Analysis of partially oriented AA′XX′ N.M.R. spin systems with the aid of sample rotation. The19F spectrum ofcis-difluoroethylene dissolved in a nematic phase

Abstract

The spectrum of an oriented AA′XX′ spin system can be analysed to give only the parameters δAX, 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.