Defect aggregation in anion-excess fluorites II. Clusters containing more than two impurity atoms

A theoretical study has been made of defect clustering in fluorite crystals. The emphasis is on the aggregation processes that can occur beyond the dimer stage which was discussed previously (J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong,Phys. Rev. B25, 6425 (1982)). The systems studied comprise CaF₂, SrF₂, and BaF₂ doped withMF₃whereM =Y, Lu, Yb, Er, Tb, Gd, Eu, or La, and also SrCl₂ doped with GdCl₃, LaCl₃, or PrCl₃. Two tetramers, one containing a single anion vacancy and two 〈111〉 relaxed F^? ions and the other with a planar arrangement of the dopant ions, are stable for certain systems while a trimer and hexamers, with an octahedral arrangement of substitutionalM³⁺, also show considerable stability. A notable feature is the increase in the relative stability of larger complexes as the ratio of the radius of the dopant ion to that of the host lattice cation is decreased. The results are discussed in relation to recent EXAFS and neutron scattering work on these systems and should provide a useful guide in future experimental studies.     

Chelatometric titration of bismuth with hedta

A new chelatometric method for the determination of Bi in bismuth-base alloys, low-melting alloys and reagent-grade bismuth salts is proposed. It is based on the chelation of Bi(III) with HEDTA, which is added in substoichiometric amount in perchloric acid medium to eliminate the interfering side-reactions of olation and oxolation, followed by pH adjustment and titration at pH 2 with Semi-Xylenol Orange as indicator. It is more selective than the classical EDTA method, and the accuracy and precision are enhanced. The coefficient of variation was found to be ˜ 0.1%.     

Reactions of Grignard reagents from 1-halogenoallenes. Allenes—XXXIII

Grignard reagents have been prepared from 1-bromo-3,3-dialkylallenes in ether or tetrahydrofuran. The allenic acids, 4,4-dialkylbuta-2,3-dienoic acids together with 4,4-dialkylbut-2-ynoic acids are formed with carbon dioxide to the exclusion of acids with a terminal acetylene group. Two coupled products, 3,4-diethyl-3,4-dimethylhexa-1,5-diyne and 3-ethyl-3,6-dimethylocta-4,5-dien-1-yne, which are formed as byproducts in reactions of 3-methylpenta-1,2-dienyl magnesium bromide with electrophiles (CO₂, H₂O, Me₂CO), are obtained in high yield by passing oxygen through the solution of the Grignard reagent.     

Automated nephelometric determination of polyvinyl-pyrrolidone in salazopyrin.

An automatic nephelometric method is described for the determination of polyvinyl-pyrrolidone (PVP) in Salazopyrin. PVP is separated from the other constituents of Salazopyrin on a two-layer anion and cation exchanger in 96—99 % yield. The PVP is determined nephelometrically after precipitation with perchloric acid, in an AutoAnalyzer II system at 30 samples per hour with a relative standard deviation of 0.5 %. The nephelometric response of the precipitation reaction is strongly dependent upon reaction time and, because of the precipitation reagent used, on the molecular weight of the sample. An investigation of the influence of these factors is presented.     

Geometry optimised ab inttio calculations on 1,5- and 1,2-dicarba-closo-pentaborane (5)

Geometry optimised ab initio LCAO- MO-SCF calculations for the isomeric carboranes 1,5- and 1,2-dicarba-closo-pentaborane (5) are reported. The optimised geometry of the 1,5-isomer agrees with the experimental geometry and for this isomer greater thermo-dynamic stability is predicted. The framework bonding is due mainly to interactions between the apical atoms and those in the equatorial plane, while within this equatorial plane there is very weak bonding.     

Vibrationally hot fluorescence in chlorophyll-a

Fluorescence from high vibrational levels of the first excited singlet state of chlorophyll-a in solution is observed at 77°K. The pronounced fluorescence efficiency dependence on pumping of specific vibrational levels may contribute to excitation-wavelength effects on photosynthesis.     

The compatibility of the cyano and alkyl chloride functions in phenylthioalkylation reactions of O-silylated enolates

The compatibility of the cyano group and of chlorides with the conditions of phenylthioalkylation O-sitylated enotates has been demonstrated.     

Quasi-molecular angle dependence of photoemission from thin films of polystyrene

Angle-resolved photoelectron spectra from thin films of randomly oriented polystyrene show a pronounced angular dependence of the emission for several peaks derived from benzene molecular orbitals in the valence band region. The analysis yields a clear molecular behaviour for the angular distribution of the uppermost π-band (1e_1g) which can be described by a β parameter of β = 0.6 for hν = 31 eV. These results support, for polystyrene, the notion of weak intermolecular interaction in a randomly oriented “condensed gas”.     

GPRS的漫游——实现移动分组接入的解决方案

首先简单介绍了GPRS的系统结构，在此基础上给出了GPRS网络中的APN（access point name）和域名系统的概念，分析了漫洲情况下GPRS是如何支持分组数据路由的过程，最后介绍了GPRS系统中的IP地址管理的问题。     

Discovery of an electrical post-pulse in the surroundings of a high voltage spark discharge

In the vicinity of a high voltage spark, an electrical pulse of negative polarity was observed. The pulse's behavior with respect to spark cathode composition, gas flow, spatial distribution, and related parameters was characterized. Suggestions as to the origin of the pulse are made. Attempts at observing laser-enhanced ionization in the post discharge time period are described.