Anisotropic shielding effects

As a continuation of studies of anisotropic shielding effects brought about by a cyclopropane ring, the large high-field shifts within the series 30-34 were examined by Tori and coworkers. In 3 1, Q is shielded by 11.1 ppm with respect to the corresponding... 

One of the most important pieces of progress in recent years is the establishment of a method for the determination of the conformation of the inclusion complex in solution. The method, which takes advantage of the change of the H-chemical shifts of cyclodextrin on complex formation with the aromatic guest compound (mostly due to the anisotropic shielding effect of the aromatic ring of the guest), is briefly described as follows [7-9]. 

The magnitudes of the anisotropic shielding effects by the aromatic rings of the guest compoimds on the protons of the cyclodextrin are evaluated by use of the results of the calculation by Johnson and Bovey [10]. 

Fig. 3 depicts the time-averaged position of phenyl acetates in the cavity of a-cyclodextrin, determined by the above method. In these time-averaged conformations, the centers of the aromatic rings of p-nitrophenyl acetate, phenyl acetate, and nj-nitrophenyl acetate, respectively, are at the heights of 2.2, 1.9 and 1.7 A with respect to the plane comprised of the 6 H-3 atoms of a-cyclodextrin. As shown in Table 1, the calculated values of the anisotropic shielding effects of the aromatic rings of the phenyl acetates on both the H-3 and H-5 atoms agree fairly well with the observed values. 

The spectra are closely similar, indicating that (37) is a completely delocalized homoaromatic species, as is the case for (36), which is contrary to previous results which showed no such charge delocalization in (37) the role of anisotropic shielding effects is discussed. 

Either chloroform or carbon tetrachloride also form the inclusion complexes as shown in Fig. 2. The formation of 2 in Fig. 2 is further confirmed by the fact that the H chemical shift of chloroform is shifted toward lower magnetic field by 0.14 ppm, which is due to the anisotropic shielding effect by the aromatic ring of phenolate ion. 

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