A convenient method of synthesis of mesomorphic 2,5-disubstituted cyclohexan-1-ones by a catalytic reduction of 3,6-disubstituted cyclohex-2-en-1-ones

The preparation of trans-2,5-disubstituted cyclohexan-1-ones from 3,6-disubstituted cyclohex-2-en-1-ones by reduction with hydrogen in the presence of palladium on carbon in a basic medium is described. The nature of the mesomorphic phases together with some physical properties of single compounds and of nematic mixtures comprising them are reported. Mesogens containing the trans-2,5-disubstituted cyclohexan-1-one ring show a small negative dielectric anisotropy and a low birefringence.     

1H-NMR studies at N-acetyl-D-neuraminic acid ketosides for the determination of the anomeric configuration II.

In D₂O solution the ¹H-nmr signal of the equatorially orientated protons at C-3 of N-acetyl-D-neuraminic acid and its derivatives shows a quite characteristic dependence on the anomeric configurations: δ [H(3e)] = 2.72 ± 0.05; δ [H(3a)] = 2.32 ± 0.08.     

Conformational studies by nuclear magnetic resonance—V: 13C spectra and structural problems of enamino carbonyl compounds

A ¹³C magnetic resonance investigation of 22 compounds of the general formula O?C¹(R¹)C²(R²)?C³(R³) NR⁴R^4′ has been carried out in order to confirm the conformational assignments estimated previously by PMR and IR spectroscopy. The effects of nonplanarity and hindered rotation have been discussed based on chemical shifts and shapes of the C¹, C² and C³ signals.     

Conformational studies by nuclear magnetic resonance?VI: C?C and C?N rotation barriers in various enamine derivatives

Delocalisation of the nitrogen lone electron pair by an acyl and acylvinylogue group simultaneously, results in a lowering of both C? N rotational barriers. MeC(O)CBr?CHNMe₂ and MeC(O)C(OCOMe)?CHNMe₂ exhibit restricted rotation around what is formally a double bond C?C.     

The spectrum of HeAr+

Two band groups near 1450 Å, first observed by Tanaka, Yoshino, and Freeman (J. Chem. Phys.62, 4484–4496 (1975)) in discharges through mixtures of helium and argon and assigned by them to the HeAr⁺ ion, were studied under high resolution. Like the similar spectrum of HeNe⁺ previously investigated, the spectrum of HeAr⁺ is a charge transfer spectrum. The upper state B²Σ⁺ of both band groups is derived from $\text{He}{}^{\mathrm{+}}\text{(}{}^2\text{S) +}\text{Ar}\text{(}{}^1\text{S)}$ while the two lower states $\text{A}{}_2{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and X²Σ⁺ are derived from $\text{He}\text{(}{}^1\text{S) +}\text{Ar}{}^{\mathrm{+}}\text{(}{}^2\text{P)}$. All three states are very weakly bound, the two lower states even more weakly than the upper state. Unlike HeNe⁺ most of the HeAr⁺ bands are violet shaded. In the longward band group each band shows only three branches while in the shortward group there are four. The former observation shows that the $\text{A}{}_2{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ state behaves like a ²Σ^? state with γ_v ≈ 0. The B, D, γ, p, and ΔG values of all states were evaluated. While the B_v values of upper and lower states are nearly equal, the D_v values are quite different and this difference accounts for the violet shading of most of the bands even when B′_v is slightly smaller than B″_v; it also accounts for some of the extraheads and linelike features in the rotational structure. As in HeNe⁺ the ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ component of ²Π was not observed.     

Influence of underlying local organisations in typical nematic phase of 6CHBT and induced nematic phase of bicomponent liquid crystalline systems 4DBT–12CB – a Fast Field Cycling NMR investigation

Variable-temperature proton magnetic relaxation dispersion (PMRD) profiles are collected in the induced nematic phases of the binary liquid crystalline mixtures composed of smectic mesogens: 5-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4′-dodecyl-4-cyanobiphenyl (12CB) and the low viscous nematogen 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to assess the influence of local organisations on the power spectrum of director fluctuation modes. Two distinct compositions of the binary mixture i.e. 50 mol% of 12CB (without a smectic phase) and 70 mol% of 12CB (with a smectic phase) are used. Fast Field Cycling nuclear magnetic resonance relaxometry is employed to measure proton spin-lattice relaxation rates (R₁) as a function of Larmor frequency (10 kHz–30 MHz). PMRD data analysis with the choice of a suitable model indicated nematic clusters of moderate size (~200 Å) found in the broad nematic region of 50 mol% 12CB, whose size is almost invariant with temperature. On the other hand, cybotactic clusters, i.e. local smectic organisations of relatively larger size (~2000–3000 Å), are observed near the nematic–smectic transition (T > T_AN) of 70 mol% 12CB. Interestingly, the shape geometry of such local organisations accessed from PMRD analysis is weakly anisotropic near T_AN, while being isotropic near T_NI for 70 mol% 12CB.     

Dielectric and electro-optic properties of 6CHBT nematic liquid crystals and silver nanoparticles composites

We have prepared the composite of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and silver nanoparticles (AgNPs). Thermodynamic, electro-optic and dielectric properties of the prepared composite have been investigated. From these investigations, parameters such as transition temperature, dielectric anisotropy, dielectric permittivity and splay elastic constant have been determined for the pristine as well as composite systems. Threshold switching voltage of the pure and composite samples has also been determined from transmission voltage curve. The effects of dispersion of AgNPs on dielectric and electro-optic parameters of LC-NPs composite have been discussed in terms of existing standard models.     

MOLECULAR AND COLLECTIVE PROCESSES IN THE ANTIFERROELECTRIC PHASE OF A CHLORINATED COMPOUND

Abstract

New liquid crystalline substance, (S) 4-(1-methylheptyloxycarbonyl)-3-chlorophenyl 4′-(4-butanoiloxybut-1-oxy)biphenyl-4-carboxylate, exhibiting ferroelectric and antiferroelectric phases have been studied by means of frequency domain dielectric spectroscopy. DSC calorimetric and electrooptic measurements were done to study its phase transitions and electrooptic behaviour. In the antiferroelectric SmC_A* phase one of the dielectric relaxation modes exhibits a non-Arrhenius behaviour. The plots of critical frequencies and dielectric increments vs. temperature show an anomalous behaviour of the relaxation processes in the neighbourhood of the ferroelectric — antiferroelectric phase transition. Reversal current method reveals one current peak in the ferroelectric phase and two in the antiferroelectric one. Temperature dependencies of tilt angle, spontaneous polarization, switching time and transmittance are also presented and discussed in scope of the mean-field theory.