Analysis of the Nuclear Overhauser Effect in the Difluoroethylenes

Abstract

Analysis of the nuclear Overhauser effect observed in proton double resonance is used to determine the probability of relaxation between spin states of different symmetry in the three difluoroethylene isomers. The probability of relaxation between states of A and B symmetry is much greater for CH2CF2 than for either cis‐ or trans‐CHFCHF. This indication of an appreciable A to B transition probability in CH2CF2 provides some detailed information on the symmetry properties of the interactions responsible for nuclear relaxation. A qualitative comparison of the Overhauser effects in these three molecules indicates that the fluorine spin—rotation interaction is probably responsible for the A to Brelaxation in CH2CF2, but further experiments are suggested to test this hypothesis. The relaxation equations for these spin systems are solved for steady‐state and transient behavior using transition probabilities given by assuming relaxation by randomly varying isotropic local fields. Overhauser effects and the transient recovery of the spin systems from the steady state in the presence of an irradiating rf field to thermal equilibrium are used to estimate quantitative proton, fluorine, and A to Brelaxation rates.