Benzene diamagnetic anisotropy

Values of the diamagnetic anisotropy of benzene and other aromatic hydrocarbon molecules are calculated on the basis of the assumption that the p, electrons (one per aromatic carbon atom) are free to move from carbon atom to adjacent carbon atom under the influence of the impressed fields. When combined with the assumed values for the contributions of the other electrons (-2.0X 10-6 for hydrogen, —4.5 X10 c for aromatic carbon, — 6.0XlO-6 for aliphatic carbon) these lead to principal diamagnetic susceptibilities of molecules in approximate agreement with the available experimental data. The diamagnetic anisotropy of graphite is also discussed. 

The H-NMR spectrum of the parent compound (4) 9,20,30.59 analyzed as an AA BB CC system the ratioJ j/Jj g = 0.73 is as expected when compared both with hetero analogs and with benzene and 1,3-cyclohexadiene. Calculations on the ring current contributions to diamagnetic anisotropy and chemical shifts have been reported. H-NMR data for other benzo[c]furans have been published.66.70,71.109,111.114,132... 

Some diamagnetic crystals (graphite, bismuth, naphthalene and other aromatic substances) show prohounced diamagnetic anisotropy. The observed anisotropy of crystals of benzene derivatives correspond to the molar diamagnetic susceptibility —54 X 10 with the field direction perpendicular to the plane of the benzene ring and —37 X lO"6 with it in the plane. This molecular anisotropy has been found to be of some use in determining the orientation of the planes of aromatic molecules in crystals.1... 

The effects of diamagnetic anisotropy on carbon and on a proton have similar magnitudes, but the much larger paramagnetic shielding renders the phenomenon relatively small for carbon. Thus, benzene (8 128.7) and cyclohexene (8 127.2) have almost identical carbon resonance positions, in contrast to the situation with their protons. The full range of alkene and aromatic resonances is about 8 100-170. 

An explanation offered for the upheld chelation shift envisions a loose association of benzene molecules about the positive end of the chelate complex dipole (21). A similar solvation model has been proposed for TMED -ZnCl2 in mixtures of dioxane and benzene (22). According to the model, the N—CH2 protons of the ligand would be near the benzene 7r-molecular orbitals, the associated diamagnetic anisotropy of which would induce an upheld shift in the N—CH2 proton resonances. 

The calculated diamagnetic anisotropy of metal-free phthalocyanine is fifteen times as great as that of benzene (148, 281). 

The acetylacetonate chelates 8°1 27 form an extensively studied class of complexes for which the suggestion that cyclic conjugation should lead to aromatic stability was applied historically for the first time 81.82), However, Musso etal. 83.84) showed by analysis of vibration spectra that the ti bonds in the chelated ligands are completely delocalized and the use of a mesityl substituent in position 3 as an indicator for diamagnetic ring currents showed no diamagnetic anisotropy comparable to that in benzene. They therefore discarded the concept of cyclic delocalization and aromatic character in these compounds. 

Diamagnetic anisotropy causes the aromatic protons of benzene to be strongly deshielded. 

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