Ring current mechanism

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-... 

L. A. Nafie and T. B. Freedman, J. Phys. Chem., 90,763 (1986). Ring Current Mechanism of Vibrational Circular Dichroism. T. B. Freedman and L. A. Nafie, Top. Stereochem., 17,113 (1987). Stereochemical Aspects of Vibrational Optical Activity. 

Figure 3 Ring current mechanism (positive VCD) in the C-H stretching vibration of an L-amino acid. Redrawn from Freedman TB, Balukjian GA and Nafie LA (1985) Enhanced vibrational circular dichroism via vibrationally generated electronic ring currents. Journal of the American Chemical Society 107 6213-6222.

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. 

Experimentally, an elegant property of the DDIF experiment is its simplicity. As the structural information is encoded by the internal field provided by the static uniform field and the material itself, the pulse sequence requires no applied field gradients and thus suffers from none of their complications, such as eddy currents, mechanical ringing, miscalibrations or non linearity. 

After solving this system of equations for the response orbitals y/ the induced electronic ring current density is obtained from the y/ using the expectation value of the standard quantum mechanical current density operator ... 

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. 

The mechanisms of substituent effects are numerous23 the most prominent being inductive, mesomeric, neighbor anisotropy, ring-current, electric-field and steric effects, as well as van der Waals compression. 

Rideal, transients, 1401, 1402, 1409 Ring-disk electrode, 1140, 1143 mechanism determination, 1263 ring current in. 1142... 

Nuclear Magnetic Resonance Many textbooks mention charge circulation, inferred from NMR measurements in a magnetic field, applied perpendicular to aromatic planes, as evidence for the delocalization of pz electrons. The only mechanism whereby such a ring current can be induced is by... 

---