Simplification of 19F NMR spectra of liquid crystalline samples by multiple-pulse COSY experiments

A general method is presented of converting second-order fluorine-19 spectra of liquid crystalline samples into first-order spectra. This is achieved by recording a 19F COSY spectrum with a multiple pulse dipolar reducing sequence (MPS) operating in the t1 period, leading to a F1 projection which is first order. The method is illustrated by recording spectra on a sample of 1,3-dichloro-4-fluorobenzene dissolved in the nematic solvent ZLI 1167. Experiments have been done in which the MPS is either the MREV-8 or Flip-Flop-16 sequence. The first-order F1 projections are analyzed to give reduced total 1H-19F couplings, KCSTHFij and the reduction factors, KCS, are obtained by comparison with the THFij available from an analysis of the ordinary 1D spectrum. The reduction factors are compared with values predicted by average Hamiltonian theory (AHT). The reasons for the differences found between observed and predicted values of KCS have been investigated by performing numerical simulations of the 2D MPS COSY experiments. Copyright 1998 Academic Press.