Synthesis and spectroscopic studies of phenyllead halide and thiocyanate adducts with hexamethylphosphoramide

Hexamethylphosphoramide (HMPA) adducts of the type Ph₃PbX·HMPA (X=Cl, Br, I, and NCS), Ph₂PbX₂·2HMPA (X=Cl, Br, and I), and Ph₂PbX₂·HMPA (X=Br and I), have been prepared and characterized by infrared, Raman, mass, and ³¹P nmr spectroscopy. Molecular weight and infrared solution data show that Ph₃PbX·HMPA adducts dissociate in benzene, the degree of dissociation being NCS«Cl<Br<I. The thiocyanate adducts Ph₃PbNCS·HMPA and Ph₂Pb(NCS)₂·2HMPA have v(CN) and v(CS) frequencies in the solid state, and v(CN) frequencies and absorptivities in benzene solution consistent with N-bonded thiocyanate in the solid state and in benzene solution. Vibrational frequencies are reported in the range 260 to 80 cm⁻¹ and assignments are made for v(Pb-X), v(Pb-O0, and v(Pb-NCS) modes. The 1:1 adducts Ph₃PbX·HMPA are monomeric and trigonal bipyramidal, whereas the 1:2 adducts Ph₂PbX₂·2HMPA are monomeric and cis-octahedral and the Ph₂PbX₂·HMPA appear to be halogen bridged polymers with lead six-coordinate. Coordination of HMPA causes a small upfield change in ³¹P chemnical shift values, and ²J(Pb-P) values vary with X in the order: NCS>I-Br>Cl for Ph₃PbX·HMPA adducts. Corresponding tin and lead adducts are compared with respect to mode of adduct formation.     

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.     

Cluster Chemistry: XXVIII. Utility of fast atom bombardment mass spectrometry for the characterisation of high molecular weight complexes containing metal clusters

Fast atom bombardment (FAB) mass spectrometry has been used to obtain spectra of HRu₃Au(μ₃-S)(CO)₉(PPh₃), Ru₃Au₂(μ₃-S)(CO)₉(PPh₃)₂ and Ru₃Au₃(μ₃-C₁₂H₁₅)(CO)₈(PPh₃)₃, none of which give metal-containing ions in conventional EI mass spectrometry. All the compounds give molecular (M⁺) or quasimolecular ([M + 2H]⁺) ions which fragment by conventional routes.     

Solving wigner’s mystery: The reasonable (though perhaps limited) effectiveness of mathematics in the natural sciences

The Viewpoint column offers mathematicians the opportunity to write about any issue of interest to the international mathematical community. Disagreement and controversy are welcome. The views and opinions expressed here, however, are exclusively those of the author, and neither the publisher nor the editor-in-chief endorses or accepts responsibility for them. Viewpoint should be submitted to the editor-in-chief, Chandler Davis.     

On vacuum metrics of type (3, 1)

A technique is described for constructing solutions of Einstein's equations for empty space, in which the Riemann tensor has a triply degenerate principal null direction with twist.     

Synthesis and crystal structure of the first scandium-containing open framework solid

A novel open framework scandium sulfate phosphate is prepared hydrothermally in the presence of the azamacrocycle cyclen (1,4,7,10-tetraazacyclododecane) in which secondary building units of formula Sc7(S,P)12O48 are linked to give a structure with supercages.     

Future cockpit displays

As aircraft systems become more complicated it is necessary to display information which was either not previously available or was provided by instruments which were unduly bulky, heavy, expensive, or unreliable. This paper considers possible display mechanisms for future cockpits and their performance in high ambient illumination. Methods of optimizing the legibility of commercially available display types using contrast enhancement techniques are described. Finally the results of subjective tests on light emitting diode displays are discussed.