Ring-current contributions molecule

By contrast, in the case of antiaromatic molecules with N = An the contribution is positive (paramagnetic ring current). 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

London orbitals were introduced by Fritz London, who in 1937 used Hiickel theory to calculate the contribution to the magnetizability from the ring currents in the 7r-orbital backbone of some aromatic molecules [13]. The great virtue of London s approach is that each individual AO—the building blocks of molecular wave functions—has been harnessed to respond correctly (to first order at least) to the application of an external magnetic field, irrespective of the choice of the external gauge origin. Moreover, since, in London s approach, only the atomic orbitals are modified, this method is fully transparent to the treatment of the electronic structure otherwise. 

But the most precise attack was probably mounted by J. Musher who argued that the N.M.R. behaviour of aromatic compounds might be described as the sum of contributions from localized electrons of both a and it charac-ter besides, he emphasizes the fact that the so-called ring current is a phenomenon which depends essentially on the group symmetry of the molecules and that this restriction greatly weakens the generality of this concept... 

We have shown, in the first part of this contribution, how the ring current concept was classically associated with the notion of aromaticity. To be consistent with our earlier discussion, we have introduced the concept of potential strobilism to describe the phenomena which are measured by all the magnetic properties of matter in the case of cyclic molecules which are the seat of a ir-electron delocalization, that is of a ring current. This new term is not very pleasing phonetically but it has the merit of deriving from a striking image ... 

Much chemistry can be performed with molecules containing a cyclopropene ring whilst leaving the three-membered ring unaffected. Although a detailed discussion of such functionalization remote from the ring is beyond the scope of the current contribution, the three examples shown below serve to illustrate the possibilities ... 

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