Conformational analysis of the alkyl spacer of a dimer liquid crystal in the solid and liquid crystalline states by13C-13C dipolar coupling

Abstract

ABSTRACT: The conformation around the bond of the alkyl spacer ofC a,u-bis[(4,4@-cyanobiphenylyl)oxy]eECe@decane (CBA-10) was evaluated in the solid and liquid crystalline states through the detection of the dipolarinteraction between and CBA-10C 13C-labelled in the d position of the alkyl spacer was prepared and for thedCd@.measurement in the solid state a variation of the CarrEPurcellEMeiboomEGill sequence was used. The spectra inthe nematic state were measured with the pulse saturation method. It was concluded that in the solid state the transconformation is assumed whereas in the liquid crystalline state the fraction of the trans conformers is around 0.6.( 1998 John Wiley & Sons, Ltd. KEYWORDS: NMR; solid-state NMR; 13CE13C dipolar coupling; dimer liquid crystal; nematic state; alkyl spacer;conformation INTRODUCTION The interest in dimer liquid crystals arises from the factthat they are often regarded as good model compoundsin their polymeric homologues. In our previousstudies,1 a scheme for determining the conformation ofthe spacer of such compounds was developed. In thatmethod, the rotational isomeric state (RIS) approachand information on molecular orientation derived fromeither 2H quadrupolar couplings or 13C chemical shiftanisotropy were employed.In chain molecules, the internuclear distance betweena pair of nuclei three bonds apart gives the conforma-tion around the central bond. Since the dipolar coup-ling between nuclei is inversely proportional to the thirdpower of the distance between them, this gives a meansfor distance measurements. Many examples in whichthis principle has been applied have been reported inthe literature. Yannoni and Kendrick2 measured theCEC bond length in a doubly 13C-labelled acetic acidusing the nutation spectroscopy method; the samesample was used by Engelsberg and Yannoni3 and thebond length was determined using a modiÐed type ofCarrEPurcellEMeiboomEGill (CPMG) sequence; Wein-traub et al.4 applied simple excitation for the dephasingof rotational-echo amplitudes (SEDRA) to obtain theNEN bond length in 3(5)-methyl-5(3)-phenylpyrazole-