The nuclear magnetic resonance spectra of fluorobenzenes. Part I. The analysis of the ABB′XX′ and ABB′XX′R spectra of some 2-substituted 1,3-difluorobenzenes

Abstract

The analyses of the ABB′XX′ and ABB′XX′R spectra of a number of 2-substituted 1,3-difluorobenzenes are given. The use of the sub-spectral analysis technique in these analyses is discussed. In the simpler ABB′XX′ spectra this enables a virtually complete analysis to be performed, but in the case of the more complex ABB′XX′R spectrum this analysis could be completed only by the determination of the transition energies and intensities of the complete spectral system plus an iterative computer programme. The relative signs obtained from these analyses confirm other relative and absolute sign determinations and indicate that the signs and ranges of values of F–F and F–H couplings (c./sec.) in the benzene ring are: J_FF(ortho)–19 to –21; J_FF(meta)–10 to +12; J_FF(para)ca. 0 to +15; J_HF(ortho)+6 to +12; J_HF(meta)+5 to +8; J_HF(para)ca. 0 to –3.