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C A Veracini

Publications and source records attributed to C A Veracini.

13 recordsLinked to original sources
PubMed2006

Deuterium NMR of the TGBA* phase in chiral liquid crystals.

A deuterium NMR (DNMR) study of the TGBA* (twist grain boundary smectic A*) phase in an NMR magnetic field of 9.4 T for the chiral compound 4-[4'-(1-methyl heptyloxy)] biphenyl 4-(10-undecenyloxy) benzoate (11EB1M7) is reported. The deuterium two-dimensional (2D) exchange spectra were observed for the first time in this phase. The present study allows us to learn how the helicoidal structures arrange in an external magnetic field. To derive quantitative kinetic parameters of this novel phase, both 1D and 2D experimental spectra were simulated by means of a jump diffusion model. By comparing the experimental and simulated spectra, an accurate determination of the dynamic parameters in the TGBA* phase was obtained. Furthermore, the twist angle between two neighboring smectic A blocks is found as 26 +/- 10 degrees, which is consistent with the X-ray results for similar chiral liquid crystals. The diffusion constant (D(parallel)) is estimated to be 3.2 x 10(-12) m(2)/s at 379.5 K.

Journal Article↗
PubMed2006

13C and 2H NMR study of structure and dynamics in banana B2 phase of a bent-core mesogen.

In this paper, the difficulty in orienting the B(2) phase of the banana mesogen 1,3-Phenylene-bis 4-[4-(10-undecenyloxy)-benzoyloxy] benzoate (Pbis11BB) in a relatively high magnetic field is reported based on some observations using both (13)C and (2)H NMR. (2)H NMR spectra recorded for the two labeled isotopomers of Pbis11BB in the isotropic and B(2) phases are shown here. Preliminary results on the deuteron spin-spin relaxation (T(2)) data are reported at 61 MHz in order to underline the peculiar slow dynamics of banana-shaped liquid crystals (BLC), and these results are discussed in the framework of recent studies on similar BLC. The molecular structure and dynamics in the B(2) and crystalline phases are also studied by (13)C solid-state NMR techniques. The results also point to the slow dynamics in the B(2) phase of Pbis11BB. In particular, two-dimensional MAS exchange experiment has been performed to shed light on the molecular conformation structure of the five-ring banana core in the crystalline phase of Pbis11BB, and to compare with that of quantum mechanical calculations reported in the literature.

Journal Article↗
PubMed2006

Nematiclike viscosity coefficients of ferroelectric liquid crystals in their smectic-C* phase.

The five nematiclike viscosity coefficients corresponding to the Smectic-C* (SmC*) phase are calculated by a combination of the existing statistical-mechanical approach and NMR theory, based on a rotational diffusion model for two ferroelectric smectogens. The order parameter S2, the smectic tilt angle Theta, and the rotational diffusion coefficient D(perpendicular), corresponding to molecular tumbling in the SmC* phase, for the ferroelectric smectogens 4-[4'-(1-methylheptyloxy)]biphenyl'-(10-undecenyloxy)benzoate and (S)-[4-(2-methylbutyl)phenyl]-4'-n-octylbiphenylcarboxylate have been obtained by means of 2H NMR spectroscopy and these data have been used to calculate viscosity quantities. For practical purposes a specific form of the functional dependence of S4 on S2 is used. As the main result, our calculations also predict a laminar flow regime in a high shear flow for these liquid crystal compounds.

Journal Article↗
PubMed2006

Structure and dynamics of chiral ferrielectric phases by deuteron NMR.

The results of studying angular dependent spectral parameters in a magnetic field are reported in the chiral smectic C phases of a smectogen 1-methylheptyl -(4-n-decyloxybenzoyloxy)-biphenyl-4-carboxylate. Our data provide direct evidence of a phase transition between two ferrielectric phases in this compound, viz. three-layer (SmC*(FiI)) and four-layer (SmC*(FiII)) superlattices. Simulation of spectral patterns of the methyl (C10) deuterons obtained after an aligned sample is rotated by 90 degrees in the magnetic field rules out the "clock model" for the three-layer and four-layer structures in these ferriphases. Instead, our data obtained under a magnetic field (9.4 T) seem to favor an "asymmetric clock model" for the interlayer packing. Interlayer jump rate is also obtained from the simulation of angular dependent spectra in these ferrielectric phases, and compared with those in the antiferroelectric SmC*(A) phase.

Journal Article↗
PubMed2005

NMR study of orientational ordering in the chiral- phase of partially deuterated smectic liquid crystals.

Carbon-13 NMR spectroscopy is used to probe the orientational ordering in the tilted chiral smectic phase of two partially deuterated liquid crystals. Quantitative analyses of the chemical shifts of the aromatic carbons in the smectic- and - phases enable us to obtain the location of the long molecular axis of the core and the off-diagonal order parameter of the molecular core in the smectic- phase. The advantage of knowing the ordering information of these molecules in their mesophases from deuteron studies is demonstrated in this work, as they can be used to guide the assignments. The temperature behaviors of the chemical shifts from the aromatic sites in the helical structure of the smectic- phase is caused by both an increase of the tilt angle and a change in the orientation of the principal frame that describes the molecular ordering.

Journal Article↗
PubMed2005

Interlayer jump diffusion in the smectic-C*A phase by angular-dependent NMR study.

A detailed 2H nuclear magnetic resonance (NMR) study at 61.4 MHz of two chiral smectic C phases of a smectogen 10B1M7, one ferroelectric (SmC*) and the other antiferroelectric (SmC), is reported. Simulation of angular-dependent spectral patterns and/or signal intensities of the methyl deuterons are shown to provide information on the synclinic or anticlinic ordering, as well as the jump diffusion rates at different temperatures in the SmC phase. The molecular self-diffusion constant is determined to be in the range of 0.2-1.2 x 10(-14) m2/s over the temperature range of the SmC phase. Molecular self-diffusion in the SmC* phase should be faster, but remains undetectable by the present approach. Furthermore, the phase biaxiality in these phases is found to be vanishingly small. The critical magnetic field for unwinding the helical structure of 10B1M7 is found to be relatively high.

Journal Article↗
PubMed2005

Solid-state NMR characterization of diastereoisomeric chiral stationary phases and their soluble models.

Two diastereoisomeric chiral stationary phases (CSPs), devised for enantioselective HPLC, showing unexplained differences in their chromatographic performances, have been characterized, together with their soluble models, by means of 13C-cross polarization/magic angle spinning (CP/MAS), 1H-MAS and 1H-free induction decay (FID) analysis. The NMR investigation of the soluble models has not highlighted significant structural/conformational differences between the two diastereoisomers, but has constituted a useful support for the analysis of the more complex NMR data of the CSPs. The organic chiral selectors of the stationary phases show a poor internal mobility and no conformational differences between the two diastereoisomers have been observed. On the contrary, interesting differences between the two CSPs have been found involving the silanols on the silica surface and the dynamics of the linking chains between the organic selectors and the silica surface itself. An explanation of the chromatographic behaviour has been proposed in terms of different proximity of the organic moieties with respect to the inorganic surface in the two CSPs.

Journal Article↗
PubMed2005

Chain dynamics in a chiral C phase by deuteron spin relaxation study.

Molecular reorientations and internal conformational transitions of an aligned chiral liquid crystal (LC) 10B1M7 are studied by means of deuterium spin-lattice relaxation in its smectic A (SmA) and smectic C* (SmC*) phase. The motional model which is applicable to uniaxial phases of many LCs is found to be adequate even when the phase is a tilted SmC* phase. The deuterium NMR spectrum in this phase cannot discern rotations of the molecular director about the pitch axis. The basic assumption is that the phase biaxiality is practically unobservable. However, the relaxation rates can be accounted for by the tilt angle between the molecular director and the layer normal in the SmC* phase. The tumbling motion appears to show a higher activation energy upon entering from the uniaxial SmA into the SmC* phase.

Journal Article↗
PubMed2005

Correlation between 1H FID and T1rho components in heterogeneous polymer systems: an application to SBS.

Wideline 1H FID and relaxation measurements of a relatively simple motionally heterogeneous system, the triblock copolymer styrene-butadiene-styrene, have been performed in a temperature range between the polystyrene and polybutadiene glass transition temperatures. The two FID and the two spin lattice relaxation time in the rotating frame (T1rho) components found at each temperature have been correlated by means of a two-dimensional approach. It is shown that this approach allows dynamic information, not accessible simply by interpreting proton T1 and T1rho data, to be revealed. In the case examined, the correlation found could be confirmed by high-resolution 1H T1rho-selective 13C Cross Polarization experiments.

Journal Article↗
PubMed2002

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.

Journal Article↗
PubMed1991

Activity and conformational changes of alpha-chymotrypsin in reverse micelles studied by spin labeling.

alpha-Chymotrypsin (CT), spin-labeled at the active site by using an acylating label which constitutes a substrate for this protein, has been investigated in reverse micelles formed by AOT in isooctane. The electron spin resonance spectra provided information on conformation, dynamics and deacylation activity. The dynamics of the label bound to CT appears to be more hindered in reverse micelles than in aqueous solution, probably owing to the effect of the micellar environment on protein conformation. The deacylation rate in reverse micelles does not show the characteristic bell-shaped dependence on water content which is generally found for CT enzymatic activity.

Acylation↗