Absolute Configuration and Detailed Structural Parameters

VCD configurational analysis of a rigid molecule typically consists of the following steps  

For rigid molecules, the procedure described above determines whether the chosen configuration is the right one. Development of the calculation methods based on DFT and the current computer technology enable to carry out an ah initio VCD calculation for isolated gas-phase molecule up to 100 heavy atoms. Solvent effects can also be included to a certain extent. 

As an example of the VCD ability to discriminate absolute configurations, the experimental spectrum of 4-fluorophenyl-3-hydroxymethyl-l-methylpiperidine and the calculated spectra of the (3R,4R)- and (3R,4S)-isomer are presented in Fig. 8.5 [109]. An almost perfect band-to-band correspondence is evident between experimental and calculated spectra of the (3R,4S)-isomer, while the calculated spectrum of the low energy conformer of the (3R,4R)-isomer provides unrealistic VCD pattern with a poor or even no coincidence with the experiment. In the calculated spectrum of the (3R,4S)-isomer, four low-energy conformers were included in the spectral envelope. For comparison, the corresponding calculated and experimental IR absorption spectra are shown. It is evident that the variance with respect to configuration is much more pronounced in VCD spectra While the calculated spectrum for the (3R,4R)-isomer significantly differs from experimental VCD, the unpolarized absorption spectrum of the (3R,4R)-isomer exhibits some common 

The VCD study of the 1,1 -binaphthyl derivatives [110] serves as an example of other structural information that can be obtained by the comparison of experimental and computed VCD. This method allows monitoring not only of absolute chirality of the molecule, but also of the contributions of individual functional groups to the spectra, molecular conformations or some important structural parameter. As an example, we discuss chiral binaphthyls which represent popular building blocks, chiral recognition receptors and catalyst. Controlling the angle between naphthyl planes is important when supramolecular complexes based on these compounds are built. 

The parent binaphthyl 1 exhibits rapid racemization (in minutes) and thus its VCD spectra cannot be obtained. However, other derivatives are stable and their VCD spectra were obtained in most cases for both enantiomers. Excellent agreement in the positions, signs and the relative intensity of the VCD bands for experimental and calculated spectra makes it possible to obtain detailed structural parameters, for example, the torsional angle between the naphthyl planes depending on the substituents given in Table 8.3. It is obvious that the chain-bridged deriva- 

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