MULTIPLE-PULSE COSY NMR-SPECTROSCOPY OF ORIENTED MOLECULES IN THERMOTROPIC CHOLESTERICS

The coherent reduction of anisotropic NMR interactions in molecules or iented in a thermotropic cholesteric phase, using a multiple-pulse COS Y-experiment, is described. The technique, which directly modifies the spin part of the nuclear spin Hamiltonian, allows the transformation of second-order spectra into first-order spectra. An experimental stud y of the evolution of the chemical shift and dipolar reduction factors is presented. The effects of the finite pulse duration in the multipl e-pulse sequence are discussed, and explicit equations for the anisotr opic interactions which take into account the pulse widths are obtaine d. The technique in cholesteric solvents is illustrated by using two s pecific examples for AB and ABC type of spectra: 2,6-dichloro-3-nitrop yridine and 2-cyanofuran.