Use of Weak Perturbing Radio-Frequency Fields in Nuclear Magnetic Double Resonance

A theory is developed to explain high-resolution nuclear magnetic resonance spectra observed in the presence of a second weak rf field at ω2. When ω2 is set on a nondegenerate transition frequency and the rest of the spectrum recorded by sweeping the frequency of the investigating rf field, any transitions that have an energy level in common with the perturbed transition will be split into doublets. If magnetic field inhomogeneities exceed the natural linewidth, the line profile of the doublets gives information about the ordering of the three energy levels involved. Good agreement with the theory has been obtained in experiments on two-spin and three-spin proton systems where the energy-level diagram is already known. The method has been used to establish the ordering of the energy levels of the ABC system of protons in methyltrivinylsilane and the analysis has been completed by two independent methods. Some of the special effects which arise when double-resonance spectra are recorded by sweeping the magnetic field have been examined and discussed.