Analysis of partially oriented AA′XX′ N.M.R. spin systems with the aid of sample rotation. The ¹⁹F spectrum of cis-difluoroethylene dissolved in a nematic phase

Abstract

The spectrum of an oriented AA′XX′ spin system can be analysed to give only the parameters δ_AX, J _AA′, T _AA′zz , J _XX′, T _XX′zz and the combinations (J _AX + J _AX′ + 2T _AXzz + 2T _AX′zz ) and (J _AX - J _AX′ + 2T _AXzz - 2T _AX′zz ). By controlled, slow sample rotation it is possible to determine all these parameters separately, and some of their relative signs. Application of this technique to cis-difluoroethylene confirms the signs of the coupling constants determined by Baldeschwieler et al. The dipolar couplings for cis-difluoroethylene are compared with values calculated assuming the geometry as determined from microwave spectroscopy and including the effects of harmonic vibrations.