Use of the Nuclear Overhauser Effect in the Analysis of High-Resolution Nuclear Magnetic Resonance Spectra
Relative intensity changes may be observed in a high-resolution proton magnetic resonance spectrum when one of the lines is partially saturated by irradiation with a radio-frequency field. This is a manifestation of the general Overhauser effect since the intensity changes result from changes in the populations of the eigenstates of the spin Hamiltonian. A simple theory is based on the approximation that population changes are negligible for energy levels not connected by the irradiated transition. This theory appears to be adequate for applications in the analysis of high-resolution spectra. Examples are given for systems with spin couplings of low or moderate strength to show how the Overhauser effect may be used to determine the relative signs of spin coupling constants, and to assign spectral lines to transitions between energy levels. The experimental techniques are discussed. Previously reported results for trans-crotonaldehyde and m-dinitrobenzene are confirmed, and all coupling constants in m-dinitrobenzene are found to have the same sign.