Systematic Use of Solvent Effects in High-Resolution Nuclear Magnetic Resonance

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Abstract
It is shown that certain characteristic features of nuclear magnetic double-resonance spectra (the coalescence of spin multiplets or the splitting of single lines into doublets) can also be induced in single-resonance spectra of molecules that contain a pair of strongly coupled nuclei. Degeneracies due to overlap of two groups of resonance lines in one region of the spectrum may cause coalescence or splitting effects in some quite different region of the spectrum. These features are particularly easily recognizable when introduced into the spectrum in a gradual manner by the systematic use of preferential solvent shifts. Expressions derived for the transition frequencies and intensities bear a strong formal resemblance to those used for ``spin-decoupling'' and ``spin-tickling'' double-irradiation experiments. The strong temperature dependence of the chemical shift of the hydroxyl resonance in 2-propyn−1-ol has been exploited to induce the coalescence of certain lines in the spectrum of the methine proton, demonstrating that J(H–C–O–H) and J(H–C≡C–C–H) have opposite signs. This is a case where standard double-irradiation methods would not be applicable because the long-range coupling J(H–C≡C–C–O–H) is vanishingly small. Control over the overlap of the AB region of the ABMX spectrum of 3-fluoro-4-methoxyacetophenone makes it possible to induce splittings in the X spectrum, permitting the subspectra to be identified and the relative signs of the coupling constants to be obtained by inspection. The ortho and meta proton—fluorine coupling are shown to have the same sign, opposite to the sign of the para coupling.

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