Diamagnetic shifts

The C-3 methyl chemical shifts in the 3-benzyl-2,3-dimethyloxaziridines 1 and 2 are 23.1 and 14.4 ppm459. It is believed that the anti orientation of the nitrogen lone pair is responsible for a significant diamagnetic shift in isomer 2 (see also Section 4.1.l.l.)458. 

These pentapyrrolic macrocycles contain a 22-jr-electron ring system, and are expected to be the largest members of the [An + 2] n aromatics. In fact, their spectroscopic characteristics, i.e. the intense absorption band in visible region, the large diamagnetic shift of peripheral H and inner NH NMR signals, and relatively strong parent (M+) and dicationic (M2+) mass peaks, indicate the existence of a delocalized cyclic aromatic n system (Table 11). 

The m = 1 states are split by the linear shift and have twice the diamagnetic shift of the m = 0 state. 

C chemical-shift differences, especially at the C-2 and C-5 (a) atoms, as well as at C-3 and C-4 (fi). Using the field effect, which causes diamagnetic shifts, especially at these two pairs of carbon atoms, it was confirmed that, for a given substituent at position 2 or 5, the shift depends mainly on whether it is in the exo or the endo orientation, (see Structure 4.)... 

Figure 2J Diamagnetic shift [E, in cm-1) of the ground state of a C4+ ion as a function of the magnetic field (B, in MG) within dense plasma at plasma electron densities n = 8.055 x 1023/cm3, 5.155 x 1025/cm3 and 1.503 x 1026/cm3. Reprinted with permission from [202] 2001, American Physical Society...

Different changes are observed when quaternization causes inversion of the quinolizidine ring from trans to cis, as is observed in the case of nupharidine (which is an N-oxide) (32) and deoxynupharidine methiodide (71) (41, 69). In such cases the tertiary carbon atoms in the 3 position (C-4, C-10) with respect to the new N+—O" or N+—C bonds exhibit a paramagnetic shift, and the secondary carbon atom (C-6) exhibits a diamagnetic shift in comparison with similar carbon atoms in the free base. 

Sensitized UV irradiation of 1,3-dimethyluracil with 1-methyl-3,4-dibromomaleimide in acetone solution gives the cis-anti-cis-[2 + 2] cycloadduct (25) (41%) (Equation (5)) <83AG(E)i6l>. The presence of methyl substituents at the C-5, C-6, or both of these loci resulted in a severe reduction of yield (1-8%). With 1,3,5-trimethyluracil as starting material, a small amount of the cis-syn-cis-[2 + 2] cycloadduct (3%) was obtained along with the cis-anti-cis-isomer. For the latter, a diamagnetic shifting of the C—Me group by the imide C=0 moiety was noted in its H NMR spectrum. 

However, already Longone and Chow pointed out that rehybridization should play a minor role in diamagnetic shift This in indicated by the 1H—NMR-spectra of the more strained members of the [mjparacyclophane series 8. Despite even a drastic ring distortion their phenylene protons resonate in the usual range (see Table 1). Triene 6 displays two singlet signals of equal intensity (5 = 6.24 and 7.37 ppm). By... 

The slight but reproducible diamagnetic shift of H-2 (0.12 ppm) on introduction of the methoxyl group ( + M, —I) in the remote p-position of the phenyl substituents (Nos. 1 and 2) has been interpreted on the basis of the limiting structures 336 and 337. A pd conjugation179 must be taken into account in a detailed quantum mechanical treatment of the 1,3-dithiolium system.180... 

The replacement of -CH=CH- of benzene by BH would lead to a 4 ti-electron system which should be antiaromatic, and it has been calculated to have a paramagnetic shift at the center of its ring, in contrast to the diamagnetic shift characteristic of the 6 n-electron systems.41... 

The H chemical shifts of phthalocyanine and porphyrine complexes with hexacoordinate siliconas well as theoretical calculations of the contribution to H magnetic shieldingshow a high degree of aromatic nature of the macrocycle compounds of this type can even be used as diamagnetic shift reagents... 

If the substituent Rx had occupied position 4, decarboethoxylation would produce an upfield shift of the 4-alkyl resonance. Instead, there was a pronounced diamagnetic shift (0.5-0.6 ppm) for the 4-proton. 

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