Molecular conformational changes in the smectic C, smectic A and nematic phases of TBBA

Abstract

2014 On discute de manière quantitative la dépendance en température de plusieurs rapports de splittings de DMR, dans les phases smectique C, smectique A et nématique de TBBA, à l'aide d'un modèle qui prend explicitement en compte les rotations internes et les changements de la conformation moléculaire la plus probable. On montre que dans toutes les phases, cette conformation n'est pas plane. La valeur de l'angle dièdre moyen entre quelques fragments moléculaires rigides est estimée et comparée à des valeurs théoriques. Dans l'Appendice, on présente une discussion du même problème à l'aide d'un modèle à deux paramètres d'ordre orientationnels. Abstract. 2014 The relative temperature dependence of several DMR splittings in the smectic C, smectic A and nematic phases of TBBA is discussed quantitatively in terms of internal rotations and changes of the most probable molecular conformation. It is shown that in these phases, the conformation is not planar. The mean dihedral angles between some rigid molecular fragments are estimated and compared with theoretical calculations. A discussion in terms of two order parameters is also made in the Appendix.

Chain Ordering in the Smectic C, Smectic A and Nematic Phases of Terephtal-bis-butyl-aniline (TBBA) and its Temperature Dependence

“…If the two conformations were not exactly energetically equivalent (as for example in a chiral molecule), then these two deuterons would generally not be equivalent (see also the discussion in Ref. [14]). 20…”

“…Following Ref. [14], assuming that (i) the e.f.g. tensor acting on one deuteron Di has cylindrical symmetry around the CDi bond (ii) the molecular frame rotates uniformly around its long axis OZO and (iii) the where cc = e2qiQ/h is the quadrupolar coupling constant, S S, , is the usual nematic order parameter and the brackets stand for an averaging over the internal motions.…”

“…These internal motions are of two kinds: rotation around the chain C-C bonds and exchange between, equivalent, most probable conformations. (i) the average over the rotations around the C-C bonds are governed by the potential V{cpi) = V ( C~I , ~2 9 ~3 , (~4 ) (13) according to the Boltzmann law: u ( V i ) ) = z-1\ d(cpi)fI~i)e-""'"' (14) where f is any function of the cpi, and Z a normalization constant P Since the all-trans conformation is a symmetry plane of the chain, the potential V(+4 is necessarily symmetric i.e. V{-cpi} = V{cpi).…”

“…[14]. We define a molecular frame O X O~O Z O that we associate to the most probable molecular conformation, i.e.…”

“…[15] and [14], where we were interested first in a description of the behavior of the molecule as a whole (external motions),'' and then to the behavior of the aromatic core (internal motion^). '^ We emphasize here that we have chosen to describe the external motions in terms of the singIe order parameter S, , , as a consequence of the analysis made in Ref.…”