Ab initio and DFT

All the techniques described above can be used to calculate molecular structures and energies. Which other properties are important for chemoinformatics Most applications have used semi-empirical theory to calculate properties or descriptors, but ab-initio and DFT are equally applicable. In the following, we describe some typical properties and descriptors that have been used in quantitative structure-activity (QSAR) and structure-property (QSPR) relationships. 

Table 1.18. Comparison of Ab Initio and DFT Calculations of Atomization Energies in keal/mol.

Table 1.19. Comparison of Ab Initio and DFT Enthalpies with Experimentai Values in kcal/mol for the Isodemie Reaction " C H + (3 - )CH4 -> (2 - /2)CjH,...

Both Fe(ll)(TPP) and Fe(II)(OEP) have positive electric quadrupole splitting without significant temperature dependence which, however, cannot be satisfactorily explained within the crystal field model [117]. Spin-restricted and spin-unrestricted Xoi multiple scattering calculations revealed large asymmetry in the population of the valence orbitals and appreciable 4p contributions to the EFG [153] which then was further specified by ab initio and DFT calculations [154,155]. 

Smallwood, C. J., McAllister, M. A., 1997, Characterization of Low-Barrier Hydrogen Bonds. 7. Relationship Between Strength and Geometry of Short-Strong Hydrogen Bonds. The Formic Acid-Formate Anion Model System. An Ab Initio and DFT Investigation , J. Am. Chem. Soc., 119, 11277. 

Table 1 Scaling factors used to improve the frequency predictions of ab initio and DFT calculations...

The electronic nature of the NSN fragment was studied, using both ab initio and DFT methods, for a series of 1,2,5-thiadiazoles and compared to the established zwitterionic structure of naphtha[l,8-rz/][l,2,6]thiadiazine 6 (Figure 1). 

The only selenium diimide to be structurally characterized in the solid state, Se(NAd)2 (Ad=adamantyl),129 adopts the cis,trans conformation. By using ab initio and DFT molecular orbital methods, the cis,Irons conformation is predicted to be most stable for E(NR)2 (E=S, Se R=H, Me, lBu and SiMe3) with the exception of the parent molecules E(NH)2 147 and the unsymmetrical... 

Iron tetracarbonyl and various related unsaturated iron complexes show a remarkably rich diversity of spin-forbidden chemistry. Various reactions of these species have been the object of a large number of experimental studies, including many careful studies of reaction kinetics. These experiments provide an excellent set of data with which to evaluate the power of computation to rationalize observations in a qualitative way, as well as to account in a more quantitative way for experimental observations. The computational work described above is largely drawn from our own published work (23-27), including some unpublished data for some of the reactions (85,87). Nevertheless, many other groups have carried out insightful ab initio and DFT studies of many of the species and processes described. 

The accuracy of the semiempirical results may be different for different classes of compounds, and there are elements that are more difficult than others. Such variations in the accuracy are again less pronounced in high-level ab initio and DFT calculations. 

Semiempirical methods can only be applied to molecules containing elements that have been parameterized, while ab initio and DFT methods are generally applicable. 

For all couplings involving hydrogen the Fermi contact term is dominant, and the other terms may be neglected. Hence this term is dependent on the molecular conformation. Recent studies using ab initio and DFT techniques to calculate the various... 

The tendency of the benzene combustion mechanism to substantially overestimate the formation of phenoxy radical suggested either flaws in the kinetic data involving C6H5O or incompleteness of the combustion mechanism. Subsequent work verified the kinetics and energetics used in modeling C6H5O decomposition. In separate studies, Liu et al. and Olivella et al. used ab initio and DFT models, along... 

Intramolecular reactions that differ from the 1,3-dipolar type are also known for olefinic diazo compounds. cc,p-Unsaturated diazo compounds are known to undergo 1,5-cyclization to give pyrazoles. This reaction type may be considered as a variant of an intramolecular [3 + 2] cycloaddition. A recent ab initio and DFT study classified the cyclization of vinyldiazomethane to 3/7-pyrazole as a monorotatory pericyclic process (318). 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

Ab initio and DFT calculations of the Diels-Alder reactions of vinylallene and diallene with dipolarophiles show that these reactions are concerted processes and that cumulated dienes are less reactive than non-cumulated dienes.153 Electron-rich... 

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