The ring current mechanism

An extreme example of the generation of a ring current is the azide stretch band at 2025 cm in the VCD spectrum of azidomethemoglobin A (Marcott et al., 1979). With an anisotropy ratio of 0.02 it shows the largest single molecule effect observed today. 

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The VCD of amino acids and transition metal complexes of amino acids has been the subject of ongoing investigation in our laboratory (82-90), from which has emeiged new information on solution geometries and on the ring current mechanism for generating intense monosignate VCD intensity. 

An explanation of the NH stretching VCD in this complex can be formulated in analogy to the ring current mechanism for VCD bias in the C H stretching... 

The spectra in this figure represent one of the strongest cases for the ring current mechanism of VCD intensity enhancement. In this case, currents increase the magnetic dipole transition moment without altering the relative arrangement of electric dipole transition moments and the relative intensity pattern is preserved. In the structures 42 and 43, we show that in the A complex, 42,... 

The allene VCD spectra can be interpreted in terms of the ring current mechanism (114). As shown in 46 and 47, for a conformation that maintains a planar orientation due to conjugation with the allene, for both the phenyl and carboxyl groups relative to the adjacent portion of the allene group, an intramolecular hydrogen bond is possible between the carboxyl hydrogen and a x orbital of the central allene carbon atom. The antisymmetric C=C=C stretch can be de-... 

Accordingly, the mechanism of conduction in metals is different from that for the ring currents in benzene, other aromatic molecules, and graphite, in which the atoms do not have the metallic orbital.16 16... 

C NMR. The membrane perturbation by PRC H+ is found to be relatively smaller than DBC H. The ring current effect on the EPC h NMR signal is not substantial at any site. Only significant NMR downfield shift is noticed in the carbonyl site, as shown in Fig. 10. The difference in the perturbation effect between DBC H and PRC H supports the difference in the trapped site in bilayers and that in the anesthetic mechanism between the two drugs. 

Thus, when one looks sufficiently critically at the ideas currently under discussion one finds that there appears to be no good reason for discarding the conventional tert.-oxonium ion as the propagating species, and that the only clearly identifiable problem is whether propagation through these conventional tert.-oxonium ions goes by the Jaacks mechanism or by the ring-expansion mechanism. This is a difficult matter to resolve experimentally because the direct discrimination between the microcyclic ions (II), (VIII), and (IX) on the one hand and the macrocyclic ion (V) on the other, requires subtle and sensitive techniques. 

A word of warning should be inserted here with regard to the model used in all these calculations. The free electron methods treat the ring current as a line current in the carbocyclic skeleton, and it may be argued that the close agreement between the quantum mechanical methods and the (single) circular magnetic shell method with respect to the calculated chemical shift of benzene implies that the same criticism may be levelled at the non-classical methods (and is probably related to the effects of... 

London < > quantum mechanical treatment of the ring current, London diamagnetism M... 

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