Nuclear Magnetic Double Resonance. Transmission of Modulation Information through the Nuclear Spin—Spin Coupling

Abstract

Nuclear magnetic double-resonance experiments are described in which the perturbing radio-frequency field is amplitude or frequency modulated, causing the precessing moment vector of a group of nuclear spins (S) to mutate at a low audio frequency. Through the action of the nuclear spin—spin coupling this modulation information is transmitted to a second group of nuclear spins (I) in the form of a selective modulation of the local magnetic field, which may be used to excite a magnetic resonance ``sideband'' signal. A general theory is given, covering both the present modulation transfer experiments and also the removal of residual splitting in spin decoupling experiments by modulation of the irradiating frequency. Numerical calculations to predict the frequencies and relative intensities of the observed signals have been carried out on a high-speed computer and presented in graphical form. In all the experimental measurements the S spins are 13C nuclei in their natural abundance, and the I spins are coupled protons, examined at 60 Mc/sec. Good agreement with the theoretical predictions has been obtained in a detailed study of the AX spin system of chloroform. The application of modulation transfer experiments to the indirect measurement of 13C chemical shifts has been illustrated for chloroform, acetic acid, and diethyl ether. The technique has also been used to discriminate between signals from molecules with different isotopic constitution; for example, in acetic acid all proton signals from molecules not containing 13C may be suppressed, clearly revealing the small long-range spin coupling between carboxyl 13C and the methyl protons. In more complex molecules the theoretical frequency and intensity diagrams can be built up by superposition of the diagrams for simpler systems. Often the relative signs of certain coupling constants may be inferred by inspection of the modulated double resonance spectrum—the relative signs of J(13CH), J(13CCH), and J(HH) in diethyl ether have been determined in this way.