Dynamics of 4,4'-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4'-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, ε*(ω)=ε'(ω)-iε'(ω), was measured in the frequency range 1 kHz-4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l. f.) process connected with molecular reorientations around the short axes, and the high frequency (h. f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l. f. relaxation time changes step-wise at the phase transitions, whereas the h. f. relaxation time passes smoothly through all the phases. The measurement of ²H spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D_‖ and D_⊥, as well as the diffusion coefficients D_R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.     

Surface molecular relaxation in smectic C* phase as studied by dielectric spectroscopy

The molecular relaxation process of a ferroelectric liquid crystal with a high tilt angle and a high spontaneous polarization in a homeotropically aligned cell has been studied by the dielectric relaxation method in the frequency range 10 Hz to 10 MHz. The measurements have been done using thin (3.5μm) cells with gold coated electrodes and samples aligned by a magnetic field. It has been observed that the molecular relaxation around the short axis of the molecule is detected in the chiral nematic and smectic C* phases. The surface molecular process is observed in the S*_c phase down to nearly 6 to 7 K below the transition temperature of the N* to the S*_c phase. The experimental results of the surface molecular process are analysed by theoretical calculations. The experimental results agree with the theoretical predictions.     

Calculation of the surface charge and potential and of the structural component of disjoining pressure in an interlayer in KCl solutions between glass samples

The dependence of the Stern potential, ψ₁, of glass samples on the distance between these, H, has been theoretically calculated, while taking into account the Stern isotherm and the electroneutrality equation. Comparison of the theoretical dependences ψ₁(C)_H→∞ with those previously experimentally obtained enables one to calculate the energy of adsorption of OH ions on glass and, further, the dependence ψ₁(H). It has been shown that for pH 4–6 and C_KCl = 10^-2-10^-5 mol/L, the value of ψ₁ practically does not depend on H. The result obtained was used to calculate theoretically the ionic-electrostatic forces and to compute (from the experimental values of the interaction forces) structural forces U_s(H). The dependence thus obtained, U_s(H), is of exponential character.     

Nuclear spin relaxation in biaxial liquid crystal phases

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J_mLm'L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the 'biaxial' spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Kinetische und mechanistische untersuchungen von Übergangsmetall-komplex-reaktionen : III. Abhängigkeit des gleichgewichts der addition von tributylphosphan an pentacarbonyl(arylmethoxycarben)wolfram vom substituenten am ring

The reaction of pentacarbonyl(arylmethoxycarbene)tungsten, (CO)₅W[C(OCH₃(p-C₆H₄R)] [R = OCH₃ (a), CH₃ (b), H (c), Br (d), CF₃ (e)], with tributylphosphane at low temperatures results in a reversible addition of the phosphane to the carbene carbon atom. The addition—dissoziation equilibrium is not only dependent on the temperature but also to a strong degree on the nature of the substituent R. ΔG, ΔH and the equilibrium constant K increase in the series from R = OCH₃ to R = CF₃. With the exception of R = OCH₃ the substituents b to e form an isentropic class. For all substituents (a to e) a linear dependency from Jaffés σ-constants was observed for ΔH. Good linear correlation for the substituents b to e was also found for log K and σ as well as for log K and the CO-force constants k_cis and k_trans.     

Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method

By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant k_s, are found to be δπ = 0.288 erg/cm and k_s, = 9·12 × 10^-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment.     

Theoretical investigation of the effects of intramolecular electron-spin relaxation on Stern-Gerlach deflections

The exact solution of a random walk model for electron spin flips in a magnetic field is presented and applied to the Stern-Gerlach (SG) deflection of a spin-1/2 molecule. It is found that when the intramolecular spin-relaxation time τ is much longer than t, the residence time of the molecule in the magnetic gradient, the SG deflection spectrum is unaffected, whereas when τ is much shorter than t, the two-line splitting pattern collapses to that of a single line, neither shifted nor broadened with respect to the line profile at zero field gradient. SG band profiles at intermediate values of λ = τ/t are especially interesting. In the range 2.0> λ > 0.5, the M_s = ± 1/2 SG bands are not shifted from their peak position at large λ, however they are strongly broadened asymmetrically toward zero deflection. This broadening assumes the nature of a broad peak at λ ≈ 0.5, which then narrows substantially and rapidly shifts to the position of zero deflection for λ between 0.3 and 0.01. Examples are given of the SG spectra of molecules exhibiting extreme values of λ.     

Magnetic-field induced biaxiality in nematic liquid crystals. Consequences for nuclear spin relaxation

When a uniaxial nematic liquid crystal is subjected to a magnetic field making a non-zero angle with the C_∞ axis, the uniaxial symmetry is broken. The principal effect is a field-induced biaxiality in the long-wavelength region of the director fluctuation spectrum. Whereas the induced biaxiality has little effect on the mean square director fluctuation amplitudes 〈n²_x〉 and 〈n²_y〉, which are dominated by short-wavelength modes, it can profoundly affect the nuclear spin relaxation behaviour, which is sensitive to long-wavelength modes. Motivated by the increasing number of nuclear spin relaxation studies of director fluctuations in thermotropic, amphiphilic, and polymeric nematic liquid crystals, we present here a theoretical analysis of the effects of field-induced biaxiality on nuclear spin relaxation.     

The position of electrophilic substitution of 2-methyl-2H-cyclopenta[d]pyridazine and the crystal structures of two substitution products

2-Methyl-2H-cyclopenta[d]pyridazine reacts with trifluoracetic anhydride to give the 5- and 7-trifluoroacetyl products in about a 1:3 ratio. A series of HMO calculations have been made for similar ten π-electron heterocycles in an attempt to predict the positions in the 5-membered rings which would be most reactive to electrophilic substitution. The predictions based on the relative π-electron stabilities of the possible cationic intermediates are in accord with the available experimental data. If the atoms in the 6-ring are numbered 1-6 with the π-excessive heteroatom at the 1-position, then the most reactive site to electrophilic substitution in the 5-ring will be the atom which is linked to either the 1-, 3- or 5-positions. The structures of the major and minor trifluoroacetyl isomers of 2-methyl-2H-cyclopenta[d]pyridazine have been determined by X-ray crystallographic analyses, and the HMO predictions agree with the finding that C(7) is the most reactive position. 2-Methyl-5-trifluoroacetyl-2H-cyclopenta[d]pyridazine crystallizes in the monoclinic space group P2₁/m, cell constants a = 8·109, b = 6·713, c = 8·768 Å, β = 91·88°, with two molecules per unit cell. All of the atoms have been located and refined by full-matrix least-squares and the final residual is 0·056. The bond lengths appear to be influenced by a contribution of a pyridazinium form to the resonance hybrid. 2-Methyl-5-bromo-7-trifluoroacetyl-2H-cyclopenta[d]pyridazine crystallizes in the orthorhombic space group Pnam, cell constants a = 12·920, b = 12·513, c = 6·752 Å, with four molecules per unit cell. Although the quality of the diffraction data was severely limited by rapid decomposition of the specimen in the X-ray beam, the gross features of the structure were revealed. The residual following isotropic refinement was 0·19.     

Characterization of lyotropic nematics in cylindrical capillaries

Procedures for characterization of lyotropic nematics in cylindrical capillaries by optical microscopy in orthoscopic geometry are presented. Typical textures due to surface alignment allow unambiguous characterization of N_c (cylindrical micelles) and N_d (discotic micelles) phases. Typical textures due to the compromise between surface and magnetic alignment allow unambiguous definition of the sign of the diamagnetic anisotropy susceptibility δχ. The textures allow also estimates of anisotropies of elastic constants for N_d lyomesophases.     

X-ray crystal structure analysis and atomic charges of color former and developer: 2. Color formers

The crystal and molecular structures of 2′-amino-6′-dibutylamino-3′-methylspiro[isobenzofuran-1(3H), 9′[9H]xanthen]-3-one (1), 2′-amino-6′-(N-cyclohexyl-N-methylamino)-3′-methylspiro[isobenzofuran-1(3H), 9′[9H]xanthen]-3-one (2) and 2′-(2-chlorophenyl)amino-6′-dibutylaminospiro[isobenzofuran-1(3H), 9′[9H]xanthen-3-one (3) have been determined by single-crystal X-ray diffraction analysis. Atom-atom non-bonded potential energy and semiempirical quantum chemical calculations have been performed. The xanthene rings of 1 to 3 are slightly bent and the phthalide rings are planar. The phthalide ring moieties are almost perpendicular (88.9(1)−93.5(5)°) to the xanthene rings. The bond lengths C(6)---O(2) are apparently extended from the normal C(sp³)---O (lactone) length. The temperature factors for one butyl group C(32)---C(35)) of 1 increase gradually toward the terminal carbon. The temperature factors for C(30)---C(33) of 2 indicate large vibrations and these are reflected in short bond lengths. Two butyl groups of 3 are disordered and these C---C bond lengths are short and long alternately. Atomic net charges around spirocarbon C(6) and toward N(1) to C(6) indicate the weak alternative system in the colorless form. As the xanthene ring has a planar geometry, the π electron density migration will easily occur from the auxochromes attached to the phthalide ring to the xanthene ring.     

X-ray crystal structure analysis and atomic charges of color former and developer: 2. Color formers

The crystal and molecular structures of 2′-amino-6′-dibutylamino-3′-methylspiro[isobenzofuran-1(3H), 9′[9H]xanthen]-3-one (1), 2′-amino-6′-(N-cyclohexyl-N-methylamino)-3′-methylspiro[isobenzofuran-1(3H), 9′[9H]xanthen]-3-one (2) and 2′-(2-chlorophenyl)amino-6′-dibutylaminospiro[isobenzofuran-1(3H), 9′[9H]xanthen-3-one (3) have been determined by single-crystal X-ray diffraction analysis. Atom-atom non-bonded potential energy and semiempirical quantum chemical calculations have been performed. The xanthene rings of 1 to 3 are slightly bent and the phthalide rings are planar. The phthalide ring moieties are almost perpendicular (88.9(1)–93.5(5)°) to the xanthene rings. The bond lengths C(6)---O(2) are apparently extended from the normal C(sp³---O (lactone) length. The temperature factors for one butyl group C(32)---C(35)) of 1 increase gradually toward the terminal carbon. The temperature factors for C(30)---C(33) of 2 indicate large vibrations and these are reflected in short bond lengths. Two butyl groups of 3 are disordered and these C---C bond lengths are short and long alternately. Atomic net charges around spirocarbon C(6) and toward N(1) to C(6) indicate the weak alternative system in the colorless form. As the xanthene ring has a planar geometry, the π electron density migration will easily occur from the auxochromes attached to the phthalide ring to the xanthene ring.     

Structure and conformational properties of carbonylbisimidosulfuryl fluoride, O=C(N=S(O)F2)2

The molecular structure and conformational properties of O=C(N=S(O)F₂)₂ (carbonylbisimidosulfuryl fluoride) were determined by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G* and B3LYP/6-31G*). The analysis of the GED intensities resulted in a mixture of 76(12)% syn–syn and 24(12)% syn–anti conformer (ΔH⁰=H⁰(syn–anti)−H⁰(syn–syn)=1.11(32) kcal mol⁻¹) which is in agreement with the interpretation of the IR spectra (68(5)% syn–syn and 32(5)% syn–anti, ΔH⁰=0.87(11) kcal mol⁻¹). syn and anti describe the orientation of the S=N bonds relative to the C=O bond. In both conformers the S=O bonds of the two N=S(O)F₂ groups are trans to the C–N bonds. According to the theoretical calculations, structures with cis orientation of an S=O bond with respect to a C–N bond do not correspond to minima on the energy hyperface. The HF/3-21G* approximation predicts preference of the syn–anti structure (ΔE=−0.11 kcal mol⁻¹) and the B3LYP/6-31G* method results in an energy difference (ΔE=1.85 kcal mol⁻¹) which is slightly larger than the experimental values. The following geometric parameters for the O=C(N=S)₂ skeleton were derived (r_a values with 3σ uncertainties): C=O 1.193 (9) Å, C–N 1.365 (9) Å, S=N 1.466 (5) Å, O=C–N 125.1 (6)° and C–N=S 125.3 (10)°. The geometric parameters are reproduced satisfactorily by the HF/3-21G* approximation, except for the C–N=S angle which is too large by ca. 6°. The B3LYP method predicts all bonds to be too long by 0.02–0.05 Å and the C–N=S angle to be too small by ca. 4°.     

Magnetohydrodynamic effects in nematic liquid crystals

The behaviour of a nematic liquid crystal when it is spun about an axis orthogonal to a magnetic field is predicted to be controlled by the critical angular velocity, ω_c. For spinning speeds below ω_c theory shows that the director makes an increasing angle with the field until at ω_c this angle is 45°. Above ω_c the director should rotate with an angular velocity slightly less than that of the sample. Observation in both regimes allows ω_c to be determined; since it depends on the ratio of the diamagnetic anisotropy to the rotational viscosity coefficient of the nematic, this ratio can be measured. However, an experimental investigation by Eastman et al. [1], suggests that the theoretical relationship between ω_c and this ratio may be in error by a factor of about four. We have reanalysed their data in an attempt to check this important claim and have found that there is in fact good agreement between theory and experiment.     

Thermal rearrangements of bis-allenyl thiosulfonates. Synthesis of novel thienothiophene and thieno-oxathiine derivatives

The synthesis and thermal rearrangement of bis-allenyl thiosulfonates are described. Bis-γ,γ-disubstituted allenyl thiosulfonates have been prepared by disproportionation of the corresponding allenesulfinic acids. On heating, these compounds unexpectedly rearrange to a mixture of 1H,3H-thieno[3,4-d][1,2]oxathiine-3-oxide 8, 1H,3H-thieno[3,4-c]thiophene-2,2-dioxide 9, and 3-alkyl-4-alkenylthiophene 10. A tentative reaction mechanism involving sequential sigmatropic rearrangements and cyclizations is suggested.     

Spectroscopic and theoretical investigations on effective and selective complexation between porphyrins and fullerenes

We have tailored some interesting mono- and diporphyrins, viz., 5, 10, 15, 20-tetrakis(octadecyloxyphenyl)-21H,23H-porphyrin (TP), 5, 15-bis(3, 5-dioctylphenyl)-2, 8, 12, 18-tetramethyl-3, 7, 13, 17-tetradodecyl-21H,23H-porphyrin (AP) and 1, 3-bis(10′,20′-diundecyl-21′H,23′H-porphyrin-5′-yl)benzene (DP) as host molecules to study their complexation behavior with C₆₀ and C₇₀. Fluorescence spectroscopic measurements showed that emission of TP, AP and DP in toluene was quenched in the presence of C₆₀ or C₇₀. Large binding constants (K) in the magnitude of 1.5 × 10⁴ dm³ mol⁻¹ have been obtained for the 1:1 complexes of C₇₀ with TP, AP and DP. However, the C₆₀ complexes have exhibited 8.5 times smaller K compared to C₇₀ complexes. Ab initio theoretical calculations give a good support in favor of strong complexation between C₇₀ and porphyrins.     

Strong antiferromagnetism in the dinuclear 2-pyridone complex with N–C–O bridges: A paddle-wheel analogue of the dinuclear tetracarboxylates

A novel dinuclear complex [Cu₂(μ-L)₄(HL)₂] (1) was isolated from starting 2-pyridone (HL) via a resonance and a tautomeric transformation. Each copper centre is in a square-pyramidal coordination sphere, defined by two oxygen atoms (Cu–O4 1.978(5), Cu–O11 1.964(4) Å) and two nitrogen atoms (Cu–N2 2.003(5), Cu–N3 2.007(5) Å) of four bridging deprotonated pyridin-2-olates and an oxygen atom on the top from a neutral 2-pyridone (Cu–O2 2.227(5) Å), analogous to tetracarboxylate paddle-wheel complexes. Compound 1 was compared with mixed pyridin-2-olato/methanoato analogues [Cu₂(μ-HCO₂)₂(μ-L)₂(HL)₂] · 2CH₃CN (2) and [Cu₂(μ-HCO₂)₂(μ-L)₂(HL)₂] (2a) (2a is an air stable form obtained from 2 outside mother-liquid). The EPR spectra of air stable 1 and 2a show three signals H_z1, H₂ and H_z2, typical for the binuclear systems with spin S = 1, both revealing strong antiferromagnetism 2J = −334 (1) and −324 cm⁻¹ (2a). Interestingly, only for 1 additional H₁ signal at 100 mT is noticed (D(1) = 0.293 cm⁻¹ < hν = 0.320 cm⁻¹ < D(2a) = 0.347 cm⁻¹). On the other hand, several broad signals in the 100–450 mT region, only in the high temperature spectrum for 2a are observed. These results are in agreement with the magnetic susceptibility analysis.     

Hartree-fock and Hückel-like orbital energies: an elementary extension to Koopmans‘ theorem

For a closed-shell MO configuration with 2n electrons which occupy n non-degenerate canonical MOs, it is deduced that the RHF energy, Σⁿ_i=1[2H⁰_n+Σⁿ_j-1(2J_ij-K_ij)], may be expressed in Hückel-like form as 2Σⁿ_i-1ε, −Σⁿ_i-1[j_i(λ+1)+1,(λ+2)] with λ=2(n-i). The l_i(λ+1) and I_i(λ+2) are the ionization potentials for the HOMO ψ, which arises after λ and λ+1 electrons have been successively removed from the initial configuration.     

Effect of humidity, Na ion exchange and silica/alumina ratio of zeolite H-Y on its physical properties as assessed by water desorption using differential scanning calorimetry

Zeolite H-Y gives two widely separated endotherms in the DSC curve which are attributed to water desorption from large supercages and small β-cages. Water is still present in the β-cages after 50% dehydration of the zeolite, whereas water in the supercages is linearly dehydrated. Increasing the Na⁺ ion exchange in H-Y, prevents water molecules from reaching the β-cages but increases the multi-layers of water in the supercages. Dealumination of H-Y appears to destroy the β-cages but decreases the internal surface area and widens the supercages and, hence, decreases the water uptake. The size of the endotherms and, hence, their δH values, together with the shift of the endotherm maxima, give useful information.     

6,6′-Bisperfluoroalkylated BINOLs promoted asymmetric allylation of aldehydes

6,6′-Bis(1H, 1H, 2H, 2H-perfluorooctyl)-1, 1′-bi-2-naphthol (Rf₆-BINOL) and 6,6′-bis(1H, 1H, 2H, 2H-perfluorodecyl)-1,1′-bi-2-naphthol (Rf₈-BINOL) were used in allylation of aldehydes in fluorous biphase system. Good enantioselectivity was obtained and the ligands could be recovered by continuos liquid–liquid extraction.