SIMPLIFICATION OF F-19 NMR-SPECTRA OF LIQUID-CRYSTALLINE SAMPLES BY MULTIPLE-PULSE COSY EXPERIMENTS

A general method is presented of converting second-order fluorine-19 s pectra of liquid crystalline samples into first-order spectra. This is achieved by recording a F-19 COSY spectrum with a multiple pulse dipo lar reducing sequence (MPS) operating in the t(1) period, leading to a F-1 projection which is first order. The method is illustrated by rec ording spectra on a sample of 1,3-dichloro-4-fluorobenzene dissolved i n the nematic solvent ZLI 1167. Experiments have been done in which th e MPS is either the MREV-8 or Flip-Flop-16 sequence. The first-order F -1 projections are analyzed to give reduced total H-1-F-19 couplings, KCSTijHF and the reduction factors, K-CS, are obtained by comparison w ith the T-ij(HF) available from an analysis of the ordinary 1D spectru m. The reduction factors are compared with values predicted by average Hamiltonian theory (AHT). The reasons for the differences found betwe en observed and predicted values of K-CS have been investigated by per forming numerical simulations of the 2D MPS COSY experiments. (C) 1998 Academic Press.