Quantum-chemical calculations initio

An excellent, up-to-date treatise on geometry optimization and reaction path algorithms for ab initio quantum chemical calculations, including practical aspects. 

Water adsorption and dissociation on Ni( 111) and Nig(6 + 2) clusters have been studied by ab initio quantum-chemical calculations.744 "746... 

The simplest discrete approach is the solvaton method 65) which calculates above all the electrostatic interaction between the molecule and the solvent. The solvent is represented by a Active molecule built up from so-called solvatones. The most sophisticated discrete model is the supermolecule approach 661 in which the solvent molecules are included in the quantum chemical calculation as individual molecules. Here, information about the structure of the solvent cage and about the specific interactions between solvent and solute can be obtained. But this approach is connected with a great effort, because a lot of optimizations of geometry with ab initio calculations should be completed 67). A very simple supermolecule (CH3+ + 2 solvent molecules) was calculated with a semiempirical method in Ref.15). 

Quantum chemical calculations (see also theory, MO calculation, ah initio methods) Quinghaosu Quinodimethanes... 

In this review, we consider the application of quantum-chemical calculations to a range of sulfur-rich compounds. The empirical, semiempirical and ab initio MO calculations published before 1980 are considered rather inadequate. Therefore, theoretical calculations before 1980 will not be mentioned in detail here. 

Extensive quantum chemical calculations have been reported for sulfur-rich compounds in the past two decades. These calculations were used to investigate molecular structures and spectroscopic properties, as well as to understand the nature chemical bonding and reaction mechanism. Many high-level ab initio calculations were used for interpretation of experimental data and for providing accurate predictions of molecular structures and thermochemical data where no reliable experimental values are available. In recent years, density functional calculations have been extensively tested and used on many first- and second-row compounds. These proven DFT methods look promising for larger systems because for their computational efficiency. 

How to Find Instanton Trajectory and How to Incorporate Accurate ab initio Quantum Chemical Calculations. 

Now, the general formulation of the problem is finished and ready to be applied to real systems without relying on any local coordinates. The next problems to be solved for practical applications are (1) how to find the instanton trajectory qo( t) efficiently in multidimensional space and (2) how to incorporate high level of accurate ab initio quantum chemical calculations that are very time consuming. These problems are discussed in the following Section III. A. 2. 

Koper MTM. 2004. Ah initio quantum-chemical calculations in electrochemistry, fir Vayenas CG, Conway BE, White RE, Gamhoa-Adelco ME, eds. Modem Aspects of Electrochemistry, No. 36, Berlin Springer, pp. 51-130. 

To judge the bonding properties of SiO and SiS, we compare their experimentally derived force constants and bond energies with those of CO and CS [10]. Further insight into the bonding characteristics is gained from molecular parameters such as geometry and force constant data as well as electron distributions (Tab. 1), which are derived from ab initio quantum chemical calculations. 

The Gaussian programs are the product of Gaussian, Inc., Pittsburgh, Pennsylvania. They perform quantum chemical calculations, using either semi-empirical methods such as AMI, MINDO/3, PM3, or ab initio calculations which have been discussed previously. 

Nyulaszi et al. <1998NJC651> investigated the fused phosphindolizine ring system 2. Their ab initio quantum-chemical calculations have shown this tricoordinated phosphorus to be essentially planar and aromatic in nature. Such planar tricoordinate phosphorus compounds can be used as building blocks for aromatic systems. 

To verify the mechanism presented, the quantum-chemical calculations of proton affinity, Aa, were carried out for modifiers, since the corresponding experimental data are quite rare. The calculations were performed for isolated molecules, since the properties of species in the interlayer space are probably closer to the gas phase rather than the liquid. The values of Ah were calculated as a difference in the total energy between the initial and protonated forms of the modifier. Energies were calculated using the TZV(2df, 2p) basis and MP2 electron correlation correction. Preliminarily, geometries were fully optimized in the framework of the MP2/6-31G(d, p) calculation. The GAMESS suite of ah initio programs was employed [10]. Comparison between the calculated at 0 K proton affinities for water (7.46 eV) and dioxane (8.50 eV) and the experimental data 7.50 eV and 8.42 eV at 298 K, respectively (see [11]), demonstrates a sufficient accuracy of the calculation. 

Our understanding of the physicochemical properties of pyrazines has deepened. The internal rotation and IR spectrum of 2,5-pyrazinedicarboxamide were studied by quantum chemical calculations <05TC73>, and ab initio MO calculations at the MP2/6-31++G( ) level were used to explain the electronic and vibrational properties of complexes of pyrazine and HX linear acids <05JMS2822>. MM and MO calculations were used to investigate the conformational and electronic properties of odor-active pyrazines <05JMS169>, and NMR, IR, X-ray, and DFT methods were used to examine the structures of pyridol l,2-a pyrazinium bromide <05JMS7>. 

The problem has also been addressed by means of ab initio quantum-chemical calculations, showing with the example of the anion radical of 4-chlorotoluene how out-of-plane vibrations may remove the symmetry constraint.93... 

These examples show that the quantum chemical calculation of nuclear spin-spin coupling constants which has been available for the general scientific community only quite recently is an additional valuable and reliable tool for the interpretation and assignment of NMR spectra of carbocations. Taken together quantum chemical ab initio calculation of chemical shifts and nuclear spin-spin coupling constants opens the possibility for a complete simulation of NMR spectra solely based on calculated carbocation structures. 

Fig. 3. Calculated potential energy curve for the rotation about the O-C-C-N bond in the muscimol zwitterion using HF/6-31+G ab initio quantum chemical calculations. Dihedral angles corresponding to the global energy minimum of muscimol and the solid-state conformations of the muscimol zwitterion and the THIP cation are denoted by arrows.

Brehm, L Frydenvang, K., Hansen, L. M., Norrby, P.-O., Krogsgaard-Larsen, P, and Lilje-fors, T. (1997) Structural features of muscimol, a potent GABAa receptor agonist. Crystal structure and quantum chemical ab initio calculations. Struct. Chem. 8,443 451. 

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