Nuclear magnetic double resonance using very strong decoupling fields

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Abstract
In order to demonstrate the predicted partial recovery of spin-spin coupling resulting when very high double irradiation fields are used, an AMX system (the vinyl group of vinyl acetate) was studied using H 2 powers of up to 100 c/s. Optimum decoupling of the low field proton from the rest of the spectrum occurred when an H 2 power of 44 c/s was used. For powers >44 c/s the predicted increase in splittings was observed, the spectrum becoming more complicated as the H 2 power was increased. A good theoretical fit of the observed spectra was obtained using simple double resonance theory (without considering relaxation effects) although the values of H 2 power used were by no means negligible compared with the chemical shifts involved. Various necessary corrections which must be applied in this type of experiment, in particular those due to shifts analogous to the Bloch-Siegert shift, are discussed. It is emphasized that in general, these effects may lead to complications when spin-decoupling experiments are carried out in order to simplify observed spectra.