Volume 37, Issue 2

This paper considers the parametric decay of an electromagnetic wave incident on a weakly-inhomogeneous magnetised plasma into a Langmuir wave and an ion-acoustic wave. The Vlasov model is used in order to calculate the low-frequency electron-density perturbation produced by the beating of the side-band modes with the pump wave. The results show that the threshold value of the pump wave to undergo a decay instability drops in the presence of an inhomogeneity in the plasma.

The dielectric loss of very diluted solutions of four aromatic and three aliphatic amines in three non polar aliphatic solvents at 20 °C has been measured. The wave numbers cover 0.1 to 200 cm -1 . It is shown, how to analyse the microwave spectra of the loss factor ε″ in terms of three absorption areas, two of Debye relaxation type and the high frequency one of Lorentz resonance type. To limit the latter at its high frequency side, the extinction coefficient a has been used to determine and separate the lowest molecular resonances. From the analysis, dispersion steps and the components of the dipole moment due to the three absorption regions are calculated. Comparison with the so called optical dispersion step insures within the limits of experimental error that the absorption due to orientation of the permanent dipole moment is covered by the measured band. The long wave region belongs to the rotation of the molecules, the two others to intramolecular reorientation. In aromatic amines not all molecules of the sample are able to invert their NH 2 group, but slower orientation by hindered inversion happens. On the other hand, in aliphatic amines the group is rotating and the high frequency region may be due to Foley absorption.

Supplementary to a preceding investigation on primary amines, the dielectric loss increments of three aromatic dimethylamino compounds, diphenylamine, N-methyldiphenylamine and triphenylamine, and of dicyclohexylamine have been measured in very diluted solutions at 20 °C. The loss data resulting from 0.1 up to 250 cm -1 are now covering the complete absorption region due to reorientation of the permanent molecular dipole, especially that part based on inversion. To resolve this part, the absorption curves are analysed in terms of three areas using the same method as done for primary amines. By this the dipole components assigned to the special reorientation processes may be discerned and discussed. So the dipole component of the dimethylamino group which reorientates by intramolecular motion is smaller than that of the amino group. The occurrence of an inversion process is demonstrated, but the molecular dipole component governing it also may reorient more slowly as the result of a hindered inversion. The additional reorientation of a mesomeric moment by twisting of the phenyl groups is suggested in triphenylamine and in the diphenyl compounds. The aliphatic dicyclohexylamine only shows a Poley absorption area, increased perhaps by intramolecular libration.

Results of ab initio SCF calculations on thioacrolein (CH 2 =CH-CH=S) and thioglyoxal (S=CH-CH=S) are reported. The geometries are optimized by the analytical gradient method using the double zeta (DZ) basis set. The trans conformers of these molecules are calculated to be more stable than the cis conformer by 2.06 and 4.31 kcal/mol, respectively.

The microwave spectrum of γ-thiobutyrolactone has been observed and measured in the vibra-tional ground state and in several excited states of the ring-bending and ring-twisting modes. From the value of the μ c component of the dipole moment and inertial-defect considerations the ring skeleton was shown to be non-planar. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state of the ring-bending and ring-twisting modes are ~ 120 cm -1 and ~ 250 cm -1 above the ground state respectively. These two out-of-plane ring vibrations are essentially independent and the ring-bending vibration is governed by a nearly harmonic potential energy function. The components of the dipole moment were determined by the Stark effect to be μ a = 3.770 ± 0.001 D, μ b = 1.818 ± 0.044 D and μ c = 0.832 ± 0.018 D, leading to μ total = 4.268 ± 0.023 D. The rigidity of the γ-thiobutyrolactone ring is discussed in relation to those of related molecules.

The F…F non-bonded distances are strikingly constant in a large series of trifluoromethyl derivatives. Semiempirical NDDO calculations have been performed for five representative molecules. From the energy partitioning it appears that the bonding interactions (resonance energy terms) are dominating in determining the molecular geometry. The constancy of the non-bonded distances may occur as the changes in the bond lengths and bond angles are compensated. A good description of the total energy, however, necessitates the inclusion of the two-center electrostatic terms of all atom pairs.

Nuclear electric quadrupole coupling constants e 2 qQ/h have been obtained by evaluation of quadrupole perturbations in 51 V NMR spectra of various vanadyl esters and esterchlorides of the general formula VOCl 3-n (OR) n (n=0-3; R = Me, Pr n , Pr n , Bu n and Bu i ). e 2 qQ/h values have been calculated from (i) 2nd. order quadrupole splittings of the 51 V NMR signals in pseudo-polycrastalline samples, (ii) 1st. order quadrupole splittings in nematic liquid crystals and (iii) line widths A ⊿ν 1/2 of the 51 V and 35 Cl NMR signals in the liquid isotropic state and e 2 qQ/h( 35 Cl) obtained by pure NQR measurements. The influences of the asymmetry parameter η and the orientation of the crystallites in powder spectra are considered, and temperature and concentration effects upon ⊿ν 1/2 and 51 V shielding (isotropic phases) and the ordering factor S a (meso-phases) are discussed.

A quantitative analysis of fluorescence self-quenching of chlorophylls a and b in ether as well as chlorophyll a in lipid vesicles and liposomes has been carried out. It is demonstrated that concentration changes of the fluorescence quantum yield can be correctly described by a Förster-type excitation energy transfer process between chlorophyll molecules in the monomeric form if part of the transfers leads to energy degradation.

A highly hydrolyzed solution of iron(III) chloride is investigated by X-ray diffraction. The occurrence of six O atoms in the first coordination shell of iron(III) is demonstrated. The structural similarity of hydrolyzed solutions of Fe(III) chloride and nitrate is pointed out.

Internal mobility ratios in melts of the system (Li-NH 4 )NO 3 were measured with the Klemm method. Densities and electric conductivities of this system were also measured. From these data the internal mobilities b Li and b NH⁴ were calculated. On account of the inevitable thermal decomposition of NH 4 NO 3 the range of temperatures and concentrations was limited. In these the mobility of NH 4 + is considerably greater than that of Li + . In contrast to other binary nitrate systems, the isotherms of the internal mobility of the larger cation distinctly decrease with a decrease of its concentration, in which the fact that NH 4 NO 3 , whose cation is moderately light though large in size, melts at relatively low temperature is reflected. The isotherms of b Li and b NH⁴ and the temperature dependence are discussed on the basis of the autor’s interpretation so far presented.

We made measurements of the temperature dependence of the static dielectric constants, the optical anisotropy and the three elastic constants of binary mixtures belonging to twelve structurally and physically markedly different liquid crystal classes. The class-specific material properties were quantitatively correlated with molecular structural elements. The changes of material properties occurring when different LC-classes are combined were investigated. Analytical approximations are presented which are shown to reproduce the static transmission characteristics of twisted nematic cells and their dependence on material constants and temperature with an accuracy comparable to exact numerical calculations. Strong smectic pretransitional effects are shown to influence not only k 22 and k 33 but also k 11 , thus suggesting that not only twist and bend but also splay deformations may be prohibitive in smectics. We show that the elastic ratio &33/&11 of different liquid crystal classes increases with increasing dielectric anisotropy thus leading to a decrease of the slope of transmission characteristics of twist cells. Furthermore, it is shown that k 33 /k 11 decreases for molecules comprising heterocyclic rings whereas hydrogenation of an aromatic ring causes k 33 /k 11 to increase.

The temperature dependence of the rotational viscosities of several physically markedly different binary mixtures belonging to ten different liquid crystal classes is measured. An attempt is made to correlate the results with class-specific structural properties such as hydrogenated rings, molecular polarity and structure of flexible side-chains. It is show that the measured small-scale dynamic properties of twist cells compare well with approximative theoretical calculations and experimentially determined class-specific material parameters. For the switching times corresponding to large-scale twist deformations heuristic approximations lead to satisfactory results.

The behaviour of the 63 Cu nuclear magnetic resonance signals in tetrahedrally coordinated copper(I) complexes has been investigated as a function of temperature, solvent and anion. From the strong dependence of the linewidth and the chemical shift on these parameters, exchange processes with complexes of lower symmetry are concluded.

IR band-shape analysis has been carried out on the 620 cm -1 deformation band of the sulphate ion in several Na 2 SO 4 -K 2 SO 4 -ZnSO 4 glasses. Variations of correlation times and second moments suggest that reorientational motions of sulphate ions begin to evolve prior to the glass-transition temperature. The correlation times may support a cluster model for the glass-transition.

A class of dissipative systems with circulatory forces is given for which a necessary and sufficient condition for stability can be expressed as an explicit Liapunov function equivalent to the Hermitian form of a Hermitian matrix. For a discretized continuous system this matrix has a band structure.

The mean fluorescence decay time for rhodamine 6G (donor) in the presence of malachite green (acceptor) has been investigated using a pulse fluorometer. The experimental results have been compared with theoretical ones obtained earlier [1].