Arrangements of hyperplanes with property D

We denote by the complement of the complexification of a real arrangement of hyperplanes. It is known that there is a certain technical property, called property D, on real arrangements of hyperplanes such that: if a real arrangement of hyperplanes is simplicial then has property D, and if has property D then is aK(, 1) space. Our main goal is to prove that: if has property D then is simplicial. We also prove that a quasi-simplicial arrangement is always simplicial.     

Self-consistent solutions of the semiclassical Einstein equations with cosmological constant

The general solution to the semiclassical backreaction equation is found for conformally invariant free quantum fields in spatially flat homogeneous and isotropic spacetime with Cosmological constant and with no classical source when the ratio of the renormalisation parameters/=9/4. It contains a two-parameter family of bouncing solutions that avoid the singularity. There are several one-parameter families which do not have particle horizons. The stability of these solutions is investigated and it is found that they are stable when and have different signs. However, when both parameters have the same sign the set of stable solutions is restricted by the condition 0 < < 1/9. In both cases these solutions have a final de Sitter stage.     

Weyl equation in some anisotropic stiff fluid universes

The Weyl equation (massless Dirac equation) is studied in a family of exact solutions of the Einstein equations whose material content is a perfect fluid with stiff equation of state (p=) and which are of Bianchi type I. The field equation is solved exactly for some members of the family.     

Effect of particle geometry on phase transitions in two-dimensional liquid crystals

Using a version of density-functional theory which combines Onsager approximation and fundamental-measure theory for spatially nonuniform phases, we have studied the phase diagram of freely rotating hard rectangles and hard discorectangles. We find profound differences in the phase behavior of these models, which can be attributed to their different packing properties. Interestingly, bimodal orientational distribution functions are found in the nematic phase of hard rectangles, which cause a certain degree of biaxial order, albeit metastable with respect to spatially ordered phases. This feature is absent in discorectangles, which always show unimodal behavior. This result may be relevant in the light of recent experimental results which have confirmed the existence of biaxial phases. We expect that some perturbation of the particle shapes (either a certain degree of polydispersity or even bimodal dispersity in the aspect ratios) may actually destabilize spatially ordered phases thereby stabilizing the biaxial phase.     

A short synthesis of the erythrina skeleton and of (+/-)-alpha-lycorane

[reaction: see text] A new nonchain 5-endo radical cyclization starting with xanthates was exploited in a short synthesis of (+/-)-alpha-lycorane and the erythrina ring system.     

Mean amplitudes of vibration of the XF+4 interhalogen cations

Mean amplitudes of vibration for the cations ClF⁺₄, BrF⁺₄ and IF⁺₄ have been calculated using the ‘Method of the Characteristic Vibrations’ and recently revised spectroscopic data. The results are briefly discussed and some comparisons with isoelectronic molecules and other related species are made.     

Surface phase separation in complex mixed adsorbing systems: an interface-bulk coupling effect

The interfacial thermodynamics and structure of ternary mixtures of the type A+B+solvent are investigated. According to the Gibbs phase rule, the coupling between the bulk phase and the interfacial region-which is related to the reversibility of the adsorption of the corresponding species-is a determinant as to whether phase separation can be observed at the interface. For an n-component adsorbing solution, at least one of the species has to adsorb irreversibly over the experimental time scales in order not to fix more intensive variables than those required to observe surface phase separation. We present results for a lattice model planar interface consisting of the ternary mixture A+B+solvent. The solvent molecules and the type A molecules have fixed chemical potentials at the interface since they are equilibrated with a bulk solution. In contrast, the type B molecules are irreversibly adsorbed at the interface and do not equilibrate with the bulk. Mean-field theory is compared with Monte Carlo simulation. Interestingly, the spinodal line in the interaction-composition plane shows a reentrant on the B-rich phase side. We discuss the implications of these results for surface phase separation of adsorbing mixtures of proteins and low-molecular-weight surfactants.     

Second-order nonlinear optical properties of the Ag(111)/electrolyte interface in the presence of self-assembled monolayers containing conjugated pi systems. I. Alpha-functionalized terthiophene films on Ag(111)

The nonlinear optical properties of self-assembled monolayers obtained from bonding two different alpha-functionalized terthiophenes (alpha-T3) to (111) silver electrode surfaces have been investigated using second harmonic generation (SHG). The two (alpha-T3) compounds used were functionalized with alkane chains of different lengths (C8 and C4), and each was terminated with a thiol anchoring group. A nitrile group was attached to the terminal thiophene ring of the (alpha-T3) compound with the C4 chain. The orientation of the polarization of the incident beam was changed systematically and gradually between "p" and "s" orientation and the SH signal (isotropic and anisotropic contributions) analyzed in both directions ("P" and "S"). The symmetry of the system was reduced by the presence of the adlayers from C3v to C3. The dependence on the applied potential and the incident wavelength has also been studied. The relative magnitudes and phases of the various second-order tensor elements have been estimated and compared with the values for a bare surface. A resonance process in the (alpha-T3) pi moiety has been investigated, and from this, the effective "band-gap" energies of the organic semiconductor SAMs (i.e., the energy difference between the pi-pi bands) have been estimated.     

Das Schwingungsspektrum von Barium-Divanadat

The infrared and laser-Raman spectra of crystalline Ba₂V₂O₇ are reported and discussed by means of a Site Symmetry analysis.

28. Mitt. der Reihe Orthovanadate und verwandte Verbindungen. 27. Mitt.:E. J. Baran undM. E. Escobar, J. Phys. Chem. Solids, im Druck; 26. Mitt.:E. J. Baran, I. L. Botto, J. F. Ramos undP. J. Aymonino, Mh. Chem.109, 1139 (1978); 25. Mitt.:E. J. Baran, J. Mol. Struct.48, 441 (1978); 24. Mitt.:E. J. Baran undI. L. Botto, J. inorg. nucl. Chem.40, 1603, (1978); 23. Mitt.:M. E. Escobar undE. J. Baran, Z. Anorg. allg. Chem.,441, 273 (1978).     

Superoxide-mediated dehydrohalogenation reactions of the pyrethroid permethrin and other chlorinated pesticides

Superoxide generated in DMF readily converts the dihalovinyl group of permethrin and related compounds to a haloethynyl moiety and yields major products from elimination reactions of DDT, $\text{cis}$-chlordane, and 1,2-dibromo-3-chloropropane (DBCP).