Chiral configuration

When isotopic substitution creates a centre of chirality, configuration is defined as for other types of substitution (see 2-Carb-16.1 to 2-Carb-16.4). 

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

Addition of organolithium compounds to chiral ( )-configured a, -unsaturated iron acyl complexes (116) proceeds with an exceptionally high degree of asymmetric induction.127-132 The initially formed... 

Fluctuations are inherent to any experimental chemical system. Even if these fluctuations are infinitesimally small, they are sufficient to drive the system away from an unstable state. The optically active state is characterized by two equivalent options starting from an unstable racemic situation, the system can evolve into either an R configuration or into an S one. However, each individual experiment remains unpredictable as to which of the optically active states the system will move towards. For a large number of experiments an equal and random distribution between R and S dominance is expected if the initial conditions do not involve any preferences. Due to this unpredictability of the chiral configuration, the phenomenon of mirror-symmetry breaking introduces another element of stochastic behavior into chemical reactions different from that occurring in clock reactions [38,39]. 

However, because traditional mechanics are based on non-chiral concepts—like the Newtonian center of mass—the effects of chirality on molecular level motion have largely been overlooked [6]. This review is concerned with the relationship between mechanical motion and chirality at the molecular level we will discuss how chirality—or its expression—can be altered through molecular motion, and how a fixed chiral configuration can help to direct motion. But first it is important to briefly describe the physics that governs motion at the molecular level since it is fundamentally different to that which governs movement in the macroscopic world and, in many respects, the differences are somewhat counterintuitive [7]. 

Fig. 10 Chirality due to molecular arrangement. Top-view on achiral and chiral configurations of AA on hopg. The nonadecyl groups stretch out into zigzag configurations on hopg (not shown)...

Keywords NMR spectroscopy RDCs residual dipolar couplings alignment media structure chirality configuration conformation... 

Figure 17 Chiral [Mz+(ox)3]<-6 ZJ building blocks assembled with (a) alternating chiral configuration and (b) equal chiral configuration.

The isomers of [Co(L-cys)(en)2](N03) AgN03 have been structurally characterized. Significantly, the stmcture derived from AL-[Co(L-cys)(en)2]+ leads to a two-dimensional sheetlike structure, while AL-[Co(L-cys)(eu)2]+ affords a oue-dimensional left-handed helix, where the left-handed heUcity is predetermined by the chiral configuration in the starting Co complex. ... 

The chiral l,T-bi-2-naphthol 1, has become one of the most widely used chiral compounds in chemistry.1 The C2 symmetry, rigid structure and highly stable chiral configuration of 1 play an important role in the use of chiral derivatives containing this moiety in stoichiometric and catalytic asymmetric organic reactions. Moreover, the 3,3 -, 4,4 - and 6,6 - positions can be selectively functionalised which further expands the scope of fine tuning the chiral discriminating ability of this moiety. 

P NMR study of the chiral configuration of the unsymmetrical substituted cyclophosphazenes (146) and (147), prepared from (136) via (144) and (145), has shown these compounds to be diastereoisomeric each consisting of two different racemic mixtures. The spermine-bridged cyclophosphazenes (148) appear to exist in meso and racemic forms, which for the gfem-diphenyl derivative were separated by column chromatography and investigated by NMR spectroscopy and X-ray structure determinations. ... 

The excimer of poly(2-napAla) showed an intense circular polarization, indicating that the excimer has a skewed chiral configuration. Therefore, the excimer in poly(2-napAla) may be formed not in the randomly coiled part (e.g., at the termini), where no particular excimer configuration is expected, but in the helix part of the polypeptide. Since the interchromophore distance in the helix part (6,0 A) may be too far to form an excimer, a small distortion of the side-chain and/or main-chain will be necessary to form the excimer. 

The three-carbon core of such molecules most often consists of glycerol, with the hydrophobic tails being made of long-chain fatty acids hnked to glycerol by ester bonds - except in archaebacteria, where they are formed by isoprenoid alcohols hnked to glycerol (in the chiral configuration opposite to that shown in Scheme... 

Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and tlie individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but w hen a majority of a substance has a certain chiral configuration the plane can be rotated in either direction. 

An interesting synthetic and structural investigation deals with the chiral configuration of cyclotriphosphazenes carrying macrocyclic substituents. Two configurations of compound (83) could be isolated from a reaction mixture of (82) and piperazine, viz. a meso- and racemate-form as proven by X-ray analysis. Further aminolysis yielded two meso-forms of (84), one with a plane of symmetry, the other with a center of symmetry. The results are consistent with inversion of configuration at a P(OR)Cl center for each substitution step going from (82) to (84). The patterns of the P NMR spectra are consistent with the X-ray structures. ... 

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