Structural feature characterization

The transition state of the [2+2]-cycloaddition of ketene and ethene is shown in Figure 15.11. In this transition state, the carbonyl C atom (C2) of the ketene approaches the ethene more closely than the methylene C atom (Cl) does. The two C atoms and the O atom of the ketene fragment no longer are collinear. Yet, all five atoms of the ketene remain in one plane. The structural changes between ethene and the ethene moiety in the transition state are minor. Besides, in the transition state of the [2+2]-cycloaddition, the four atoms that will eventually form the cycloadduct are still far removed from their positions in the cycloadduct. All these structural features characterize this transition state as an early one. Therefore, much as in the case of the transition states of the one-step [4+2]-cycloadditions in Section 15.2.2, the bonding situation can be described by means of the MOs of the separated reagents. 

The quadratic form, however, rests on the very general assumption that whatever structural features characterize the environment, conformational, polarization, density, or other physical properties respond linearly to the field changes, that is, changes in electric field, pressure, or other forces. The quadratic form of the activation free energy is thus broadly vahd independently of the nature of the solvation and reorganization. 

What is an alkene What structural feature characterizes alkenes Give the general formula for alkenes. 

What is a polyester What structural feature characterizes polyesters Give an example of a common polyester. 

Linear low-density polyethylene is a copolymer in which ethylene is polymerized under Ziegler-Natta conditions in the presence of a smaller quantity of a second alkene such as 1-hexene. What structural feature characterizes the resulting polymer ... 

What is meant by the term "unsaturated hydrocarbon" What structural feature characterizes unsaturated hydrocarbons ... 

The structural feature that Figures 3 2 and 3 3 illustrate is the spatial relationship between atoms on adjacent carbons Each H—C—C—H unit m ethane is characterized by a torsion angle or dihedral angle which is the angle between the H—C—C plane... 

Perform an optimization of these two derivatives at the PM3 or RHF/STO-3G level in order to discern which is the more favorable isomer (the latter is a very long job). What are the most dramatic structural features that characterize these two isomers Do the bridging carbons remain bonded in the derivative ... 

The rheological behavior of storage XGs was characterized by steady and dynamic shear rheometry [104,266]. Tamarind seed XG [266] showed a marked dependence of zero-shear viscosity on concentration in the semi-dilute region, which was similar to that of other stiff neutral polysaccharides, and ascribed to hyper-entanglements. In a later paper [292], the flow properties of XGs from different plant species, namely, suspension-cultured tobacco cells, apple pomace, and tamarind seed, were compared. The three XGs differed in composition and structural features (as mentioned in the former section) and... 

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

Similarity searching requires the specification of an entire molecule, called the target structure or reference structure, rather than the partial structure that is required for substructure searching. The target molecule is characterized by a set of structural features, and this set is compared with the corresponding sets of features for each of the database structures. Each such comparison enables the calculation of a measure of similarity between the... 

Carbene reactivity is strongly affected by substituents.117 Various singlet carbenes have been characterized as nucleophilic, ambiphilic, and electrophilic as shown in Table 10.2 This classification is based on relative reactivity toward a series of both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituent to act as an electron donor. For example, dimethoxycarbene is devoid of electrophilicity toward alkenes because of electron donation by the methoxy groups.118... 

Because the goal of hit triage is to identify chemical series that hold promise for further optimization, an approach to characterize the ADME properties of a series, not just individual compounds is often useful. Where possible, characterizing the structure-ADME property relationship, in much the same way that a structure-potency relationship is defined, can be valuable for assessing the probability that a given structural series can be successfully optimized. The goals of this ADME property characterization are twofold (1) to identify specific structural features that may be liabilities (benefits), and (2) to identify general structure-ADME property correlations. 

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