Determination of order parameters from carbon–fluorine dipolar coupling

Abstract

A convenient NMR method for the determination of the order parameters for liquid crystals containing a fluorinated phenyl ring is described. The technique consists of measuring the carbon-fluorine dipolar coupling constants in the onedimensional C-13 spectrum of the molecule. The order parameters may then be calculated for the fluorine-containing ring with a high degree of precision because of the excellent resolution afforded by the 1-D C-13 spectra. The method is used to determine the core order parameters for 4-n-hexyloxybenzilidene-4'-fluoroaniline (FAB-OC6). The results of the carbon-fluorine dipolar coupling method are compared with two established methods for determining the core order parameters of phenyl rings, namely deuterium NMR spectroscopy and SLF/VAS, a twodimensional (2-13 NMR spectroscopy. Some comments about the orientational properties of the fluorinated liquid crystal FAB-OC6 are made.

Orientational ordering studies of fluorinated thermotropic liquid crystals by NMR spectroscopy

“…The orientational order of rigid fragments of the aromatic core of FABOC6 was also investigated by Magnuson et al . by measuring either 13 C– 19 F couplings in 1D proton‐decoupled 13 C NMR spectra or 13 C– 1 H couplings in 2D SLF/VAS experiments.…”

“…The obtainment of order parameters from 13 C NMR experiments is often more complex than for 2 H NMR due to the fact that the spectra can be quite complex and difficult to interpret and several assumptions are needed in linking experimental observables (either anisotropic chemical shift or dipolar couplings) to order parameters (vide infra). Nevertheless, following the first paper by Magnuson et al ., 13 C NMR spectroscopy has been widely applied for the investigation of orientational order on F‐LCs. In particular, in the older studies, only 13 C– 19 F couplings were employed to determine orientational order parameters owing to the difficulty in experimentally determining chemical shift tensors (CSTs) useful for chemical shift anisotropy data analysis.…”

Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR

“…Heteronuclear dipolar couplings are dependent on distance and motion, 60 and the magnitude of this dipole–dipole coupling is given by the following expression for 13 C 1 H where d CH is the dipolar coupling constant between the 13 C and 1 H nuclei in Hz, μ 0 is the vacuum permittivity, ℏ is the reduced Planck constant, γ C and γ H are the respective gyromagnetic ratios for the 13 C and 1 H nuclei, and r CH is the distance between the carbon and hydrogen atoms. Motional averaged dipolar couplings ⟨ d CH ⟩ can be obtained by two-dimensional (2D) proton detected local field (PDLF) 43 , 44 , 61 experiments that correlate the 13 C isotropic chemical shifts with their corresponding 13 C 1 H dipolar spectra, providing site-selective heteronuclear dipolar coupling constants (see the Experimental Section for further details).…”

Ordering in nematic liquid crystals from NMR cross-polarization studies

“…One of the first demonstrations of this approach has been made with the study of 2 H- 13 C dipolar couplings in N-(4-methoxybenzylidene)-4-n-butylaniline (MBBA) deuterated at a specific site [4]. Similar studies have been performed on systems containing 19 F and using 19 F- 13 C dipolar couplings [5,6]. However, a more general and attractive approach would be to utilize 1 H- 13 C dipolar couplings.…”