The fluorine chemical shifts and spin-spin coupling constants of 65 pentafluorophenyl derivatives with widely varying organic substituents were examined. Useful correlations of the three meta coupling constants with the chemical shifts of thepara fluorine were found. It is suggested that these relationships be extended to all compounds of the type considered in order to determine the signs and approximate values of meta coupling constants.
Equations for correlation of the fluorine chemical shifts with the Taft constants are presented. The possibility of calculating the Taft constants from the 19F NMR spectra of pentafluorophenyl compounds is being discussed.
THE FLUORINE NMR spectra of pentafluorophenyl derivatives have been examined previously.1tog In this work the fluorine NMR spectra of such compounds with more widely varied organic substituents are analysed for the purpose of identifying and predicting the NMR spectra of similar compounds. Moreover, unifying the data for many compounds of this type might become a foundation for calculating the correlations of fluorine chemical shifts and spin-spin coupling constants with the electronic parameters of the molecule which are to a considerable extent determined by the substituent influence. The unification of numerous data about similar compounds in the same work permits more precise definition of existing regularities and the discovery of new ones between the various parameters of 19F NMR spectra. It was also assumed that a careful analysis of the correlation relationships of chemical shifts with the Taft constants for the compounds C,F,R affords the possibility of determining these constants from the corresponding fluorine NMR spectra as is possible fos the meta-and para-substituted fluorobenzenes.
METHOD
The comparison of results from different sources is complicated by variations due to the conditions of measurement. Hence most of the compounds were measured under identical conditions in the CCI, solvent, which has obvious advantages over chloroform,6 trichlorofluoromethane,2 or benzene,' the last forming complexes with polyfluoroaromatic compounds.ll In the case of insufficient solubility only (R = --COOH, -CH,CQOH), ethanol was used as a solvent.
Chemical shifts were measured at the same concentration of 5 mole-%, because the difference between chemical shifts for less than 10 mole-% concentration and the values obtained by extrapolation to infinite dilution is negligible,' even for such substituents as -QH and -NH,, where molecular association might be expected.
Our results for previously studied compounds are in accordance with the data of ~t h e r s . l . ~* ~