Anisotropy of diamagnetic susceptibility of alternant hydrocarbons

Semi-empirical calculations of the magnetic properties of condensed hydrocarbons. Part 2

“…However, so far as is known at present, the role of these effects cannot be quantitatively determined. Furthermore, because of the difficulties in determining accurate experimental values of magnetic susceptibilities and anisotropies, several definitions have been proposed for the various quantities.5a* 28- 31 The formulae adopted here are listed in the Appendix : the approach of ref. (12) is used throughout the calculation together with the values of Pascal's constants for C and H there reported.…”

“…The results are also close to those obtained by UCHF,l where a suitable origin of coordinates must be chosen by a supplementary calculation.1s Again, the comparison with other calculations shows that SCF-CHF gives more reliable values than ASMO 28 and SCM0. 31 In conclusion, these calculations show that CHF theory provides organic chemists with a simple and useful tool for interpreting the magnetic properties of aromatic hydrocarbons, and, at a semi-empirical level, there is no reason to prefer the simpler UCHF method, which in turn seems more suitable for other purposes.8* Furthermore, the inclusion of the dipolar term of vector potential in the gauge factor, seems to give the matrix elements an essentially correct dependence on the geometry.…”

Topological theory of the electronic currents in benzene

“…This sheds new light on the concept of localization of molecular properties, especially for the interpretation of magnetic properties in terms of the current density. The r61e of delocalized currents in the properties of cyclic, conjugated hydrocarbons is an unresolved problem [1,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”

Aromaticity and Ring Currents

“…Third, as we have alluded, Professor Paolo Lazzeretti of the University of Modena has just given the benefit of his own 30 years' experience of the subject via a scholarly, authoritative, and highly readable account, simply entitled Ring Currents , published in a recent issue of Progress in Nuclear Magnetic Resonance Spectroscopy 4 which is enthusiastically commended to readers of this present issue of Chemical Reviews for the very latest information on the status of the ring-current hypothesis. This policy of merely referring to refs −4 instead of formally reviewing the detailed history of ring currents has the added advantage that we, and our readers, are spared the ritualfelt to be obligatory, it seems, by the majority of writers on ring currents over the last 35 yearsof rehearsing and retailing the arguments against the ring-current concept put forward in the mid-1960s by the late Jeremy Musher − and the refutation of them by several authors − (one of the present included 11,12a,13 ). Instead, we shall here simply confine ourselves to a consideration of how workers in this field have tried to correlate the notion of ring currents with the idea of aromaticity.…”