Investigations on the ordering of five p, p'-di-n-alkyloxy-azoxybenzenes in their nematic phases by proton NMR

Abstract

We present the results of proton NMR investigations of the ordering of five members of the homologous series of the p, p'-di-n-alkyloxy-azoxybenzenes in their nematic phases. The difficulties connected with the determination of the accurate absolute value of the order parameter by NMR is critically discussed. The results are compared to those obtained by other methods (Δχ-measurements, X-ray diffraction analysis). The lineshapes of four compounds are calculated theoretically permitting the derivation of conclusions concerning intramolecular mobility and/or order

Thermotropic Liquid Crystals, Fundamentals

“…In spite of the problems mentioned under (ii) 1 H-NMR has been useful, often in combination with analysis of the shape of the broad lines (see, for example, Limmer et al 1981). In spite of the problems mentioned under (ii) 1 H-NMR has been useful, often in combination with analysis of the shape of the broad lines (see, for example, Limmer et al 1981).…”

Proton nmr studies of smectic phases of three n-(4-n-alkyloxybenzylidene)-4'-n-alkylanilines (n0 . m's)

“…compared to T2 ) reorientations (cf. [5,6]), and the same should be valid for Avthis means that Av -M2.…”

“…Since S cannot be greater than 1 this can only mean that either the proton distance is distinctly less than 2.45 A (cf. also [9]) or that other interactions of near-neighboured protons superimpose on the ortho-proton doublet. This could be, for example, the interaction between the bridge methine proton and the inner ortho-proton of the aniline ring (Fig.…”

“…The details of the computation of the theoretical proton NMR spectra have already been described elsewhere [5,6,9,20]. As discussed in [6,9] the use of standard bond lengths (which give rise to a ortho proton distance r = 2.47 A) generally leads to splitting (and accordingly second moments) clearly too small compared to the experimental values. If, however, atom positions from an X-ray crystallographic analysis are available even the quantitative agreement between computed and experimental spectra is quite good (cf.…”

“…Otherwise one can only estimate S with limited accuracy assuming (as is often done) an ortho proton distance of about 2.45 A yielding a maximum splitting Avm., = 24.5 kHz (neglecting the small angle between the phenyl ring para-axis and molecular director). For a more detailed discussion see [6,9].…”