Congeners, definition

Other congeners of phosphinins—arsenin, antimonin, and bismin—have been shown to be definitely less aromatic than benzene by diverse theoretical treatments that have been reviewed.236 For instance, the Bird aromaticity index for arsenin was found to be 67, compared to 100 for benzene.123 Table 4 summarizes a few parameters used to estimate the aromaticity of heterobenzenes resonance energies... 

Note No definitive structure can be illustrated. Toxaphene is a complex mixture of at least 175 chlorinated camphenes of this number, less than 10 structures are known (Casida et ah, 1974). The following congeners were isolated from environmental samples 2 exo, i endo,5-evo,9,9,10,10-heptachlorobornane, 3-e sro,5-enr/o,6-ej o,8,8,10,10-heptachlorobornane 2 endo, 3-e o,5-e/3c/o,6-e o,8,8,10,10-octachlorobornane, and 2-exo,3-endo,5-exo,6-endo,9,9,l0,l0-octa-chlorobornane (Vetter et al, 2000). According to Pollock and Kilgore (1978), the average chlorine content is 67 to 69%. 

The indispensable contribution of crystallography has been to establish the molecular formulas, the stereoconfigurations, and the absolute configurations of five quite different toxins representative of the five major classes of dendrobatid alkaloids. Each of the five substances had unexpected and novel chemical bonding. Knowledge of the structures from crystal structure analyses led to reevaluations of the spectral properties of analogs and congeners and to the definitions of the structures of more than 200 dendrobatid alkaloids. 

Additionally, there is the crucial consideration of the extent to which such external exposure translates into internal exposure or body burden. To date, there is only one paper that makes a definitive link between indoor contamination (specifically of dust) and body burden for PBDEs (Wu et ah, 2007). While there are few data relating to such bioavailability issues for indoor air and dust, the situation is not appreciably less certain than for dietary exposure. An important recent study (Huwe et al, 2008) has compared the bioavailability of PBDEs administered to rats in both indoor dust and in corn oil. While there was considerable congener-specific variation in uptake, there were no significant differences in uptake for a given congener regardless of whether it was administered in dust or corn oil. 

The three-dimensional steric disposition of the flexible acetylcholine molecule and those of its congeners can be defined on the basis of torsion angle t. Summations and definitions of nomenclature incident to use of torsion angles are found in refs. 257 and 258. Structure (220) defines relevant torsion angles in the acetylcholine molecule. 

Molecular Similarity and QSAR. - In a first contribution on the design of a practical, fast and reliable molecular similarity index Popelier107 proposed a measure operating in an abstract space spanned by properties evaluated at BCPs, called BCP space. Molecules are believed to be represented compactly and reliably in BCP space, as this space extracts the relevant information from the molecular ab initio wave functions. Typical problems of continuous quantum similarity measures are hereby avoided. The practical use of this novel method is adequately illustrated via the Hammett equation for para- and me/a-substituted benzoic acids. On the basis of the author s definition of distances between molecules in BCP space, the experimental sequence of acidities determined by the well-known a constant of a set of substituted congeners is reproduced. Moreover, the approach points out where the common reactive centre of the molecules is. The generality and feasibility of this method will enable predictions in medically related Quantitative Structure Activity Relationships (QSAR). This contribution combines the historically disparate fields of molecular similarity and QSAR. 

Although the constitution of nicotin and of morphin and its congeners is not definitely established, sufficient has already been determined to render it certain that nicotin contains a dipyridyl nucleus and morphin a phenanthrene nucleus. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

Figure 3. An interactive session with CONGEN including definition of the reaction site (SITE), the reaction transform (TRANSFORM) and constraints on the reaction site (CONSTRAINTS) for the example reaction, dehydrochlorination. A summary of the complete reaction is provided by the SHOW command. User responses to CONGEN are underlined (carriage-returns terminate each command).

As previously discussed, the physical-chemical property data for the individual PCB congeners are fragmented, with few properties defined for many congeners, and infrequent definition of the variations of these properties with temperature. As a result, the PCB isomer groups were selected as the fundamental or primary level for defining PCB physical-chemical properties. This reduced the number of fundamental PCB types to ten monochlorobiphenyl, dichlorobiphenyl, trichlorobiphenyl, tetrachlorobi-phenyl, pentachlorobiphenyl, hexachlorobiphenyl, heptachlorobiphenyl, octachlorobiphenyl, nonachlorobiphenyl and decaclorobiphenyl. 

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