Relative signs of nuclear spin coupling constants by means of transitory selective irradiation experiments

Abstract

A new method for the determination of energy level arrangements in nuclear spin systems is described. In this method, a transitory selective irradiation (TSI) of some chosen line is used to produce a redistribution of the spin level populations within a time that is short compared with the spin lattice relaxation time T 1. These population changes give rise to intensity changes for lines which have an energy level in common with the irradiated line—and only for such lines—when the spectrum is recorded within a time that is short compared with T 1. The application of the TSI method for the determination of the relative signs of spin coupling constants is described. As an illustration the method has been used for the determination of the relative signs of the ethylenic coupling constants in the simple three-spin systems of styrene and vinylbromide. In three-spin spectra with only two observable coupling constants the relative signs of these cannot be determined by double resonance experiments, but with an extension of the TSI method this problem can be solved. In this particular case three transitory irradiations are required. The procedure to be followed is described, and as an application we have studied the case of trans-cinnamic aldehyde where it was found that the two vicinal coupling constants in the side chain have the same sign.