Circular dichroism spectra complexes

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

M(A-A)3 complexes are optically active, and the problem can be remedied if a circular dichroism spectrum of one enantiomer can be measured and the sharp electronic lines identified. The bite and twist angles have very different effects on the rotational strengths. The twist angle has a marked effect that is quite plausible when one considers that a 60° twist causes conversion to the opposite enantiomer. Sign changes in the rotational strength can also occur at twist angles near / = 0°. 

Fig. 8 Circular dichroism spectrum of the calcium complex of tetracycline in methanol-water (50 50)...

Fig. 9 Circular dichroism spectrum of the magnesium complex of tetracycline in aqueous solution...

The magnetic circular dichroism spectrum of thioxanthone and the circular dichroism spectrum of its inclusion complex with cyclodextrin have been measured and interpreted with the aid of PPP and CNDO/S calculations. The first jtJt state exhibits intramolecular charge transfer characteristics <1994JPC10432>. 

Saito and his co-workers have determined the absolute configuration of a number of complexes including D-(—)-[Co(tn)3]Br3 H2O (38), the configuration of which was contrary to that suggested by the circular dichroism spectrum, and D-(+)-[Co(penten)]3+ (39), which conflicts with assignments of the C.D. spectrum based on ion pairing, showing the deficiencies at present inherent in spectroscopic methods. 

B Ligand field circular dichroism spectrum of optically active tetragonal copper(II) complex (adapted from Ref. 11 o). C Magnetic circular dichroism of the ligand field region of copper(II) complex in (A) scale normalized to 10 K Gauss (adapted from Ref. 11 n)... 

Fig. 2. The axial single crystal circular dichroism spectrum (Ae/10) of 2 A-t+XCofenlslCls NaCl 6 H2O (solid curve) and the absorption and CD spectrum of A-(+)-[Co(en)3l(a04)3 in water (upper and lower broken curves, respectively). The axial crystal CD spectrum characterises electronic transitions of the complex ion to upper states of symmetry, polarized perpendicular to the threefold rotation axis of the complex...

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

For soluble and immobilized bromelain, temperature increase is accompanied by a decrease in residual enzyme activity. A more complex form of denaturation occurs with the immobilized enzyme, which may involve a two-phase process. Immobilization offers more resistance to denaturation at the higher temperature of 60°C where the second phase is prolonged by a factor of three [60]. Differential scanning calorimetry experiments showed that bromelain is an exceptional protease among the cysteine proteases, illustrated by the fact that its thermal denaturation is consistent with an irreversible two-state model [61]. Also, the far UV circular dichroism spectrum of bromelain differs from those of papain and chymopapain and therefore represents a third spectral class within the cysteine proteinase family [62],... 

The circular dichroism spectrum of the isomer designated as A-fac-Fe(mphen)32+ is shown in Figure 4. The facial designation is based on the sharp XH NMR signal observed for the resolved complex relative to the broader signal observed for the synthetic mixture of fac and mer isomers. The shielding is analogous for all protons, thus only a small difference is observed. Thus the absolute purity of the facial isomer cannot be assured. 

Theoretical Studies on the Magnetic Circular Dichroism Spectrum of Permanganate and Related Tetroxo Complexes... 

The y-CD - pyrene (1 2) complex shows an exciton coupling type of circular dichroism in the pyrene absorption regions, which indicates that the two pyrene molecules included in the y-CD cavity are facing each other, taking S-helical relationship. On the other hand, designed guests which have two naphthyl moieties at both ends show enhanced excimer fluorescence in the presence of y-CD, taking a folded form in the y-CD cavity (Type Circular dichroism spectrum of... 

C6oPd (-l-)DIOP, was the first optically active fullerene complex reported. Its circular dichroism spectrum shows many Cotton effects. The C70 analog has also been reported and its reactivity to attack by radicals has been studied... 

Enantiomeric (E)-cyclooctene (20E) was first resolved in 1963 through its diastereomeric platinum(II) complex. Synthesis of optically active 20E has been the subject of intensive study since 1968. The first preparation involves the treatment of enantiopure (E)-cyclooctane-l,2-thiocarbonate with triisooctyl phosphate or of (E)-cyclooatane-l,2-trithiocarbonate with l,3-dibenzyl-2-methyl-l,3,2-diazaphospholi-dine. Following analogous synthetic routes, enantiomeric (E)-cycloheptene (18E) can be produced and trapped by 2,5-diphenyl-3,4-isobenzofuran as an optically active adduct. In 1973, the circular dichroism spectrum of enantiopure 20E vapor was recorded in the vacuum UV region down to 150 nm. The first enantiodifferentiating Z-E photoisomerization of cyclooctene sensitized by chiral benzenecarboxylates appeared in 1978. Transfer of chiral information from sensitizer to substrate occurs within the exciplex intermediate. ... 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

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