Conjugate points and Maslov index in locally symmetric semi-Riemannian manifolds

We study the singularities of the exponential map in semi Riemannian locally symmetric manifolds. Conjugate points along geodesics depend only on real negative eigenvalues of the curvature tensor, and their contribution to the Maslov index of the geodesic is computed explicitly. We prove that degeneracy of conjugate points, which is a phenomenon that can only occur in semi-Riemannian geometry, is caused in the locally symmetric case by the lack of diagonalizability of the curvature tensor. The case of Lie groups endowed with a bi-invariant metric is studied in some detail, and conditions are given for the lack of local injectivity of the exponential map around its singularities.     

The preparation of 4-oxo-N-aryl-4H-1-benzopyran-2-acetamides by the condensation/cyclization of trilithiated acetoacetanilides with lithiated methyl salicylates

Several acetoacetanilides were trilithiated in excess lithium diisopropylamide, and the resulting polylithiated intermediates were regioselectively condensed with lithiated methyl salicylates followed by acid cyclization to substituted 4-oxo-N-aryl-4H-1-benzopyran-2-acetamides (benzopyranone-2-acetamides).     

Determination of propazine by differential pulse polarography in micellar and emulsified media

An electroanalytical study of the herbicide propazine's reduction process in micellar solutions and oil-in-water emulsions is reported. The anionic surfactant sodium pentanesulphonate was chosen as the most suitable. The differential pulse polarograms of micellar solutions had two reduction peaks below pH 2.0, whereas only one peak was obtained above pH 2.O. Ethyl acetate was chosen as the organic solvent to form propazine emulsions. Unlike in micellar solutions, the DPP polarograms of propazine emulsions showed only one peak even at pH < 2.0, suggesting that propazine hydrolysis was hindered in the emulsified medium. The limiting current is diffusion-controlled and the electrode process is irreversible. Propazine can be determined by differential pulse polarography over the 1.0 × 10^–1 – 1.0 × 10^–1moll^–1 and 1.0 × 10^–15 – 4.0 × 10^–1 moll^–1 concentration ranges and the limit of detection was 2.8 × 10^–1 moll^–1. Of the potential interferents simazine, methoprotryne and terbutryn (alls-triazines), thiram (a dithiocarbamate), dinoseb (nitrophenolic), and heptachlor (chlorinated cyclo-diene herbicide), only the first two were significant (10% error for equimolar concentrations). The method was applied to the determination of propazine in spiked drinking water. At a concentration level of 2.0 × 10^–1 moll^–1 a recovery of 94 ± 6% was obtained, after tenfold concentration on Sep-Pak.     

Benzo[d]-1,2-oxaphospholes as precursors of stabilized C-centered radicals

[reaction: see text] Several benzo[d]-1,2-oxaphosphole 2-oxides were examined as potential precursors of stabilized C-centered radicals. The transient absorption spectra obtained after laser flash photolysis in the presence of di-tert-butyl peroxide showed the features of benzylic radicals with formation and decay kinetics not significantly influenced by the presence of oxygen. In the case of compounds with two possible diastereomeric forms, the C-H bond of the trans-isomers [corrected] is more reactive toward hydrogen abstraction.     

Coupling continuous flow systems to instruments based on discrete sample introduction

The interfaces between low-pressure continuous sample treatment systems and high-level information instruments such as gas chromatographs, capillary electrophoresis equipment and graphite furnace atomic spectroscopic instruments, which are characterized by conventional discrete sample introduction devices, are presented. The present and future developments are discussed of (directly) linking real samples and those analytical equipments with the main objective of avoiding or minimizing manually implemented preliminary operations of the analytical process. Received: 27 January 1998 / Revised: 5 March 1998 / Accepted: 7 March 1998     

Peptoids as source of compounds eliciting antibacterial activity

N-Alkylglycine oligomers (peptoids) constitute a family of non-natural peptidomimetics attractive for the early drug discovery process because of their physicochemical features, easy of adaptation to combinatorial chemistry approaches and their proteolytic stability. Consequently, peptoid libraries have found application for discovering hits against a wide diversity of pharmaceutical targets, among which different examples of antibacterials are found. In the present work, research efforts addressed towards the identification of peptoids as antibacterial agents are discussed.     

Relaxation processes in room temperature ionic liquids: the case of 1-butyl-3-methyl imidazolium hexafluorophosphate

A detailed investigation on the nature of the relaxation processes occurring in a typical room temperature ionic liquid (RTIL), namely, 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]), is reported. The study was conducted using both elastic and inelastic neutron scattering over a wide temperature range from 10 to 400 K, accessing the dynamic features of both the liquid and glassy amorphous states. In this study, the inelastic fixed energy scan technique has been applied for the first time to this class of materials. Using this technique, the existence of two relaxation processes below the glass transition and a further diffusive process occurring above the glass-liquid transition are observed. The low temperature processes are associated with methyl group rotation and butyl chain relaxation in the glassy state and have been modeled in terms of two Debye-like, Arrhenius activated processes. The high temperature process has been modeled in terms of a Kohlraush-Williams-Watts relaxation, with a distinct Vogel-Fulcher-Tamman temperature dependence. These results provide novel information that will be useful in rationalizing the observed structural and dynamical behavior of RTILs in the amorphous state.     

Studies on the synthesis of pentacyclic strychnos indole alkaloids. photocyclization of n-chloroacetyl-1,2,3,4,5,6-hexahydro-1,5-methanoazocino[4,3-b]indole derivatives

Photocyclization of 2-chloroacetyl-1,2,3,4,5,6-hexahydro-1, 5-methanoazocino[4,3-b]indole ($\text{5}$) takes place at the indole 4-position to give a 1 ,2 ,3 ,4 , 5 ,6-hexahydro-2 ,11-ethano-1 ,5-methanoazocino [4 , 3-6] indole system. Consequently, the method appears to be unsuitable for constructing the pyrrolidine ring of pentacyclic Strychnos indole alkaloids.