Role of Multiple Quantum Transitions in NMR: A Three‐Spin System

Abstract

The double and triple quantum (DQ and TQ) transitions in molecules containing three weakly coupled nuclei of spin ½ are examined theoretically and experimentally. Owing to the rapid growth and saturation of these transitions with increasing rf, it is practicable to observe each individual resonance only within a limited rf range. Based on the theory of Yatsiv, expressions at a given T₁=T₂ are computed for the rf fields at which the various MQ absorption signals will attain their maxima and the intensity of these absorptions relative to the maximum intensity for SQ signals. It is pointed out that the usual MQ signal measured in liquids has a linewidth almost equal to that of a SQ signal, since a considerable saturation of the former is necessary for its easy observation. A frequency shift only significant for high‐rf MQ signals is also computed. The DQ spectrum of a camphor derivative containing three weakly coupled protons is examined and the experimental results for the observable peaks agree rather well with the theory. The geminal J is opposite in sign to the vicinal J's.