Determination of Complex Stability and Stoichiometry by CD

The concentration dependence of ICD has widely been used to investigate intermo-lecular interactions in solution [5, 7, 18]. Several techniques for the calculation of association constants (K ss) have been applied, for instance Benesi-HUdebrand [19], Scatchard plots, [20, 21] and, with the growing capacities of computer calculations, nonlinear curve-fitting procedures [22, 23]. As already mentioned, CyD complexes of type A seem especially well suited for determination of complex stabilities by ICD. Of course, the reliability of the calculated K ss largely depends 

In these cases the agreement is quite good. Landy et al. in their very accurate work [27] come to the concJusion that in the case of substituted phenols, NMR and UV titrations give mucdi more accurate results than ICD, due to the quite unfavorable g fador for these aromatic systems. In general, ICD and CD have more favorable g factors, if magnetically allowed but weak transitions are involved. This holds true for the n n transition of carbonyl compounds, and also for the n — 71 transition of the azo chromophore in diazirines and other cyclic azocompounds. Indeed, in the latter cases exceptionally good results were obtained by ICD [17, 21, 23, 28, 29]. 

If 1 1 and 2 1 complexes are formed, a fact frequently noticed for a-CyD complexes, a change in phenotype (i.e. the shape of the spectrum) with increasing concentration of CyD should be expected. But in certain instances no such changes occur. This is the case for the a-CyD complexes of 2-aziadamantane, as the ICD for the 1 1 complex is too weak to be observed at all, when compared with the very strong ICD for the 2 1 complex [21]. In the complexation of 2,6-diaziadamantane neither phenotype changes, as both diazirine chromophores experience similar effects and, therefore, an additive model can explain the experimental data [28]. 

For complexes of type C the measurement of racemates makes no sense, but only of pure enantiomers [8, 9]. In any case, the CD observed is very complex, as stated in Refs. [41, 42]. If the racemization reaction is quite rapid, as for bilirubins [8, 43] or binaphthyl [44], type D applies, which can give rise to interesting effects. In Ref [44] the very fast association reaction accompanied by the appearance of an ICD (of type A) is followed by the slower diastereoisomerization reaction, yielding a partially resolved racemate complexed by the CyD and thus a strongly modified overall CD spectrum. 

Even if ICD is not applied for calculations of fCass, h is often used to assess the 

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