Shielding effect of the aromatic

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]. 

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 1H—NMR spectrum of [2.2]paracyclophane 13-18> consists of two singlets of equal intensity for the methylene (t=6.96) and aromatic protons because of the shielding effect of the two benzene rings, the latter absorb at comparatively high field strength (r =3.70). 

The strong shielding of the -COOCH3 group by approximately 1 p.p.m., which is not solvent-dependent, is due to the anisotropic effect of the aromatic system of the dihydroeburnamenine component. 

The chemistry of cyclophanes has been studied extensively by Cram and others (12), and their following spectroscopic properties are revealed Characteristic absorption around 11 y in IR spectroscopy (13), high field shift of aromatic protons in 1NMR spectroscopy due to the shielding effect of the opposite aromatic nucleus (14), abnormal absorption at ca. 240 nm and red-shifted, broad B-band in UV spectroscopy (15), red-shifted CT-band with TCNE in visible spectroscopy (VS) (14, 16), and characteristic properties in fluorescence spectroscopy (FS) (17, 18) and 13CNMR spectroscopy (19), which are discussed below. 

R = CHjPh), which showed a marked shielding effect of the 8a proton by the phenyl ring. In the unsubstituted compound (81, R = H) the triplet due to the 8a proton is found at S 4.93, whereas in both of the dl isomers of the benzyl compound (81, R = CH2Ph) the 8a proton resonates at S 3.07. The explanation of this remarkable shielding effect is the proximity of the aromatic ring of the pseudoaxial 3-benzyl substituent to the 8a... 

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]. 

The methano-bridged aza[10]annulene 20 [9], a 10 r-analogue of pyridine, is a stable, yellow compound with a quinoline-like odour and a flattened perimeter (UV Amax 364 nm). The NMR data confirm the aromatic character of 20. The perimeter protons combine the features of a-substituted 1,6-methano[10]annulenes and quinoline (see p 318) the upheld shift of the bridge H-atoms (< ch2 = -0.40 / +0.65) is characteristic. It is caused by the shielding effect of the diatropic heteroarene system. The heteroannulene 20 is less basis (pX = 3.20) than pyridine (pX = 5.23) or quinoline pKa = 4.94). 

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