Molecular fragment, definition

Debska, B. and Guzowska-swider, B. Fuzzy definition of molecular fragments in chemical stmctures./. Chem. Inf. Comput. Sci. 2000, 40, 325-329. 

Localization of an unpaired electron in the framework of a definite molecular fragment can sometimes lead to the formation of ion-radicals with spatially separated charge and radical sites. They can be considered free radicals with an appended, remote charge. These species form a particular class of distonic ion-radicals. Distonic is from the Greek word diestos and the Latin word distans, both meaning separate. Yates et al. (1984) introduced this term for ions that formally arise by the removal of an electron from a zwitterion or a biradical. 

Triazoline aromatization to triazoles can be achieved by oxidation, isomerization reactions, and elimination of stable molecular fragments, all of which require the presence of a free hydrogen at position 4 and/or 5 of the triazoline ring. The aromatization reaction affords a selective, synthetic route for the preparation of triazoles of definitive structure, inasmuch as azide addition to acetylenes is not regioselective.33... 

As seen, the formation of a closed contour for unpaired electron delocalization is a widespread phenomenon that detains the electron in the framework of a definite molecular fragment and really defines the physical and chemical properties of ion radicals. 

Table 2.5 defines the 40 molecular descriptors and provides their values. Figure 2.1 provides further definition of the different types of molecular fragments used while Figure 2.2 provides further definition of the hydrogen bonding and biphenyl ring corrections. Simamora and Yalkowsky (1994) consider the values in parentheses in Table 2.5 insignificant, based on the statistical analysis used to derive the molecular descriptor values. 

Mathematics and statistics, graph theory, computational chemistry and molecular modelling techniques enable the definition of a large number of theoretical descriptors characterizing physico-chemical and biological properties, reactivity, shape, steric hindrance, etc. of the whole molecule, molecular fragments and substituents. 

In the preceding chapters we have seen how to define and describe the structure of a molecule or molecular fragment (Chapters 1 and 2), how to retrieve the relevant information from electronic databases (Chapter 3) and how to analyze this information with a variety of statistical methods (Chapter 4). Fragment definition, data retrieval and statistical analysis in structure correlation have their analogs in ordinary chemical experiments First a problem is identified, then the data are acquired from an appropriate experiment, filtered to distinguish the relevant from the irrelevant, and processed into a clear summary form. However, this is not the final step. The data still need to be understood. 

If this protocol is applied to disconnect fragments 20 - 23, either carbon at the point of disconnection can be assigned as a donor (d) or an acceptor (a), giving four different possibilities. Disconnect products 20 and 21, for example, become 25 or 26 and 27 or 28, respectively. Fragments 25 - 28 are still not real molecules, and another step is required before the best donor-acceptor pair can be chosen each fragment must be correlated with a synthetic equivalent. Table 1.2 provides a list of common synthetic equivalents, leading to a modified definition of synthetic equivalent, which is a molecular fragment that is equivalent to a real molecule... 

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