V. Rutar

13C NMR in ferroelectric smectic liquid crystals

Abstract The temperature dependence of the 13C NMR chemical shifts has been measured in the isotropic, the smectic A and the smectic C* phases of chiral DOBAMBC (p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate) and HOBACPC (hexyloxybenzylidene p′-amino 2-chloro α propyl cinnamate). The measurements allowed for a microscopic determination of the temperature dependence of the molecular tilt angle and the dipolar ordering of the C=O groups-i.e. the in-plane spontaneous polarization-without destroying the helical structure.

Proton spin–lattice relaxation in nematic microdroplets

The frequency and temperature dependences of the proton spin–lattice relaxation rate have been studied in nematic droplets embedded in a solid polymer matrix. The comparison of these data to those for bulk nematic and pure polymer samples shows that cross‐relaxation between liquid crystal and polymer protons dominates the liquid crystal relaxation in the MHz frequency region, while at low frequencies the translationally induced molecular rotation becomes important. The cross‐relaxation rate is found to be ≈103 s−1 giving an order of magnitude estimate 10−4–10−3 s for the time for which a liquid crystal molecule is bonded to the surface.

13C NMR study of the ‘‘bilayer’’ phase transitions in (C10H21NH3)2CuCl4

The two structural phase transitions in the perovskite type layer structure compound (C10H21NH3)2CuCl4 have been studied by single crystal 13C NMR. The paramagnetic shifts due to the Cu++ ions have been evaluated and subtracted from the experimental data to yield the 13C chemical shift tensors as a function of temperature. The results show in agreement with the DTA data that the transition from the low temperature to the intermediate temperature phase is connected with a partial melting of the alkylammonium chains and the vanishing of the average chain tilt, whereas the transition to the high temperature phase is accompanied by the onset of flipping of the polar heads and an associated rotation of the chains around the long axis. The two transitions are thus analogous to the ones found in lipid bilayer membranes and (C10H21NH3)2CdCl4 but the sequence of the two successive transitions is exchanged.

NMR lineshape and phase soliton effects in incommensurate Rb2ZnCl4

In incommensurate systems the NMR, NQR or EPR resonance frequency varies in space in a way which reflects the spatial variation of the incommensurate modulation wave. An evaluation of the magnetic resonance lineshape for the case of a multi-soliton lattice is presented for different ratios between the soliton width and the inter-soliton spacing as well as for different relations between the resonance frequency and the incommensurate order parameter. The results are compared with the experimental data in Rb2ZnCl4. It is found that in this system the soliton width is large compared with the inter-soliton spacing over most of the incommensurate phase. The temperature variation of the phase soliton density has been determined. The non-classical value of the critical exponent beta =0.35+or-0.03 for the amplitude of the order parameter demonstrates that the whole incommensurate phase is critical in this one-dimensionally modulated system.

Deuteron spin-lattice relaxation mechanisms in partially deuterated nematic MBBA

The frequency and temperature dependence of the spin-lattice relaxation time, T 1, of the various benzene ring deuterons in partially deuterated p-methoxybenzylidene-p-n-butylaniline have been measured. It was found that in the nematic phase the deuteron T 1 is independent of frequency between 4 and 41 MHz and thermally activated. The results seem to demonstrate that benzene ring rotation is the dominant deuteron relaxation mechanism in the MHz region and that the aniline and benzylidene rings reorient at different rates around the para axes.

On the nature of spin-lattice relaxation in nematic MBBA

The measurements of the frequency dependence of the proton T1 in nematic MBBA have been extended to 270 MHz, and the results have been compared with the predictions of the order fluctuation and self-diffusion relaxation theories. Both the frequency and the temperature dependences of T1 are shown to be characteristic of a mechanism controlled by molecular self-diffusion.

Proton NMR relaxation of adsorbed water in gelatin and collagen

The dependence of the water self-diffusion coefficients as well as of the proton spin-lattice and spin-spin relaxation rates on the concentration have been studied in the gelatin-water system and in hydrated native collagen. The bound and free water fractions and the corresponding spin-spin and spin-lattice relaxation rates have been determined within the multi-phase water proton exchange model. Various theoretical models for the water proton cross-relaxation to the biopolymer have been studied and the results compared with the observed Larmor frequency dependence of the water proton spin-lattice relaxation rate.

87Rb NMR lineshape study of the incommensurate phase in Rb2ZnBr4

On going from the paraelectric (P) to the incommensurate (I) phase the 87Rb 12 → 12 NMR lines in Rb2ZnBr4 broaden into a continuum which is limited by two edge singularities. The observed line shape and its temperature dependence are well described by the frequency distribution function predicted by the “plane wave” modulation model of the I phase. The anomalous temperature dependence of T1 shows the presence of the phason branch in the I phase.

NMR observation of phasons and amplitudons in incommensurate phases

Abstract The study of the local spectral density of amplitudon and phason excitations in incommensurate phases by nuclear magnetic spin-lattice relaxation is reviewed. The variation of the spin-lattice relaxation rate over the incommensurate line allows for a separate determination of the spectral densities of both amplitudon and phason excitations.

13 C nuclear magnetic resonance study of phase transitions in a lipid bilayer embedded in a crystalline matrix: (C10H21NH3)2CdCl4 and (C10H21NH3)2CuCl4

The hydrocarbon parts of (C10H21NH3)2CdCl4 and (C10H21NH3)2CuCl4 represent smectic lipid bi-layers exhibiting two structural phase transitions analogous to those in biomembranes. A13C nuclear magnetic resonance study showed that in the Cd compound the low-temperature transitions is connected with a disordering of the polar heads whereas the high-temperature transition corresponds to a partial melting of the alkyl chains. In the Cu compound the sequence of the two successive transitions is reversed. A Landau theory describing the two transitions in terms of order parameters used in the theory of smectic liquid crystals is presented.