“…For a peripheral proton in a conjugated molecule subjected to an external magnetic field B 0 , at right angles to its molecular plane, ,−
Here, B core derives from the inner-shell and σ-electrons and can be thought to include a ‘local' contribution from the π-electrons (thoughas one of us and Haigh have pointed out previously 2 this cannot be rigorously defined, even classically) and B ‘ is the contribution 2,147-150 from the induced π-electron ring currents. The latter is according to the familiar models put forward by Pople, − very much in the spirit of the Pauling 131 /Lonsdale 132 approaches and much extended by others, both classically ,− ,− andthough still semiempiricallyquantum mechanically. − When B ‘ is positive, as for a benzene proton, then at constant frequency (and, hence, constant B local ), a lower value of B 0 would be required for resonance. This term thus makes the benzene proton resonate at comparatively low field; the opposite is true for the cyclophane protons above a benzenoid ring, which therefore absorb at high field. , In, at least, semiempirical work, it is customary to subtract the resonance position of a given proton in a conjugated molecule from that of a hypothetical olefinic proton (similar, perhaps, to the olefinic proton in cyclohexa-1,3-diene 154 ) in the optimistic hope that both B 0 and B core in eq 14 may be eliminated, as localized effects would be expected to be very similar in the two cases.…”