Internal reference, absolute configuration

To obtain the Hamiltonian at zeroth-order of approximation, it is necessary not only to exclude the kinetic energy of the nuclei, but also to assume that the nuclear internal coordinates are frozen at R = Ro, where Ro is a certain reference nucleai configuration, for example, the absolute minimum or the conical intersection. Thus, as an initial basis, the states t / (r,s) = t / (r,s Ro) are the eigenfunctions of the Hamiltonian s, R ). Accordingly, instead of Eq. (3), one has... 

Determinations of absolute configuration by internal comparison are becoming rather frequent (see Table 5) but they may be unreliable since occasionally authors seem to only assume the absolute configuration of the chiral reference component in the structure. 

Table 5. Determinalion of Absolute Configuration by Internal Comparison to a Chiral Reference...

Absolute configurations can be determined by X-ray crystallography, either by the method of anomalous X-ray scattering or by that of internal comparison with a reference of known absolute configuration. 

Tartaric acid serves as the stereochemical connection between the carbohydrates and glyceraldehyde, and is a chiral butanediol that exists in three forms. L-( + )-Tartaric acid, (27, 3/ )-2,3-dihydroxybutanedioic acid 1 (R=H), is referred to as the natural form, and it is widely distributed in nature and classified as a fruit acid. D-( —)-(2 S, 35)-2,3-dihydroxy-butanedioic acid 2 (R=H) is often called unnatural , although it does occur in nature as well. It has the same absolute configuration as D-glyceraldehyde. A third form, called meso-tartaric acid, possess an internal plane of symmetry and is thus inherently racemic and unresolvable. The name tartaric acid is derived from Tartarus, and is probably of medieval and alchemical origin. 

Besides the chiral resolution of pollutants, GC determination of enantiomeric ratios (ERs) can be used for the quantification of the enantiomers in complex matrices, with a known quantity of the pure enantiomer added as an internal standard. With a method of enantiomer labelling, a known quantity of an enantiopure standard is added to the mixture (or an aliquot of it) and the amount of the enantiomer originally present is calculated from the change in the ER after addition of the standard. This method provides precise knowledge of the ERs of the sample and the standards [87]. In addition, the absolute configurations of the pollutants may be determined directly, and free of chiroptical evidence, by GC via co-injection of reference pollutants with known stereochemistry [87]. 

FIGURE 55.6. Absolute configuration of chiral Cgo-fullerene cis-3 bisadduct 9 by X-ray internal reference method. 

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