Orientational Order of a Liquid Crystal with Three Chiral Centers by a Combined ¹³C NMR and DFT Approach

Abstract

In this work, the liquid crystal (S)-2-methylbutyl-[4'-(4' '-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/*) was investigated by means of 13C NMR spectroscopy. This compound has a very peculiar mesomorphic behavior, showing the following phases: paraelectric SmA, ferroelectric SmC*, antiferroelectric SmC*A, re-entrant ferroelectric SmC*re, and ferroelectric hexatic Sm*HEX. The structural and orientational ordering properties of ZLL 7/* have been determined by exploiting the nuclear chemical shielding properties of 13C. To this aim, solid-state NMR techniques such as CP, SPINAL-64, and SUPER have been used in combination with DFT calculations. The agreement between experimental and in vacuo DFT shielding parameters appears to be satisfactory. The orientational order parameters obtained from the 13C shielding analysis have been discussed, taking into account different data analysis approaches and comparing them to those previously obtained from an independent 2H NMR study.

From the SmA to the Hexatic, Including the SmC*, SmC*A and SmC*re Phases: A 2H NMR Relaxation Study

“…[9][10][11][12][13] On the basis of previous results, [22] the order parameter S was fixed in the range 0.68-0.65, the para axes of the two fragments are almost parallel, the angles between the CÀD bonds and the relative para axes are fixed to 608, and the molecular tilt angle is in the range 12.0-18.58. [22][23][24] Even though, in the SmC* phase, the experimental data are affected by larger errors (see Figure 5) than in the SmA phase (in particular at the lowest frequency), the fitting is quite good and the best fitting parameters, also reported in Table 2, are in agreement with those found in the SmC* phase of similar FLC systems. [11-13, 49, 50] Concerning the diffusion motions, if no sensitive changes are observed for the spinning and internal biphenyl ] for the smectogen ZLL7/* in the SmA, uSmC*, and SmC* phases.…”

“…According to previous studies, [22,23] the long molecular axis is almost coincident with the biphenyl para axis.…”

“…The synthesis, chemical characterization, physical, and orientational properties of the unlabeled ZLL 7/* compound are reported in refs. [20], [23], and [29]. The physical and orientational ordering properties of the two labeled isotopomers are reported in ref.…”

“…[20] and [21], respectively. The structural and ordering properties of this smectogen have been previously investigated, by exploiting both 2 H [22] and 13 C NMR [23] spectroscopy combined with quantum mechanical calculations of relevant NMR parameters, such as 13 C chemical shielding tensors. Moreover, a recent 2 H NMR study [24] allowed us to investigate the effect of external magnetic fields [25] in unwinding the supramolecular structure of the chiral SmC* phase of the ZLL 7/* mesogen.…”

“…[22,23] In this case the contribution of ODF was neglected, due to the fact that a single frequency was available. However, since the uSmC* phase has a structure more similar to the SmA than the SmC* phase, due to the lack of a helical supramolecular structure, this approximation is justified.…”