Analysis of A2B2 High-Resolution NMR Spectra. I. Furan, Thiophene, and Disubstituted Benzenes

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Abstract
The features of certain classes of A2B2 spectra are investigated systematically relative to the parameters involved, and these characteristic spectra are given as an aid to future interpretation. The effects upon the spectra of the relative signs of the coupling constants are discussed. Examples are provided by the analysis of spectra for the aromatic protons in furan, thiophene, three symmetrical ortho disubstituted benzenes, two nonsymmetrical para disubstituted benzenes, and a symmetrical 4,4′ disubstituted biphenyl. Better resolution than was obtainable in previous studies permitted a more detailed analysis for compounds cited earlier. In most examples the 24 main spectral lines expected for four spin systems of this symmetry were observed. It is found that all four of the H–H coupling constants are of the same sign in thiophene, and Jαβ and Jαβ′ in furan. Appreciably different values, 1.55 and 2.85 cps for Jαα and 1.8 and 4.7 cps for Jαβ, are found in furan and thiophene, respectively, while Jββ and Jαβ′ are virtually identical: 3.53 and 3.35 cps, and 0.8 and 1.0 cps. In the substituted benzenes, the values found for J0, Jm, and Jp fall in the ranges 7.4 to 8.8 (9), 1.24 to 3.05 (9), and 0.3 to 0.6 cps (6), the number of cases being given in parentheses. The disubstituted benzenes studied have two nonequivalent ortho or meta coupling constants; they are found to differ by as much as 0.6 cps. The spectra of these substituted benzenes show that the coupling constants J0, Jm, and Jp are all of the same relative sign.