Chemical shift additivity fluorobenzenes

Fifolt [ 130] reported this chemical shift additivity method for fluorobenzenes in two deuterated solvents d6 acetone and d6 dimethyl sulfoxide (DMSO) Close correlations between experimental and calculated fluorine chemical shifts were seen for 50 compounds Data presented in Table 18 result from measurements in deuterochloroform as (he solvent [56] Fluorine chemical shifts calculated by this additivity method can be used to predict approximate values for any substituted benzene with one or more fluorines and any combination of the substituents, to differentiate structural isomers of multisubstituted fluorobenzenes [fluoromtrotoluenes (6, 7, and 8) in example 1, Table 19], and to assign chemical shifts of multiple fluorines in the same compound [2,5 difluoroamline (9) in example 2, Table 19] Calculated chemical shifts can be in error by more than 5 ppm (upfield) in some highly fluonnated systems, especially when one fluonne is ortho to two other fluorines Still, the calculated values can be informative even in these cases [2,3,4,6-tetrafluorobromobenzene (10) in example 3, Table 19]... 

As with other nuclei, theoretical explanations for chemical shift are well developed but are nevertheless unable to predict with any certainty the structural influences on the chemical shift. Thus, there is still widespread reliance on empirical correlations for such problems, which is reflected in the discussion below. Despite the widespread use of fluorine-19 NMR spectroscopy, there are no published, widely applicable additivity rules that enable the chemical shift to be calculated conveniently as they are for carbon-13 spectra. A number of studies have concentrated on the correlation of fluorine chemical shifts in substituted aromatic fluorobenzenes, because of the well defined inductive and resonance... 

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