Diamagnetic current

Meisenheimer s rearrangement, 2 466 Meissner diamagnetic current, 23 813 Meissner effect, 23 802—803 Meissner process, 77 163-164 Meitnerium (Mt), 7 492t MEK (methyl ethyl ketone) dehydration, 78 515... 

In saturated cyclic hydrocarbons, diamagnetic currents are induced due to circulating bond electrons. Although, the effect is weak, but this is sufficient to distinguish between axial and equatorial protons is cyclohexane. The axial protons are shielded and they absorb 0.5 ppm upfield as compared to the equatorial. 

Diamagnetism current, 22 199-201 in metal-metal bonding, 5 26 shielding, 22 200 states, 32 16 terms, 22 207 theory, 22 204 Xpes and, 22 207-210... 

The scheme named continuos transformation of origin of the current density , see (27t21t22t28) proves that the transformed diamagnetic current density tensor,... 

The Larmor precession produces a diamagnetic current I induced by H0 ... 

This aromatic time-honoured species, first prepared by Faraday in 1825, is of course a highly symmetric stable molecule, C6H6, featuring six equivalent protons, yielding an uninteresting single NMR line. The term aromatic implies presence of enhanced diamagnetic currents provided by electrons in the species.75 6... 

Diamagnetic currents on the atom in question. This depends on the absolute electron density on, for example, the proton and gives a measure of the ionic character of the bond this is therefore of direct relevance to a discussion of cis and trans effects. 

This result is to be equated to the time rate of change of A and the flux in its paramagnetic and diamagnetic currents through the surface of the atom using eqn (8.228). Thus the forces exerted on the electrons in an atom Q arising from the electric fields within and external to the molecule and from... 

Diamagnetic currents arise around spin vortices given by (36). Then, after the vortex flow from x = 0 to x = L c, the phase change of x for is given by... 

So it is that the two cyclazines, 33 and 34, formally similar in structure, have n.m.r. spectra dominated by the ring currents which are surely present in their peripheral K-systems a diamagnetic current in the case of the [10]annulene-like cycl[3,2,2]azine and a paramagnetic current in the [12]annulene-like cyd[3,3,3]azine. 

Thus, by the mere addition of two electrons to [16]annulene the inner protons are shifted from r —0.61 (—140 °C) to t 18.17, whilst the outer protons t 4.70 (—140 °C) are shifted downfield to t 1.17 and t 2.55, such noticeable shifts paralleling the change from a system possessing a paramagnetic ring current to one with a diamagnetic current. 

Similarly, the monohomocyclooctatetraene di-anion, 74, (see Table 15) 212,218) deshielded peripheral protons, and a differential shielding of the methylene protons, whereby that proton situated above the ring (H(a), t 10.0) is more shielded than the "outer" proton (H(b), t 8.0). The chemical shifts of the peripheral protons (centered at ca. t 4.5 to 4.8) are shifted upfield from those of the parent hydrocarbon by only 0.4 to 0.7 ppm. As the upfield shift expected from charge density considerations alone is 2.5 ppm, a deshielding effect associated with a diamagnetic current is clearly implicated 218>. 

The current density is an observable which can be visualised for a molecule through oriented contour lines. Usually the applied field is perpendicular to the plane of the paper, and points inward. Clockwise circulation corresponds to a diamagnetic current since it induces an internal magnetic field pointing against the applied field [21]. 

Circulation of electrons around a nucleus can be viewed as being similar to the flow of an electric current in an electric wire. From physics, we know that the flow of a current through a wire induces a magnetic field. In an atom, the local diamagnetic current generates a secondary, induced magnetic field which has a direction opposite that of the applied magnetic field. 

This salt forms colourless crystals which are stable in solution in tetrahydrofuran at room temperature. The dianion has 10 ir-electrons. Its H-n.m.r. spectrum [signals at S 4.98 (4H) and 5.72 (2H)] reflects the offsetting of the shift due to a diamagnetic current by an upfield shift due to the double negative charge. It react.s similarly to the cyclopentadienide ion with methyl lithium and dichloromethane to give a benzvalene derivative which explodes violently at temperatures as low as -40° [187] ... 

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