Johnson-Bovey model

The spectrum of l,4,5,8-tetraphenyl-9,10-dimethylanthracene (20) shows methyl resonances shifted 1 -3 p.p.m. upheld from the position in 9,10-dimethylanthracene (21). The calculated shift (Johnson— Bovey model) is much larger (47 p.p.m.) presumably because the ring... 

Other studies on systems claimed to be aromatic include calculations on di- and triphenylcyclopropenium cations, in which modifications of the conventional Johnson-Bovey-Waugh-Fessenden model... 

The magnetic dipole approach used earlier gives a ring current contribution calculated as 1.46 ppm. Local contributions can be estimated from the chemical shift of the protons of a nonaromatic reference such as 1,3-cydohexadiene, which resonates at 5.8 ppm (protons in 2- and 3-position). With 5.8 +1.46 = 7.26 ppm, the calculated shift is quite close to the experimental value of 7.3 ppm. Waugh and Fessenden [42] and Johnson and Bovey [43] improved the dipole model by assuming a pair of current loops above and underneath the benzene ring separated by a distance of 0.9 or 1.28 A (Figure 13.5). 

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