Molecular rotation and conformational transitions studied by 2H NMR in a smectogen

Comparison of the dielectric and NMR results for liquid crystals: dynamic aspects

Critical analysis of the results of studies of molecular rotational dynamics in liquid crystalline substances with the aid of the dielectric spectroscopy (DS) and nuclear magnetic resonance (NMR) is given. Both methods are known to be sensitive to different aspects of molecular rotations: the polarization vector and the relaxation time τ(DS) in the case of DS, a tensor describing a nuclear interaction and the correlation time τ(NMR) for NMR method. Furthermore, both methods provide correlation functions with different rank values. A common basis for the comparison between τ(DS) and τ(NMR) is postulated. Several examples of the temperature dependence of the correlation times coming from the two spectroscopic methods are presented. Qualitative agreements of the correlation times were achieved in most cases.

Deuterium NMR relaxation in the smectic-A phase of a chiral smectogen

Correlated internal rotations are studied for a deuterated decyloxy chain in the chiral liquid crystal, 1-methylheptyl 4(')-(4-n-decyloxy-benzoyloxy)biphenyl-4-carboxyate (10B1M7) using deuteron spin-lattice relaxation and quadrupolar splitting measurements. The present study is limited to the uniaxial smectic-A phase of 10B1M7, in which the observed deuterium nuclear magnetic resonance spectra show well-resolved peaks. The deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times are simultaneously determined for the methylene deuterons using the Wimperis broadband pulse sequence at two different Larmor frequencies. The quadrupolar splittings at each temperature are modeled by the additive potential method to obtain the orienting potential of mean torque. A decoupled model using 683 conformations for the decyloxy chain is used to explain the observed relaxation rates. In this model, the overall reorientation is described by a small step rotational diffusion model, while both gauche migration and gauche pair production as well as the so called one- and two-bond motions are allowed to occur in the chain. Transition rates for the chain dynamics and the overall rotational diffusion constants are obtained using a global target analysis of the relaxation data of 10B1M7.

Conformational dynamics of a metallomesogen studied by 2H-NMR spectroscopy

In this work we present a quantitative analysis of both quadrupolar splittings and deuterium Zeeman and quadrupolar spin-lattice relaxation times reported in the literature for two isotopomers of Azpac, an acetylacetonate derivative of the cyclopalladated 4, 4'-bis(hexyloxy) azoxybenzene. Azpac-d(4) is deuterated at the aromatic rings and Azpac-d(26) is deuterated on the alkoxy chains. The additive potential method is used to model the splittings, while the derived spectral densities are interpreted using the decoupled model in conjunction with the Nordio model. The two side chains are assumed to be noninteracting and identical in their conformations in order to limit the size of the transition rate matrix needed to describe correlated internal bond rotations in the chains. Rotational diffusion constants and internal jump rate constants are derived for this metallomesogen.

Deuteron NMR study of a long-chain smectic liquid crystal: molecular order and dynamics

Deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times and quadrupolar splittings were measured in the nematic and smectic A phases of a chain-deuterated 4-n-octyloxy-4'-cyanobiphenyl (8OCB-d17) at 15.1 and 46 MHz. To model the NMR observables, the so-called pentane effect is used to limit the number of possible conformations in the chain, and is found to be a good approximation. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings. A decoupled model is used to describe correlated internal motions in the chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Nordio (Tarroni and Zannoni), while the former motion is described using a master rate equation. In the nematic phase, order director fluctuations are found to be necessary. The relaxation data in both mesophases were treated using a global target method, and the derived motional parameters are acceptable in comparison with those found for 6OCB.