Donata Catalano

A maximum‐entropy analysis of the problem of the rotameric distribution for substituted biphenyls studied by 1H nuclear magnetic resonance spectroscopy in nematic liquid crystals

The statistical principle of maximum entropy is applied to the analysis of dipolar couplings from 1H NMR of nonrigid molecules dissolved in liquid‐crystalline phases. A distribution function for the orientational and inter‐ring angles is so obtained. The most probable internal angle φ is determined for 4,4’‐dichlorobiphenyl (φ=34°) in the nematic phase of I52, 4‐pentyl‐4’‐cyanobiphenyl (φ=32°) and 4’‐Br‐4‐Cl‐2,6‐difluorobiphenyl (φ=42°) in EBBA. The physical reliability of the distributions determined is discussed. The maximum‐entropy treatment seems to indicate a limit for the information on the internal motion obtainable from the experimental data.

A rigid core‐flexible chain model for mesogenic molecules in molecular dynamics simulations of liquid crystals

A model of a mesogenic molecule, built up as a rigid anisotropic Gay–Berne site mimicking the aromatic core, connected to an array of isotropic sites mimicking a flexible chain, is proposed and tested in molecular dynamics calculations. Simulations have been performed on a system composed of 256 molecules with three different numbers of methylenic units in the chain, in order to explore the effect of chain length on static and dynamic properties. The systems are all at the same mass density and temperature and result in nematic liquid crystalline phases. The order parameters for various molecular fragments and the T1z nuclear magnetic resonance (NMR) relaxation times of deuterons are in agreement with previous molecular dynamics simulations on atomistic systems and, at least qualitatively, with 2H‐NMR experimental results. The intermolecular interactions are always dominated by the anisotropic site simulating the molecular core. The influence of the phase order on the chain static and dynamic properties is put in evidence. Extensions of the model are suggested in order to have a better reproduction of the dynamical features of such systems.A model of a mesogenic molecule, built up as a rigid anisotropic Gay–Berne site mimicking the aromatic core, connected to an array of isotropic sites mimicking a flexible chain, is proposed and tested in molecular dynamics calculations. Simulations have been performed on a system composed of 256 molecules with three different numbers of methylenic units in the chain, in order to explore the effect of chain length on static and dynamic properties. The systems are all at the same mass density and temperature and result in nematic liquid crystalline phases. The order parameters for various molecular fragments and the T1z nuclear magnetic resonance (NMR) relaxation times of deuterons are in agreement with previous molecular dynamics simulations on atomistic systems and, at least qualitatively, with 2H‐NMR experimental results. The intermolecular interactions are always dominated by the anisotropic site simulating the molecular core. The influence of the phase order on the chain static and dynamic properties i...

Orientational Order in Nematic and Smectic A Phases of Three Closely Related Compounds 4 4-di-n-Heptylazobenzene 4 4-di-n-Heptyl- and 4 4-di-n-Octylazoxybenzene

Abstract The orientational ordering in the three related compounds 4,4-di-n-heptyl-d2-azobenzene-d, (HB-d12), 4,4′-di-n-heptyl-d2-azoxybenzene-d, (HAB-d12) and 4,4-di-n-octyl-d2-azoxybenzene-d8 (OAB-d12) has been monitored using deuteron N.M.R. spectroscopy. In these non-rigid molecules it is possible to obtain only the elements of local order matrices. For the aromatic rings these are found to be axially symmetric within the precision of our measurements. The temperature dependences of the local order parameters for the aromatic rings are compared with those predicted by McMillans theory of smectic A phases.

Solute-Solvent Interactions in the Nematic and Smectic A Phases of 4 4-di-n-Heptylazoxybenzene Studied by Deuteron NMR

Abstract The orientational order parameters have been measured by deuteron N.M.R. of both solute (at low dilution) and solvent in various binary mixtures involving the liquid crystal 4,4′-di-n-heptylazoxybenzene (HAB). The solutes studied were azoxybenzene-d10 and n-heptylbenzene-d7 which are fragments of HAB, azobenzene-d 10 because of its similarity to azoxybenzene, and anthracene-d10 because of its known structure and symmetric shape. The major and biaxial order parameters of the solutes are analysed in terms of a molecular field model for the potential of mean torque for biaxial particles. The behaviour of the solute order parameters on approaching and entering the smectic A phase is interpreted in terms of a temperature and phase dependent partitioning of the solute between aromatic and aliphatic regions of the solvent.

2H-NMR and SAXS of a ferroelectric liquid crystal : unwinding of the ferroelectric chiral helix by high magnetic fields

Abstract 2 H-NMR in three different magnetic fields and small angle X-ray scattering (SAXS) are employed to investigate the orientational order, molecular organisation and phase transitions of the chiral smectic liquid crystal (−)-(S)-[4-(2-methylbutyloxycarbonyl)phenyl] 4- n -heptyl-biphenyl-carboxylate (MBHB) suitably deuterated for 2 H-NMR measurements. Unwinding of the ferroelectric chiral helix by high magnetic fields is observed and discussed. Tilt angle values are obtained with the two different techniques and compared. The results show the importance of molecular conformation for the structural properties of this mesogen.

Structure of biphenyl in a nematic liquid-crystalline solvent

The proton NMR spectra of biphenyl dissolved in two nematic liquid-crystalline solvents have been analysed to yield 12 independent dipolar coupling constants, Dij. The Dij have been used to obtain the probability distribution, Pnem(ϕ), for the angle ϕ between the ring normals. Two methods of relating Dij to Pnem(ϕ) have been used: the ME method, which invokes the maximum-entropy principle, and the AP method, which is based on the construction of a potential of mean torque from an addition of contributions from the rigid subunits in a molecule. The value of ϕ when Pnem(ϕ) is a maximum is found to be essentially independent of the solvent, and is determined to be 34 ± 1° by the ME method, and 37.2 ± 0.1° by the AP method.

Molecular dynamics simulation of biphenyl dissolved in a liquid crystalline solvent: A test of theoretical methods of deriving rotational potentials from partially averaged nuclear spin dipolar couplings

A molecular dynamics simulation has been carried out of biphenyl dissolved in a solvent comprised of rigid particles interacting with the Gay–Berne potential. The solution is investigated in isotropic, nematic, and smectic phases, and the probability distribution, PLC(φ) obtained, where φ is the angle between the two ring normals. This is compared with Pmol(φ), the distribution calculated for an isolated molecule, and it is found that the positions, φmax of the maxima of the two distributions differ by about 2°. The molecular dynamics trajectory is used to calculate averaged nuclear spin dipolar couplings, Dij, and these are used to test the maximum entropy (ME) and additive potential (AP) theoretical models which have been used previously to obtain the distribution PLC(φ) from dipolar couplings obtained on real solutions of biphenyl in liquid crystalline solvents. It is concluded that the AP method is able to recover the true distribution PLC(φ) from the simulated Dij with good precision at all the tempe...

2H NMR orientational study of a probe dissolved in nematic solution and, used as crosslinker, in a liquid crystalline elastomer

The orientational order parameters, Szz and Biax, of UB-d4 [1,4-bis(undec-10-en-1-yloxy)benzene-d4 ] in the ZLI1167 nematic mixture are studied by means of 2H NMR at variable temperature. The Szz(T) trend fairly follows the Maier–Saupe theoretical curve; therefore, UB-d4 can be considered an excellent probe to monitor the ordering of the nematic phase itself, provided that Biax is not neglected and the complete analysis of quadrupolar and dipolar splittings is performed. UB-d4 is a typical crosslinker in liquid crystal elastomers (LCEs): the order parameters here collected help us to discuss the peculiarities of the crosslinker as a probe for the study of internal order in LCEs. From the analysis of our data and of data in the literature, we infer that the order parameter Biax of the crosslinker in the LCE matrix is low enough to be neglected without severely perturbing the determination of . The crosslinker UB-d4 is a reliable probe to monitor the nematic order degree inside the LSCE-UB-d4 sample.

A constrained maximum entropy method for the interpretation of experimental data: Application to the derivation of single particle orientation‐conformation distributions from the partially averaged nuclear spin dipolar couplings of n‐alkanes dissolved in a liquid crystalline solvent

A new method is described of deriving a distribution, P(x), in a set of variables, x, which is consistent with a set of data, A(x), and which is the least biased modification (maximizes entropy) of a constraining distribution, r(x). The method is demonstrated using nuclear spin dipolar couplings obtained for n‐alkanes dissolved in a liquid crystalline solvent. The constraining distributions are obtained from computer simulations of the molecular dynamics.

An investigation of the conformation of 4-chloroethylbenzene as a solute in a nematic liquid crystalline solvent

The conformation of the ethyl group relative to the phenyl ring plane in 4-chloroethylbenzene has been investigated by recording and analysing the proton NMR spectrum of a sample dissolved in a nematic liquid crystalline solvent. The dipolar couplings obtained are compared with values calculated by the additive potential (AP) and maximum entropy (ME) molecular mean field theoretical models. In both cases good agreement between observed and calculated couplings could be obtained only by adopting a model for rotation about the ring-C bond which allows for geometry relaxation. The ME method produces a conformational distribution with an absolute maximum at 90° and a secondary maximum at 0, which is also the shape found to give the best fit to the data by the AP method. However, constraining the shape of the conformational distribution to that found by the molecular orbital calculations and using the AP method yields an acceptable, if not the best, fit to the dipolar couplings, and so is the preferred solutio...