Optical isomer correction

T/K < 5000). The Rotator program takes into account the entropy of mixing and the optical isomer corrections. 

Gas-phase data, 25°C. Entropy changes are corrected for symmetry and optical isomers. (Data from Allinger and Zalkow, 1960) b For closure to trans form c For closure to cis form... 

Under all applied conditions only C 0F4g is formed, probably as three different optical isomers [24—26] (Figure 9.1). Gakh and co-workers determined the correct structure on the basis of F NMR spectroscopy [24]. The structure was confirmed by resolution of the X-ray crystal structure [25, 26]. This shows that, probably, all three optical isomers - namely the two enantiomeric forms with Dj-symmetry and the mesoform with Sg-symmetry- can be foimd in the crystal (Figure 9.1, structures 11 and 12). 

The correct answer is (D). Optical isomers are non-superimposable mirror images. In choice (D), all four groups are different and the molecular structure does not allow for the mirror images to be superimposed. 

The symmetry numbers also allow for the degeneracy of possible return paths from the transition state, which has caused problems in the past [34,32]. The only corrections necessary to the use of symmetry numbers are the additional possibility that the reactant or the transition state may have a number of optical isomers. In this case Gold has shown that the rate coefficient needs to be corrected by a factor /m equal to the ratio of the number of optical isomers in the transition state to that for the reactant [31]. 

Thus, eight optical isomers (tour enantiomeric pairs) are possible. Of these, only the naturally occurring alkaloid, (2S,3R,5S)-(+)-muscarine (also called L-(+)-muscarine), is correctly referred to as muscarine. The 05 carbon of (+)-muscarine has the same absolute configuration as the analogous chiral p carbon in S-(+)-methacholine. 

Optical isomers are molecules that cannot be superimposed onto each other, but are a mirror of each other and these also require an added consideration for the entropy calculation. Hydroperoxy species have one pair of optical isomers. The thermodynamic property that is affected by optical isomers is entropy. The correction for a molecule with two optical isomers is ... 

Where cxopuso is the optical isomer. This correction entropy value is added to the total entropy... 

The principal cardiac glycosides (from digitalis, strophanthus, and squill) have a direct and selective action on heart muscle, increasing the force of the contraction (for details, see Section 14.1). Quinidine, which is an optical isomer of quinine (10.33), and procainamide (7.56) have a therapeutically useful depressant effect directly on the heart and are used to correct arrhythmias. This effect is shown by many other substances which have a relatively lipophilic aromatic ring linked (by an ester, ether, ketone, or carbinol bridge) to a basic group (Thomas, 1981). 

As the four substituents bound to the central carbon are distinct, these are two optical isomers. The entropy correction is therefore equal to 8S = R In 2 = 5.8 J mol K T... 

Another area in which CE is of major use is the isolation of the correct optical isomer. This is important because the biological activity of a drug is often restricted to just one isomer. The other isomer is either inactive or, worse, can have deleterious consequences. CE makes it possible to separate optical isomers even when the conventional electrophoretic techniques fail. 

Methylmalonate is an asymmetric compound. The chemically prepared material should contain two optical isomers, only one of which would be expected to be utilized enzymically. This has been shown to be correct by Beck and Ochoa 96), who found that the chemically prepared methylmalonyl CoA was only half converted to succinyl CoA, while the enzymically formed compound was converted in toto. The authors suggest that the isomerization occurs through a transcarboxylation, wherein the carboxyl group of methylmalonyl CoA is transferred to an acceptor molecule of propionyl CoA, yielding succinyl CoA. 

Normally, the Friedel pairs of structure factors, i.e. F(hkl) = F(-h-k-1), are identical. However, around the absorption edges this law breaks down since the imaginary term adds differently in the phase relation and the correct optical isomer can be determined. 

The correct value for any temperature between 5° and 37° may be calculated from the equation [a] = 85 — 0.1846/. The melting point of the compound varies according to the proportion of a and 0 isomers present. After prolonged digestion with petroleum ether, the a isomer predominates and the melting point rises. A sample of the product recrystallized once melted at 90-93°, and after five recrystallizations it melted at 98°, without change in the optical rotation. 

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