On the Use of Double-Irradiation Methods for the Assignment of High-Resolution NMR Spectra with Repeated Spacings

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Abstract
An analysis of double‐resonance experiments on pseudo‐AX systems (i.e., pairs of doublets, the four transitions of which form a closed polygon in the energy‐level diagram) based on the formalism of Freeman and Anderson, is presented. Expressions are given for the transition frequencies and the transition probabilities in the general case. It is demonstrated that double irradiation may lead to the collapse of a repeated interval under comparatively general conditions and that double irradiation may alternatively lead to enlargement of the doublet splitting in spin systems which are not ideal for ``decoupling'' experiments. The collapse or the enlargement of (repeated) doublet splittings may be used to establish the correct level arrangement in a general spin system. As illustrative applications of these methods, the ABX spectra of 5‐methylthio‐2‐thiophenethiol and trans‐cinnamaldehyde have been studied experimentally. In 5‐methylthio‐2‐thiophenethiol selective ``decoupling'' and selective enlargement as well as general double irradiation experiments were performed and it was shown that the sign of the side‐chain coupling J SH−3 is opposite to that of the ring‐coupling constant J 34. The implications of this result with regard to the mechanism of long‐range thiol proton spin couplings are indicated. In trans‐cinnamaldehyde selective enlargement of an unresolved repeated spacing was produced in order to corroborate the earlier result that the two vicinal couplings in the side chain carry the same sign.