Deuteron magnetic resonance in liquid crystals : the interplay of molecular ordering and conformational changes

Abstract

2014 Un examen des résultats de résonance magnétique des deutérons dans différents systèmes thermotropes remet en question l'interprétation récemment proposée par Bos et Doane suivant laquelle (i) la conformation moléculaire moyenne ne change pas avec la température, même aux transitions de phase, et (ii) les mésophases uniaxiales et biaxiales (Smc) sont respectivement caractérisées par deux et trois paramètres d'ordre. Quelques contre-exemples indiquent que ces points ne sont pas généraux et, dans certains cas, ambigus. Le rôle des changements conformationnels ne peut être négligé. Abstract. 2014 Examination of deuteron magnetic resonance data in different thermotropic systems calls into question the explanation recently proposed by Bos and Doane wherein (i) the average molecular conformation does not change with temperature, even at phase transitions, and (ii) uniaxial and biaxial (Smc) mesophases are characterized by two and three molecular order parameters, respectively. Some counterexamples indicate that these points are not general, and in some cases ambiguities are present in their analysis. The role of conformational changes cannot be neglected.

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

“…(ii) This latter point is supported experimentally by counter examples where some ratio plots are found to be discontinuous where they should not and not discontinuous where they should [6]. (iii) Finally, even if Sxxo-syoy, were non zero in the SmC, SmA and nematic phases of TBBA, it t is too small (by a factor we estimate to be of the order of 10-100) to explain the observed temperature dependence of some splitting ratios [5].…”

“…From symmetry arguments, it has been argued that the uniaxial phases are characterized by the two order parameters S,oZO and Sxoxo-Sy,oyv and for the biaxial phases a third one, namely Sy,oZo is also needed [3]. These results have been criticized by us [5] and other authors [6]. These criticisms are :…”

“…1 versus 4>c.r.. The curve is the theoretical variation calculated using equations (5) and (6) and the values of the parameters given in the text (section 4). The symbols represent the values of c.r.…”

Chain orientation in deformed networks via NMR

“…This possibility was suggested in the first report of WNMR studies of PDMS networks (ref. 4). This concept of dynamic heterogeneity implies that simulated lineshapes are a superposition of spectra that are partially averaged to different degrees; e.g., a superposition of spectra of the types shown in Figures 1 and 2. The superposition is determined by the number of components (number of dynamically distinct species or rates R) and their associated populations P,.…”

A deuterium nuclear magnetic resonance study of chain disorder in lamellar potassium palmitate: the effect of long and short chain guests

“…Ratios of nmr splittings of one part of a chain to those of another part have recently been used in an interesting way (13). The basis of the treatment is to describe the order parameter for a CD bond as the product of molecular and conformational order parameters (13,14). In practice, this separation cannot be performed quantitatively because there is no way to measure one of the order parameters separately.…”