Preferred configuration

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

Unsubstituted isopenams have been prepared as shown in Scheme 67 (80CC928). It is interesting but not unexpected that the thermodynamically preferred configuration has the 3-carboxylate and the H(5) cis. 

Recently the subject of conformational analysis20 has acquired some importance as a branch of organic chemistry. This is the study of the preferred configurations of molecules involving one or more possibilities of internal rotation. A better understanding of the... 

If the reaction center adopts a preferred configuration with respect to the configuration of the penultimate unit in the chain (Scheme 4.1 km kr) then Bernoullian statistics apply. The stereochemistry of the chain is characterized by the single parameter, P m) or P r) [= 1 The -ad concentrations can be... 

Diastereomeric ratios as high as 20 1 can be observed for some of the substrates, e.g., salt [72] [A-8] [41,141]. The selectivity strongly depends upon the polarity of the solvent medium. An increase in the diastereoselectivity is usually observed upon the decrease of solvent polarity. This is interpreted as the result of closer interactions between the ions. In most cases, induced CD spectra could also be measured allowing the determination of the preferred configuration of the chiral cations. 

Recently, the In situ Raman scattering from Fe-TsPc adsorbed onto the low Index crystallographic faces of Ag was examined and the results obtained are shown In Fig. 5 (15). On the basis of the similarities of these spectra with those obtained for the macrocycle In solution phase, as well as the polarization behavior characteristics, It has been concluded that the most likely configuration Is that with the macrocycle edge-on with respect to the surface. This Is In agreement with conclusions reached from the UV-vlslble reflectance spectra. The preferred configuration, however, may depend on the particular macrocycle, as well as on the nature of the adsorption site. 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

I. The preferred configuration is that which achieves the maximum number and strength of bonds, with ct > n > -v relative strengths. 

III. All else being equal, the preferred configuration is that which minimizes the spin polarization (spin-charge) at each atom. 

As it was mentioned in the formation of the NH3 molecule, compounds prefer configurations in which the electron pairs are as far apart as possible. Therefore oxygen undergoes sp3 hybridization resulting in a tetrahedral shape. 

Until now I have discussed the methods of synthesis of optically active polymers from chiral monomers. As is well known in organic chemistry, it is also possible to produce chiral molecules with one preferred configuration by reaction of achiral molecules in the presence of some chiral influence. These reactions are known as asymmetric syntheses (36, 323-325) when an unsatuiated compound is involved, the term enantioface-differenriating reaction is often used (281). 

The stereochemistry of 1,3-dipolar cycloadditions of azomethine ylides with alkenes is more complex. In this reaction, up to four new chiral centers can be formed and up to eight different diastereomers may be obtained (Scheme 12.4). There are three different types of diastereoselectivity to be considered, of which the two are connected. First, the relative geometry of the terminal substituents of the azomethine ylide determine whether the products have 2,5-cis or 2,5-trans conformation. Most frequently the azomethine ylide exists in one preferred configuration or it shifts between two different forms. The addition process can proceed in either an endo or an exo fashion, but the possible ( ,Z) interconversion of the azomethine ylide confuses these terms to some extent. The endo-isomers obtained from the ( , )-azomethine ylide are identical to the exo-isomers obtained from the (Z,Z)-isomer. Finally, the azomethine ylide can add to either face of the alkene, which is described as diastereofacial selectivity if one or both of the substrates are chiral or as enantioselectivity if the substrates are achiral. 

N.M.R. studies on diene models show that the preferred configuration of the lithium compound in hydrocarbons is trans although some cis structure does exist, in equilibrium i.e. the active center itself does exist in two forms (9,10). 

Since the new IUPAC recommendations [5], the nomenclature of quinic acid isomers is very confusing in the literature. Therefore, the latest IUPAC nomenclature is used throughout this paper instead of the older, but still useful, nomenclature. In the IUPAC nomenclature quinic acid is now treated as cyclitol. In the preferred configuration, the carboxy group and the C-4 and C-5 hydroxy groups are equatorial, with the C-l and C-3 hydroxy groups are axial. In the IUPAC system, the former 3-O-acylquinic acids are now renamed 5-O-compounds, and the... 

MDGC performed using a single oven is not a preferred configuration for the analysis of complex mixtures of food volatiles. It is important to keep in mind that no chromatographic system has yet been devised that is capable of completely resolving all the components of a complex mixture. 

The syn-anti interconversions and facial rearrangements necessary to achieve this preferred configuration are generally believed to be accomplished via -q1 -intermediates (equation 147). 

At this point mechanistic studies have reached an impasse. All of the observable intermediates have been characterized in solution, and enamide complexes derived from diphos and chiraphos have been defined by X-ray structure analysis. Based on limited NMR and X-ray evidence it appears that the preferred configuration of an enamide complex has the olefin face bonded to rhodium that is opposite to the one to which hydrogen is transferred. There are now four crystal structures of chiral biphosphine rhodium diolefin complexes, and consideration of these leads to a prediction of the direction of hydrogenation. The crux of the argument is that nonbonded interactions between pairs of prochiral phenyl rings and the substrate determine the optical yield and that X-ray structures reveal a systematic relationship between P-phenyl orientation and product configuration. 

The third case is by far the preferred configuration and also allows longer inlet runs. 

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