Line Profiles in Nuclear Magnetic Double Resonance

Abstract

In nuclear magnetic double‐resonance experiments where the irradiated resonance has an appreciable linewidth due to spin—spin relaxation, some broadening may be transferred to the observed resonance line. The expected line profiles have been calculated by the density‐operator method for double irradiation in the ``spin‐tickling'' and ``spin‐decoupling'' modes, and are found to be in good agreement with the experimental results. In the ``spin‐tickling'' approximation, where only two energy levels are appreciably perturbed by the second radio‐frequency field, the observed line shapes are also obtainable by a simple convolution integral, but this method is not generally valid for ``spin decoupling.'' One application is the measurement of the width of a line that is not directly observable in the spectrometer, either through low inherent sensitivity or because of overlying resonances. As a practical example, the line profiles of the 13C satellites of the proton spectrum of chloroform during heteronuclear double irradiation indicate that the 13C resonance is 1.8 cps wide. This broadening is attributed to spin coupling to the chlorine nuclei which are rapidly relaxed through quadrupolar interaction with the environment, permitting an estimate of the magnitude of this spin coupling constant J(13C35Cl)≈49 cps.