Experimental CD spectra

Racemic l,ll-dimethyl-5,7-dihydrodibenzo[c,< ]thiepine A-oxide 24 and A,A-dioxide 25 and 10,12-dihydro-4/7,6/7-[2]benzothiepino[6,5,4- i/][2]benzothiepine 30, based on bridged biphenyl structures, were resolved into enantiomers by chromatography on swollen microcrystalline TAC <2000HCA479>. The first-eluted enantiomers were assigned to have the (A)-configuration by comparison of the experimental CD spectra with spectra calculated by the CNDO/S method (see also Sections 13.03.3.1 and 13.03.3.3). 

Helix. The a> helix possesses a fourfold axis of symmetry, while retaining the i to i+4 hydrogen bonding scheme [77, 78], Therefore, it is looser helical structure than the a helix, and has been identified in forms of poly (L-Phe) [79] and poly (benzyl-L-Asp) [80]. The canonical 6 and y angles are -38° and -78°, respectively. No experimental CD spectra have been reported, but a theoretical CD has been computed [38]. 

Fig. 38. The agreement between simulated and experimental CD spectra for the PD complex was very good after a red-shift of 0.25 eV was applied to the excitation energies. Analysis of the computed spectrum showed the intense bands to originate predominately from n-to-n transitions within an extended n framework of the phosp-hole-helicene ligands. Unlike initially expected, the various bands in the CD spectrum cannot be assigned to transitions centered separately on the helicene and phosphole moieties, respectively. The experimentally measured molar rotation of the Pd complex was 23.1 103 deg cm2 dmol 1 2% in dichloromethane. For an analogous Cu complex it was 13.1 103 2%, a staggering 104 deg cm2 dmol 1 lower.

Fig. 38 Top Structure of the Pd-bis(phosphole-helicene) complex and a comparison of the BHLYP/SV(P) computed and experimental CD spectra. Bottom Two-dimensional structure of the phosphole-helicene ligand as well as the experimental UV-Vis vs the Boltzmann averaged spectrum computed at the BHLYP/SV(P) level of theory, red-shifted 0.25 eV. Data to prepare the plots were taken from [260]...

The CD spectra of enantiomerically enriched [7]paracyclophane 4 was also reported. The observed Cotton effects were slightly weaker than those observed for [6]paracyclophane 2 [18]. The CD spectrum of 9,12-dimethyl-4-oxa[7]para-cyclophane 5 was measured and compared with the theoretical one calculated with the DFT/SCI method. The IAsI values at the main band (40-70 M 1 cm-1) are large relative to those of nonoxa-[7]paracyclophane 4 [19]. It is to note that the calculated deformation angle of [7]cyclophane 5 is similar to those of [2.2]paracyclophanes. The theoretical study has been updated by using the more recent TD-DFT method at the B3-LYP/TZV2P level and all of the four resolved bands in the experimental CD spectra were very well reproduced [8]. 

Several 4,7,12,15-tetrasubstituted[2.2]paracyclophanes were also resolved successfully by chiral HPLC (on a Daicel OD column). The experimental CD spectra were compared with the theoretical ones at a variety of theoretical levels. The simple exciton coupling method [32] was proved to be applied only with careful precautions to the determination of the absolute configuration in most cases the method... 

The experimental CD spectra of these donor-acceptor cyclophanes were very well reproduced by the theoretical calculations at the TD-DFT-BH-LYP/TZV2P level [32]. The amount of the exact exchange in the functional to be used turned out to be important for better agreement with experiment. The analysis of the configuration contributions showed that the observed CD spectra essentially resulted from simple overlap of the Cotton effects of pure n-n transitions in the donor and acceptor parts, in addition to the CT transitions. Thus, the exciton chirality method cannot be applied to the absolute configuration determination for these cyclophanes. The experimental CD spectrum of more congested donor-acceptor cyclophane 22 was reproduced in a less satisfactory manner compared to the cases of 20 and 21. The absolute configuration was safely determined by a comparison of the experimental spectrum with the theoretical data at the TD-DFT level however, the satisfactory reproduction of the whole CD spectrum of 22 was only possible by... 

Fig. 8 Theoretical and experimental CD spectra of (Rp,3 R)-23. Reprinted with permission from [35], Copyright 2009 American Chemical Society...

Fig. 11 Experimental CD spectra of (Rp,Sp)-47, compared with the theoretical ones for (aR)- and (aS)-rotamers. Reprinted with permission from [56]. Copyright 2003 Elsevier Ltd...

Figure 3. Experimental CD spectra of poly[Lys(Z) -1-nag la] in trimethyl phosphate at room temperature, [Nap] = 5.0x10 M. Numbers in the figure indicate the number of LysCZ) units, m.

The experimental CD spectra of two proteins (called ABA-l and RS ) of unknown structure are shown below. What is the likely dominant secondary structure in each case ... 

The atropisomers of 336 were resolved by diastereomer crystallization with (S)-a-phenethylamine. The ee of the process can be monitored by chiral HPLC after methylation of the acids. X-ray structure analysis allowed the assignment the R configuration to the (+) enantiomer. The absolute configuration was also confirmed by comparison of the experimental CD spectra using ethanol with the ZINDO calculated one. 

An example of structnral assignment by the comparison of theoretical and experimental CD spectra is the determination of the helicity of (- -)- and (—)-2-aza-hexahelicene (15) (Fignre llb). In detail, its (P)-enantiomer was arbitrarily... 

The CD characteristics of the different conformations are given and can be used to inter-prete experimental CD spectra. 

Comparison of averaged experimental CD spectra of nucleic acid monomers with whole DNA (E. coli) in the range... 

Fig. 42 Experimental CD spectra of poly(DBF) having a menthoxy terminal group measured at different temperatures (in THE) (a) and theoretical CD spectra (b) of model dimer having different dihedral angles (c). Reprinted with permission from Nakano et al. [36], Copyright 2009, Wiley...

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