GENERAL FEATURES OF THE X PART OF AN ABX SPIN SYSTEM IN ISOTROPIC ANDLIQUID-CRYSTALLINE PHASES AS ILLUSTRATED BY THE C-13-(H-1) SPECTRA OF2,2'-DIFLUOROBIPHENYL

The C-13-{H-1} NMR spectra of 2,2'-difluorobiphenyl dissolved in isotr opic and liquid crystalline solvents have been obtained and analyzed. They are examples of the X part of an ABX spectrum. It is shown that t he spectrum of the isotropic solution yields J(AX), J(BX), J(AB), delt a(AB), and delta(X), but only if all the transitions are detected, and that intensities as well as frequencies of the transitions are used i n the analysis. It is demonstrated that for 2,2'-difluorobiphenyl this requires that for some of the carbons it is necessary to detect very weak transitions. For the spectra of liquid crystalline solutions of A BX systems it is shown that the dipolar couplings D-AX, D-BX, and D-AB are obtained only if these couplings are in a certain sensitive range of relative values. The sensitive range can be adjusted by using vari able angle sample spinning (VASS). It is demonstrated that VASS spectr a taken near the magic angle can be used to obtain the absolute signs of the scalar couplings. (C) 1997 Academic Press.