Magnetic double resonance studies of complex spin systems: the sign of the P–O–P coupling constant

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Abstract
Double resonance experiments are shown to establish the relative signs of J(A–X) and J(AA′) in the AA′XX′ spin system, even if J(XX′) is zero. The method is capable of extension to AA′XnXn′ systems. In the pyrophosphite anion 2J(31P–O–31P) and 3J(31P–O–P–H) are found to be negative and positive respectively.