Ab initio Quantum Chemical Calculation

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... 

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. 

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. 

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. 

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... 

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.

Using 3G and 4-31G type basis sets (39-41), ab initio quantum chemical calculations have been carried out for several small structural units of zeolites, with a variety of observed and hypothetical Si-Al distributions (29-32). The results of these studies can be summarized in a series of hypothetical Si - A1 exchange reactions within these structural units. The calculated internal energy changes for the reactions involving two neighbouring tetrahedra, are as follows ... 

The solvent effect on the azo-hydrazone equilibrium of 4-phenylazo-l-naphthol has been modelled using ab initio quantum-chemical calculations. The hydrazone form is more stable in water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form, The calculated results were in good agreement with the experimental data in these solvents. Similar studies of l-phenylazo-2-naphthol and 2-phenylazo-l-naphthol provided confirmation. Substituent effects in the phenyl ring were rationalised in terms of the HOMO-LUMO orbital diagrams of both tautomeric forms [53]. 

The cartoon-like drawing of the structure of the parent bicylobutonium ion C4H7+ 36 is adopted from an ingenious forward-looking paper of Olah and coworkers in 1972, 61) long before routine 13C-FT-NMR spectroscopy and routine ab initio quantum chemical calculations were available, which envisaged correctly the stabilization mode of the parent bicyclobutonium ion to arise from the interaction of the backside lobe of the Cy-Hendo sp3 orbital with the empty carbenium carbon p-orbital at Ca. 

It is beyond the scope of this chapter to review the numerous attempts to model 8, 2 reactivity. Besides models describing the reaction qualitatively in terms of valence bond configurations and mostly aiming at interpreting substituent effects, anomalous Bronsted slopes and appearance of intermediates on the reaction pathway (see Pross, 1985 8haik, 1985, and references therein), essentially two types of quantitative treatments have been developed. The more rigorous one, based on ab initio quantum chemical calculations (Berthier et al., 1969 Jorgensen, 1988) has reached quantitative... 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

The second chapter introduces the student to orbitals proper and offers a simplified rationalization for why orbital interaction theory may be expected to work. It does so by means of a qualitative discussion of Hartree-Fock theory. A detailed derivation of Hartree-Fock theory making only the simplifying concession that all wave functions are real is provided in Appendix A. Some connection is made to the results of ab initio quantum chemical calculations. Postgraduate students can benefit from carrying out a project based on such calculations on a system related to their own research interests. A few exercises are provided to direct the student. For the purpose of undergraduate instruction, this chapter and Appendix A may be skipped, and the essential arguments and conclusions are provided to the students in a single lecture as the introduction to Chapter 3. 

Ab initio quantum-chemical calculations are reported at the level of second-order many-body perturbation theory aimed at the equilibrium between the all -trans (ttt) and the trans-gauche-trans (tgtl conformations of dlmethoxyethane. It is concluded that the gauche effect in dimethoxyethane and by analogy POE is mainly due to the presence of a polarizable environment and not to some intrinsic conformational preference. 

Binary Interaction-Induced Dipoles. Ab initio quantum chemical calculations of interaction-induced dipole surfaces are known for some time (Section 4.4, pp. 159 ff.) Such calculations were recently extended for the H2-He [17] and H2-H2 [18] systems, to account more closely for the dependencies of such data on the rotovibrational states of the H2 molecules [19]. New calculations of the kind and quality are now available also for the H-He system [20], the H2-H system [21], and the HD-He system [22]. New computational methods, called symmetry adapted perturbation theory (SAPT), were shown to be also successful for calculating interaction-induced dipole surfaces of such simple, binary van der Waals systems [23-26]. 

Ab initio quantum chemical calculations show the existence of bonding interactions between acetylene and an alkali metal cation, the complex formed being a nonclassical bridged structure with the electron density transferred to a large extent onto the cation (82IZV891, 82IZV1474, 82IZV1477). 

The problem of tautomeric equilibriums in annelated 1,5-diazepines was studied in [68] by means of NMR spectroscopy, X-ray analysis and quantum-chemical calculations. It was shown that the electron-withdrawing rings (e.g., pyrimidine moiety) fused with the diazepine cycle increase the stability of the antiaromatic enamine tautomeric forms A and C, while in the case of benzodiazepine, a diimine tautomer B was found to be the most stable. Ab initio quantum-chemical calculations and NMR spectroscopic data showed that solvation of seven-membered heterocycles with polar solvents contributes considerably to the stabilization of the enamine forms A and C. This assumption was also proven by X-ray analysis, which showed that in the solid state these diazepines exist in the diimine form B. 

Ab initio quantum-chemical calculations were used in this study for the interpretation of the experimental data (EPR, IR, and optical). The main purpose of the calculations was to elucidate the question of how the geometric factors (spatial structure of the defect) and chemical ones (nature of the substituents) influence the spectral properties of the defects. At present time such calculations can be carried out on acceptable level for systems comparatively small in size. This makes it necessary to use molecular models for calculations of defects in solids. 

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