Quantum-chemical Approaches

1 can t understand why people are frightened of new ideas. I am frightened of the old ones. 

Description is praise. Imitation is love. John Updike 

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The important role of radicals and radical ions in various branches of chemistry (e.g., electrochemistry, radiation chemistry, macromolecular chemistry), their remarkable physical properties and reactivity, as well as the specific problems in a quantum chemical approach, make this region interesting from the theoretical point of view. 

It is thus obvious that among numerous computational methods, first principles quantum chemical approach is indispensable. However, initially first principles quantum chemical calculations required the use of models consisting of a few atoms (clusters) and the range of properties was limited. Since the advent of modem computing resources, as well the models could be extended to cover larger variety of structures as the methodology has been... 

The ultimate goal of most quantum chemical approaches is the - approximate - solution of the time-independent, non-relativistic Schrodinger equation... 

Equation (2) was also used to calculate previous results [19], calculated electrostatic potentials were computed from ab initio wave functions obtained in the framework of the HF/SCF method using a split-valence basis set (3-21G) and a split-valence basis set plus polarisation functions on atoms other than hydrogen (6-31G ). The GAUSSIAN 90 software package [20] was used. Since ab initio calculations of the molecular wave function for the whole... 

The complete theory of catalysis, which would start with the isolated reaction participants, was not available until now because of the lack of adequate knowledge of the participants themselves (even the complete theory of the isolated participants, starting from the first principles, is still lacking). However, in analogy with the homogeneous chemical reactions one can expect that the quantum chemical approach, based on the semiempirical quantum mechanical methods, could be a prospective one. 

Mukherjee, S., Chattopadhyay, A., Samanta, A. and Soujanya, T. (1994). Dipole-moment change of Nbd group upon excitation Studied using solvatochromic and quantum-chemical approaches - Implications in membrane research. J. Phys. Chem. 98, 2809-2812. 

Despite several decades of studies devoted to the characterization of Fe-ZSM-5 zeolite materials, the nature of the active sites in N20 direct decomposition (Fe species nuclearity, coordination, etc.) is still a matter of debate [1], The difficulty in understanding the Fe-ZSM-5 reactivity justifies a quantum chemical approach. Apart from mononuclear models which have been extensively investigated [2-5], there are very few results on binuclear iron sites in Fe-ZSM-5 [6-8], These DFT studies are essentially devoted to the investigation of oxygen-bridged binuclear iron structures [Fe-0-Fe]2+, while [FeII(p-0)(p-0H)FeII]+ di-iron core species have been proposed to be the active species from spectroscopic results [9]. We thus performed DFT based calculations to study the reactivity of these species exchanged in ZSM-5 zeolite and considered the whole nitrous oxide catalytic decomposition cycle [10],... 

Jiang J, Kula M, Luo Y (2006) A generalized quantum chemical approach for elastic and inelastic electron transports in molecular electronics devices. J Chem Phys 124 034708-034710... 

While in principle all of the methods discussed here are Hartree-Fock, that name is commonly reserved for specific techniques that are based on quantum-chemical approaches and involve a finite cluster of atoms. Typically one uses a standard technique such as GAUSSIAN-82 (Binkley et al., 1982). In its simplest form GAUSSIAN-82 utilizes single Slater determinants. A basis set of LCAO-MOs is used, which for computational purposes is expanded in Gaussian orbitals about each atom. Exchange and Coulomb integrals are treated exactly. In practice the quality of the atomic basis sets may be varied, in some cases even including d-type orbitals. Core states are included explicitly in these calculations. 

Of the many quantum chemical approaches available, density-functional theory (DFT) has over the past decade become a key method, with applications ranging from interstellar space, to the atmosphere, the biosphere and the solid state. The strength of the method is that whereas conventional ah initio theory includes electron correlation by use of a perturbation series expansion, or increasing orders of excited state configurations added to zero-order Hartree-Fock solutions, DFT methods inherently contain a large fraction of the electron correlation already from the start, via the so-called exchange-correlation junctional. 

In what follows, standard quantum chemical approach is summarized. The modifications required to describe chemical processes by using an energy basis are... 

Bredas J-L, Comil J, Beljonne D, Santos DAD, Shuai Z (1999) Excited-state electronic structure of conjugated oligomers and polymers a quantum-chemical approach to optical phenomena. Acc Chem Res 32 267-276... 

ArQule provides professional services and products including metabolism models for CYP 3A4, 2D6, and 2C9. The metabolism models are based on combined empirical/quantum chemical approaches and are aimed at predicting the site of metabolism, enzyme-substrate binding affinities (2D6 and 2C9), and relative rates of metabolism at discrete sites within a molecule (274). 

Different quantum chemical approaches can be invoked to calculate the electronic couplings. In many cases one can reliably estimate electron-transfer matrix elements on the basis of a one-electron approximation [27-29]. 

The g-tensor and the hfcs of tyrosine radicals have also been calculated by DFT methods.393,406 412 The agreement between the calculated and experimental data is very satisfying in general, which shows the great improvement of modern quantum chemical approaches. 

Overall the present article seeks to meld chemical graph-theoretic (chemicalbonding) ideas with conventional quantum-chemical approaches, all within the framework of traditional VB theory here extended to encompass more recent results and models. Thence use is made of some quantum-chemical nomenclature, which, however, is standard fare in any of a number of quantum chemistry texts, though they seldom seriously discuss VB models for Jt-network systems. Some effort is made to incorporate solid-state theoretic results on one of the models which has arisen with different applications in mind. As such, the present article offers a novel global perspective which (as is so often the case) emphasizes the author s own work in the area. 

These important, but not completely understood, problems are considered here by using the novel, quantum chemical, approach to the microscopical theory of ferroelectrics and related materials [1], The isomorphous H-bonded crystals M3(H/D)(A04)2 (M = K, Rb A = S, Se) are taken as examples. There are two reasons of such choice. This family is investigated actively at present. Moreover, it is a suitable subject of theoretical examination because of simple chemical constitution of the TKHS-like compounds (zero-dimensional H-bond network). 

Our quantum chemical approach has been applied to explain the thermodynamics of the TKHS-like materials on the level of simple mean (molecular) field approximation [1,4] which, however, is not well adapted to provide for quantitative estimations of the critical temperature of phase transitions. To examine the... 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

The concerted bimolecular /3-elimination reaction of substituted alkanes (X-Ca H2-Cp H2-...) has been studied using the semilocalized quantum chemical approach.3... 

It is clear that in detailed modeling studies, a flexible quantum chemical approach is required to accommodate the variety of electronic states which may be involved. Figure 3.23 displays free energy profiles along the ET reaction coordinate (17) for some examples of the CS and CR type. The definition of 17 and the formulation of the energy surfaces are discussed in Section 3.5.4. 

J.L. Rivail, D. Rinaldi, A quantum chemical approach to dielectric solvent effects on molecular liquids. Chem. Phys. 18, 233-242 (1976)... 

Schuurmann, G., Quantum chemical approach to estimate physicochemical compound properties application to substituted benzenes, Environ. Toxicol. Chem., 14, 2067-2076, 1995. 

Satohiro Yoshida, Shigeyoshi Sakaki, and Hisayoshi Kobayashi, Electronic Processes in Catalysis A Quantum Chemical Approach to Catalysis, Kodansha, Tokyo, 1994. 

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