Fragments, Chemical Functional Groups

Frequently the chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their H and C shifts. The ammonia scale of N shifts used in... 

Quantum chemical functional groups have been defined as fuzzy electron density fragments (AFDF fragments) associated with a family of nuclei fk,... 

Frequently the chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their 77and C shifts. The ammonia scale of N shifts used in Table 2.3 shows very obvious parallels with the TMS scale of C shifts. Thus, the N shifts (Table 2.3) decrease in size in the sequence nitroso, nitro, imino, amino, following the corresponding behaviour of the C shifts of carbonyl, carboxy, alkenyl and alkyl carbon atoms (Table 2.2). 

Classification of features of the fragments fragments are then classified and coded by taking into account skeletal features, type and multiplicity of chemical functional groups (including masked groups), degree of unsaturation, and carbon number. The... 

Martin and co-workers counted the total number of substructural fragments up to seven bonds long found in entire libraries as an estimate of chemical functional group diversity. They showed that although synthetic oligomeric combinatorial peptoid libraries were far more diverse than combinatorial biopolymer libraries, they still contained only about as many substructures as just the 45 top selling small molecule drugs. 

In solutions the chemical behavior of solute molecules is strongly dependent on the interactions between the solute and solvent molecules. These interactions are critically dependent on those molecular fragments and functional groups of the solute that are accessible to the solvent molecules. If in a particular conformation of a folded chain molecule some functional groups do not appear in the peripheral regions of the molecule, then their interactions with the solvent molecules are severely hindered. It is of some importance to study the question, which parts of a solute molecule are accessible by solvent molecules. ... 

Until the second half of the twentieth century, the structure of a substance—a newly discovered natural product, for example—was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by catalytic hydrogenation and subsequent determination of its location by ozonolysis. After-considering all the available chemical evidence, the chemist proposed a candidate structure (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an independent synthesis. 

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