Quantum chemical indices

Empirical evidence seems also to support the preference for AG. Ingold based all of the structure-reactivity discussions in his famous book on AG values and believed that this choice is in accord with overall experience, at least as long as very small differences are disregarded (22). Values of AG usually correlate better with quantum chemical indices (199) and with empirical reactivity... 

GAI calculation has been demonstrated for dimethyl methylphosphonate and has been applied to quantitative structure-density correlations for organophosphorus compounds [30]. For an introduction to other quantum-chemical indices, the review of Balasubramanian [31] should be consulted. Ab initio descriptors used in the GIPF approach (see Section 1.5) have been discussed by Murray, et al. [32]. 

Lukovits, I. (1983). Quantitative Structure-Activity Relationships Employing Independent Quantum Chemical Indices. J.Med.Chem., 26,1104-1109. 

Recent studies [325—328] resulted in a revival of quantum-chemical indices their important advantage is that much more heterogeneous sets, e.g. aromatic and heteroaromatic compounds, can be combined in a single QSAR equation (e.g. eq. 50, chapter 3.5). 

Other fields than those implemented in the CoMFA program have been proposed for 3D QSAR analyses, e.g. different interaction fields calculated by the program GRID [33, 909, 910] or hydrophobic fields derived from HINT [918 — 921] (chapter 9.2). In addition, any other parameters, e.g. physicochemical properties like log P or quantum-chemical indices, may be added to the X block, if they are properly... 

To interprete chemical shifts in terms of substituent constants or quantum-chemical indices it suffices to start from the most general expression (111) ( Ramsey s formula (60a)) for the nmr shielding constant a a of an atom A in a molecule, where a a represents the trace of a tensor. 

The most general correlations of the X parameters with quantum-chemical indices, however, are those that involve the tt HOMO CNDO/S electron densities at the allenic central atoms, that is, the squares of the CNDO/S LCAO coefficients for the ir HOMO (1 c). In this case the correlation for mesomeric groups includes substituents bonded via heteroatoms to the allenic skeleton. Equation 145Z> is no longer valid, if also heterosubstituents are taken into account. 

It is noteworthy that high quality QSARs cannot be developed if the critical factors related to bio activity are not present in the set of descriptors chosen. This is well illustrated with the activity of diastereoisomeric insect repellents. These molecules differ only in the spatial configuration of atoms. All topological, geometrical, and quantum chemical indices will have identical (redundant) values for the various diastereomers corresponding to the same empirical formula. Such situations call for novel approaches. As evident from our results on hierarchical overlay, the comparison of good quality structures generated by quantum chemical methods is needed in such cases. 

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