Indirect Detection of Weak or Hidden Nuclear Magnetic Resonance Signals

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Abstract
High‐resolution nuclear magnetic resonance signals that may not be directly observable can be detected indirectly in a double‐resonance experiment that applies a weak perturbing radio‐frequency field to each hidden line in turn while recording the splitting produced on transitions of a second nucleus which is spin coupled to the first. The method is illustrated for the simplest model which may be regarded as typical, three weakly coupled spin‐½ nuclei, and the importance of the ``common energy‐level rule'' is emphasized. The example studied is the C13 spectrum of 1,1,2,2 tetrabromoethane and it is found that the three coupling constants J(HH), J(C13H) and J(C13CH) all have the same sign, presumably positive. The chemical shift of C13 in this compound has been related to the resonant frequency of protons in tetramethylsilane in the same applied field, with an accuracy of ±1 part in 108.