Field Inhomogeneity Correlation Effects in Nuclear Magnetic Double-Resonance Detection of Very Small Spin Coupling Constants

Abstract

Double-irradiation techniques may be used to detect nuclear magnetic resonance splittings that are so weak as to be obscured by the broadening due to spatial inhomogeneity of the applied magnetic field. The basis for these experiments is the fact that the observable double-resonance effects result from interactions on the molecular scale (such as spin—spin coupling) with the result that the two nuclear sites, A and X, experience almost exactly correlated local fields due to nonuniformity of the applied magnetic field. One such correlation effect is the partial compensation of field inhomogeneity broadening of lines that are regressively connected to the irradiated line. A second correlation effect may be observed if the intensity of a given line with a radio-frequency field so weak that its influence is restricted to sample regions very close to the resonance condition. A comparative study of double irradiation methods based on these effects has been carried out by applying them to the problem of resolving the 0.05-Hz splitting in the 60-MHz proton magnetic resonance spectrum of 3-bromothiophene-2-aldehyde, attributed to long-range spin coupling of the aldehyde proton with the ring proton at Position 4.