Structural criterion

A linear relationship has been shown to exist between structural aromaticity indices and resonance energies (92T335). From this so-called unified aromaticity index, IA, has been proposed. It makes for a more appropriate comparison the aromaticity of heterocycles of different size. 

Charged CgQ systems with completely filled shells, especially are much 

The aromatic stabilization of a molecule is the energy contribution due to the cyclic bond delocalization. This contribution is defined as the resonance energy (RE) 

FIGURE 12. MP2/6-31G optimized geometries of 75d (a), 76d (b) and of their corresponding silanes 77a (c), 78 (d) and of 2-silabenzimidazol-2-ylidene 79b (e) the experimental structure of 79a is given in parentheses. Reprinted with permission from References 100 and 99c. Copyright (1996) American Chemical Society 

---

In the next step, the suggested models are translated into 3D space by subsequently combining the templates. Again, each model is assessed and ranked according to various structural criteria, such as the geometric fit of the 3D templates and non-bonding interactions (steric clashes). If none of the solu-... 

Many studies have been undertaken to establish the structural criteria for the activity of polyhydroxy flavonoids in enhancing the stability of fatty acid dispersions, lipids, oils, and LDL. " As for phenolic acids, the inhibition of oxidation by flavonoids is related to the chelation of metal ions via the... 

According to Cameron and Papike (1982), pyroxenes contain Cr " and TF" in rocks equilibrated at low fo (lunar specimens, meteorites). However, spectroscopic evidence is ambiguous and insufficient for a safe attribution (Rossman, 1982). Some authors (Bocchio et ah, 1979 Ghose et al., 1986 Griffin and Mot-tana, 1982) report the presence of Mn " in Ml sites in clinopyroxene. Davoli (1987) reexamined this hypothesis, proposing precise structural criteria to detect the presence of Mn " in the monoclinic phase (the ratio Mn /Mn may be a potential /02 barometer). 

The formulation of structural criteria rests essentially on the idea that the 7r-delocalization is the factor that causes the aromatic stabilization. The following manifestations of 7r-delocalization are considered in the connection the planar geometry of the ring as a factor dictated by the requirement for better overlap of the p -orbitals, equalization of the lengths of the bonds in the ring, and the correspondence of the most symmetrical structure to a minimum on the potential energy surface (PES). 

According to structural criteria, in particular, to the SINDOl-calculated (90JPC4467) values of RCI, the excited states of pyridine, as distinct from the ground state, do not possess aromatic character. This finding confirms the aromaticity antiaromaticity (nonaromaticity) inversion in the excited state relative to the ground state. 

A compared to the same bond in cyclopropene. These features of the geometry of the borirene ring may be regarded, in accordance with the structural criteria, as evidence for the aromaticity of borirene. 

One answer to this dilemma is to use computational methods to identify the most interesting and informative compounds to pursue. Several workers have devised rules to identify undesirable or desirable compounds (3-12). These applications generally rely upon hard rules or structural criteria applied with limited focus to define interesting compounds. For example, the rule-of-five utilizes a score of 0-4 and is meant to assess bioavailability issues alone. Triage methods eliminate compounds that violate one or more of a variety of rules, which results in a limited set of compounds passing all filters. This is in some... 

Since the formulation of precise and well defined substructure queries is not trivial, other approaches to identify unwanted substructures are used as well. If the decisions made by medicinal chemists whether to accept or reject individual screening hits based on purely structural criteria has been captured, this can be used to train a statistical model predicting medicinal chemists judgment on chemical structures. The consensus among medicinal chemists has been demonstrated to be limited [48]. Therefore, this exercise must be based on the decision of a larger group of chemists in order not to bias the model towards the preferences of any individual chemist. In a similar way such models can be trained on experimental toxicity data for an individual experimentally determined toxicological endpoint. 

Cope rearrangement, 33 301, 303 P = C bond energy estimation, 33 299-300 phospha Cope rearrangements, 33 300-301 ring expansion, 33 300-301 structure criteria for 3,3-diphospha Cope rearrangement, 33 301-302 valence isomerization, 33 303-305... 

Generation of a virtual combinatorial library by finding substituents of a custom scaffold that can be accommodated in the binding site of a molecular target or meet other 3D structural criteria. Once the virtual library is synthesized in the computer, individual members can be selected using structural or additional criteria and synthesized using automated equipment. 

Specific Structural Criteria of Different Classes of Chemical Carcinogens and Mutagens... 

Limited data suggest that depression of brain noradrenaline levels subject to similar structural criteria as ADH activity, not catalase activity (but see (76MH0505))... 

Fig. (2). Structural criteria for effective free radical scavenger activity.

---