Friction of materials based on PTFE in vacuum

The authors give the results of an investigation of full-scale slide bearings made of material based on PTFE in vacuum under conditions which are realized in ordinary engineering. They show that unfilled PTFE possesses insufficient wear resistance, and that very hard mineral fillers cause a marked rise in the coefficient of friction owing to frictional heating.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 447–452, May–June, 1974.     

Nuclear quadrupole resonance of bromothiophenes

Br⁷⁹ NQR frequencies of a number of bromothiophene derivatives have been measured at 77°K. The frequencies correlate with Hammett δ values and this correlation demonstrates the importance of the inductive effect. The influences of more than one substituent on the thiophene ring are additive.     

Synthesis of 5-bromopentanoic acid through catalytic decomposition of the hydroperoxides of cyclopentanone

Conclusions A synthetic method of preparing 5-bromopentanoic acid was develope by oxidation of cyclopentanone with aqueous solutions of H₂O₂. Subsequent decomposition of the resulting hydroperoxide of cyclopentanone was accomplished with a catalytic quantity of copper salt in the presence of alkali bromides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 106–108, January, 1989.     

Light-induced formation of nitroxyl radicals by organic Bi(V) compounds in the presence of 2-methyl-2-nitrosopropane and <Emphasis Type="Italic">C</Emphasis>-phenyl-<Emphasis Type="Italic">N</Emphasis>-<Emphasis Type="Italic">tert</Emphasis>-butylnitrone

The organic bismuth compounds Ph₃Bi(O₂CCH=CHCH₃)₂ and Ph₃Bi(O₂CEt)₂ decompose under scattered light in the presence of spin traps (2-methyl-2-nitrosopropane and C-phenyl-N-tert-butylnitrone) in organic solvents (acetonitrile, benzene, toluene) to form adducts of phenyl radicals with the traps.     

X-ray emission study of the electronic structure of binuclear niobium complexes with (S<Subscript>2</Subscript>)<Superscript>2</Superscript>–disulfide bridging ligands

The electronic structure of binuclear niobium complexes [Nb₂S₄(acac)₄] and K₄[Nb₂S₄(ox)4] is studied by X-ray emission fluorescent spectroscopy and quantum chemistry techniques. Data on the partial atomic composition of highest occupied molecular orbitals of the complexes are obtained. The energy positions of bonding and antibonding frontier molecular orbitals observed in the X-ray emission spectra of binuclear [Nb₂((S₂)^2–)₂]⁴⁺ clusters are determined by the analysis of overlap populations.     

Electron–nuclear motions in a series of meta-substituted iodobenzene derivatives as probed by <Superscript>127</Superscript>I NQR

A relation between the quadrupole coupling constant e ² Qq _zz and asymmetry parameter η has been found for a series of meta-substituted iodobenzene derivatives. The relationships e ² Qq _zz(η) for electrondonor and electron-acceptor substituents have the same form but different numerical values of parameters. The average time of full electron transfer via tunneling has been evaluated.     

Novel Oxidovanadium Complexes with Redox-Active R-Mian and R-Bian Ligands: Synthesis,Structure, Redox and Catalytic Properties

A new monoiminoacenaphthenone 3,5-(CF₃)₂C₆H₃-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl₂(dpp-mian)(CH₃CN)] (3) and [VOCl(3,5-(CF₃)₂C₆H₃-bian)(H₂O)][VOCl₃(3,5-(CF₃)₂C₆H₃-bian)]·2.85DME (4) from [VOCl₂(CH₃CN)₂(H₂O)] (1) or [VCl₃(THF)₃]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex 4 has an unexpected structure. Firstly, it contains 3,5-(CF₃)₂C₆H₃-bian instead of 3,5-(CF₃)₂C₆H₃-mian. Secondly, it has a binuclear structure, in contrast to 3, in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures 2 and 3, non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36–3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes 3 and 4 has been proven by EPR spectroscopy. Complexes 3 and 4 exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex 3) and 27% (complex 4). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh₃). According to the DFT calculations, the difference in the catalytic activity of 3 and 4 is most likely associated with a different mechanism for the generation of ^●OH radicals. For complex 4 with electron-withdrawing CF₃ substituents at the diimine ligand, an alternative mechanism, different from Fenton’s and involving a redox-active ligand, is assumed.