliable properties for weakly bound clusters in which dispersion is the dominant attractive component of the interaction. For example, Hartree-Fock supermolecule computations are able to provide qualitatively correct data for hydrogen-bonded systems like (Fi20)2 even with very small basis sets, but this approach does not even bind Ne2- What is the origin of this inconsistency Dispersion is the dominant attractive force in rare gas clusters while the electrostatic component tends to be the most important attractive contribution near the equilibrium structure (H20)2- As London s work demonstrated,dispersion interactions are inherently an electron correlation problem and, consequently, cannot be described by Flartree-Fock computations. To this day, dispersion interactions continue to pose a significant challenge in the field of computational chemistry, particularly those involving systems of delocalized n electrons." ... 

Acrolein can be used as an example O ig. 13.1). The major structure is evidently stmcture I, and stmcture II is a reasonable stmcture involved in the electronic delocalization because of the oxygen s larger electronegativity compared to carbons. For the same reason, stmcture III has a priori less chemical relevance. The Huckel-Lewis methods can compute the weights of each stmcture and help to determine if a stmcture is important. 

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