Guests camphor

Figure 3. ITC-measurement of the complexation of (-)camphor by a-cyclodextrin in water. Left Primary heat pulse trace (CFB = cell feedback current) the saw-tooth shape arises from changing the aliquots of titrand solution. Right The time integral of the heat pulses furnishes the titration curve. The solid line represents the best fit to a 2 1 host-guest sequential binding model.

Figure 5. Enantiodifferentiation of camphor by host-guest complexation with a-cyclodextrin. Left The temperature dependence in light water Right comparison of the differences in the enantiodifferentiation in light versus heavy water.

The cavity volume varies from 190 to 390 A3 and the average size of the guest is in the range 160-225 A3 for camphor derivatives and 145-175 A3 for pinane derivatives. Therefore, the proper matching of host and guest was crucial to obtain complexes characterized by the proper fitting and packing... 

Nuclear relaxation rates have been used in studies of CyD complexes for the determination of correlation times, thus giving insight into the dynamics of the host and/or guest in the complex. H selective and nonselective longitudinal relaxation rates enable us to characterize the dynamics of the acridine-yS-CyD complex [9]. Proton longitudinal, Ri( H), and transverse, R2( H), relaxation rates were used to determine the motion of (+)camphor guest molecules in both diastereomeric complexes with a-CyD [10]. The Ri( C) rates were used as early as 1976 to obtain correlation times for guest and host molecules in complexes formed by a-CyD with three aromatic compounds [llj. The Ri( C) rates were also used for the determination of correlation times of constituents and complexes of several CyDs with azo dyes [12]. 

For CyD complexes a number of stoichiometric ratios has been observed [2]. The most commonly reported ratios are H G = 1 1 and H G = 2 1. However, other stoichiometries as well as ternary CyD-containing complexes [47] are known. An example of 2 1 stoichiometry is the camphor-a-CyD complex in which the guest molecule is embedded inside a capsule formed by two host molecules [48]. Fenbu-fen (y-oxo-[l,l -biphenyl]-4-butanoic acid) is an interesting example of a compound which shows stoichiometry dependence on the CyD cavity size. It does not form an inclusion complex with a-CyD, but displays H G = 1 1 stoichiometry with f-CyD and H G = 1 2 stoichiometry with y-CyD [49, 50]. Metoprolol is another such compound which forms 1 1 complexes with a-CyD and f-CyD but with y-CyD it forms an H G = 1 2 complex [51]. A similar phenomenon detected using HPLC for a complex with a first-generation dendrimer is presented in Chapter 5 [52]. On the other hand, 1-adamantanecarboxylic acid and f-CyD form a complex with temperature-dependent stoichiometry, H G = 1 1 at 25 °C and H G = 1 2 at 0 °C [28]. For the complexation of dodecyltrimethylammonium bromide with a-CyD two competing associations with stoichiometries of H G = 1 1 and H G = 2 1 have been reported [53]. Use of the method of continuous variations in such situations becomes questionable and information about the complex stoichiometry is revealed directly from the titration measurement described in Section 9.2.3. 

Quantitative analysis of relaxation rates can also be used for the elucidation of inclusion complex geometry. Longitudinal and transverse H relaxation rates were exploited to determine the orientation of the camphor 4 molecule inside the CyD capsule in the (+)camphor-a-CyD 1 2 complexes applying the model of anisotropic tumbling of the guest molecules [10]. It is noteworthy that in this particular case the complex geometry could not be obtained from analysis of NOE correlations. 

Guest-induced chirality within the cavity of hydrogen-bonded softballs 307, 308, 359, and 360 (a racemic mixtnre of the latter is formed as a resnlt of dimerization of nonsymmetric ligand syntone 148) after encapsnlation of chiral camphor derivatives (Scheme 3.37) has been examined [39] by H NMR method. 

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