Ordering and intramolecular mobility in the nematic phase of PAA investigated by means of NMR lineshape analysis and computer simulations of the lineshape

Abstract

2014 Nos études expérimentales par RMN de 1H et 2H dans la phase nématique de PAA, PAA-d6 et PAA-d8 suggèrent que le paramètre d'ordre orientationnel S = Szz = 1/2. 3 cos2 03B8 -1 > est suffisant pour expliquer les spectres obtenus. On présente une méthode approximative de calcul des spectres des protons. L'ordre orientationnel des protons dans la molécule PAA est seulement caractérisé par les paramètres Sj qui permettent une description d'ordre intramoléculaire. Abstract. 2014 We have recorded the 1Hand 2H-NMR spectra of PAA, PAA-d6, and PAA-d8. The order parameter S = Szz = 1/2. 3 cos2 03B8 -1 > has been determined from the different types of NMR experiments and shown to be quite sufficient for the interpretation of both proton and deuterium NMR data since the values of the tentatively calculated second order parameter Sxx-Syy (from 2Hresonance of PAA-d8) are very small. From a comparison of the proton NMR spectra of PAA-d6 and PAA and theoretical spectra computed using an approximative procedure for different kinds of intramolecular motions some conclusions are drawn concerning intramolecular mobility of the PAA molecules.

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.…”

“…Simulation of the proton NMR lineshapes for 50.6. 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.…”

“…molecule, bracket indicates averaging), which gives rise to a scaling of the frequency spectrum in the case of fast reorienting motions (cf. [5]). There is only a slight decrease in the ratio of the heights of central peak and outer doublet peaks (from 1.65 in the nematic phase to about 1.54 in the smectic-G phase) and possibly a weak « smearing out » of the structure.…”

“…The orientational order parameter S (neglecting all other possible order parameters which, in principle, could be introduced, cf. for example [5,7,8], because of their minor influence on proton NMR spectrum) could be derived from the dipolar doublet splitting in a simple manner with great accuracy if the dipolar splitting for S = 1 (perfect ordering) were known, or, alternatively, if one could determine the exact ortho proton distances at the phenyl rings which to a first approximation are responsible for the doublet peaks. 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).…”

Proton NMR investigation of a hydrogen-bonded liquid crystal gel

“…J-couplings can also be neglected when compared with the other interactions. Since the direct calculation of the proton lineshape f h (n) from the molecular geometry requires considerable effort [18][19][20], we use instead a measured spectrum of the aligned nematic phase (S h~1 ) of the pure compound to determine f 0 (n). Given some simplifying assumptions, it is possible to find the absolute order parameter scale from the oriented spectrum: the ortho-protons of the benzene ring are sufficiently isolated from the aliphatic chain protons, and the assumption of a predominantly pairwise interaction between neighbouring orthoprotons is rather good.…”