. Methylarnrnonium lead(I1) halides, CH3NH3PbX3 (X = C1, Br, I), have been investigated by single-crystal X-ray diffraction, 2~ and ' 4~ nnu, adiabatic calorimetry, and other methods. The chloride (CL) has transitions at 171.5 and 177.4 K, the bromide (BR) at 148.4, 154.2, and 235.1 K, and the iodide (10) at 162.7 and 326.6 K. The respective entropies of transition (J K-' mol-I) are 11 .O and 5.1 for CL; 8.7,3.4, and 5.3 for BR; and 16.1 and 1.9 for 10. The highest-temperature phase, phaseI, of each halide is of the cubic (Pm3m) perovskite type. The cation in CL(4 and BR(4 could not be localized in the electron density maps; the thermal motion of the halogen atom is highly anisotropic. The In T,(~H) vs. T' plots (N-deuterated samples as well as CD3NH3PbC13) show significant departures from linearity: the temperature variation of T,(~H) in BR(II) and IO(Il) can be represented by functions of the type In T1(H) = ko -k 2 r 2 , which give adequate analytical representations of T~(~H ) a n d~, ( '~N ) in phase I as well. On cooling, BR(II) and IO(II) exhibit small quadrupole splittings QS(2H), which can be represented to a high degree of correlation by QS(2H) = k(T,, -T)", i.e. they appear to exhibit critical behaviour with respect to T. The I4N nrnr results indicate that the C-N bond in phase I reorientates in an isotropic potential at a rate approaching that of the freely rotating methylammonium ion. Below phase I this motion takes place in an increasingly anisotropic potential in BR(II) and IO(II) and is essentially arrested in CL(II), BR(II4, and IO(II4. The temperature dependence of the activation energy E, for the cation reorientation and other aspects of the non-Arrhenius behaviour are discussed, and the CH3NH3PbX3 perovskites are compared with the corresponding (CH3NH3)2TeX6 halides, utilizing preliminary ' H nrnr results on (CH3ND3)2TeBr6. The electrical conductivity, between 0 and 95"C, of CH3NH3Pb13 increases with temperature and exhibits no discontinuity at T,, = 326.6 K; the activation energy for the conduction process is estimated as -0.4 eV.

The development of green, selective, and efficient catalysts, which can aerobically oxidize a variety of alcohols to their corresponding aldehydes and ketones, is of both economic and environmental significance. We report here the synthesis of a novel aerobic oxidation catalyst, a zeolite-confined nanometer-sized RuO(2) (RuO(2)-FAU), by a one-step hydrothermal method. Using the spatial constraints of the rigid zeolitic framework, we sucessfully incorporated RuO(2) nanoparticles (1.3 +/- 0.2 nm) into the supercages of faujasite zeolite. Ru K-edge X-ray absorption fine structure results indicate that the RuO(2) nanoclusters anchored in the zeolite are structurally similar to highly hydrous RuO(2); that is, there is a two-dimensional structure of independent chains, in which RuO(6) octahedra are connected together by two shared oxygen atoms. In our preliminary catalytic studies, we find that the RuO(2) nanoclusters exhibit extraordinarily high activity and selectivity in the aerobic oxidation of alcohols under mild conditions, for example, air and ambient pressure. The physically trapped RuO(2) nanoclusters cannot diffuse out of the relatively narrow channels/pores of the zeolite during the catalytic process, making the catalyst both stable and reusable.

The yellow (CH3NH3)4PbI6•2H2O(P21/n, a = 10.421(3) Å, b = 11.334(2) Å, c = 10.668(2) Å, β = 91.73(2)°, Z = 2) contains isolated PbI64− octahedra, CH3NH3+ cations of two types and H2O molecules. The cations and the water molecules are hydrogen-bonded to form [Formula: see text] units arranged in centrosymmtric [Formula: see text] pairs; the centres of these composite units and the Pb atoms form a distorted NaCl-type lattice. The Pb—I bond lengths in the PbI64− anions are compared with those in [Pb(II)I6]ε complexes containing shared I atoms and the effect of the sharing on the bond lengths is discussed. A scheme is proposed for the extensive three-dimensional hydrogen bonding in the structure.

The attempt to prepare hitherto unknown homopolyatomic cations of sulfur by the reaction of elemental sulfur with blue S8(AsF6)2 in liquid SO2/SO2ClF, led to red (in transmitted light) crystals identified crystallographically as S8(AsF6)2. The X-ray structure of this salt was redetermined with improved resolution and corrected for librational motion: monoclinic, space group P2(1)/c (No. 14), Z = 8, a = 14.986(2) A, b = 13.396(2) A, c = 16.351(2) A, beta = 108.12(1) degrees. The gas phase structures of E8(2+) and neutral E8 (E = S, Se) were examined by ab initio methods (B3PW91, MPW1PW91) leading to delta fH theta[S8(2+), g] = 2151 kJ/mol and delta fH theta[Se8(2+), g] = 2071 kJ/mol. The observed solid state structures of S8(2+) and Se8(2+) with the unusually long transannular bonds of 2.8-2.9 A were reproduced computationally for the first time, and the E8(2+) dications were shown to be unstable toward all stoichiometrically possible dissociation products En+ and/or E4(2+) [n = 2-7, exothermic by 21-207 kJ/mol (E = S), 6-151 kJ/mol (E = Se)]. Lattice potential energies of the hexafluoroarsenate salts of the latter cations were estimated showing that S8(AsF6)2 [Se8(AsF6)2] is lattice stabilized in the solid state relative to the corresponding AsF6- salts of the stoichiometrically possible dissociation products by at least 116 [204] kJ/mol. The fluoride ion affinity of AsF5(g) was calculated to be 430.5 +/- 5.5 kJ/mol [average B3PW91 and MPW1PW91 with the 6-311 + G(3df) basis set]. The experimental and calculated FT-Raman spectra of E8(AsF6)2 are in good agreement and show the presence of a cross ring vibration with an experimental (calculated, scaled) stretching frequency of 282 (292) cm-1 for S8(2+) and 130 (133) cm-1 for Se8(2+). An atoms in molecules analysis (AIM) of E8(2+) (E = S, Se) gave eight bond critical points between ring atoms and a ninth transannular (E3-E7) bond critical point, as well as three ring and one cage critical points. The cage bonding was supported by a natural bond orbital (NBO) analysis which showed, in addition to the E8 sigma-bonded framework, weak pi bonding around the ring as well as numerous other weak interactions, the strongest of which is the weak transannular E3-E7 [2.86 A (S8(2+), 2.91 A (Se8(2+)] bond. The positive charge is delocalized over all atoms, decreasing the Coulombic repulsion between positively charged atoms relative to that in the less stable S8-like exo-exo E8(2+) isomer. The overall geometry was accounted for by the Wade-Mingos rules, further supporting the case for cage bonding. The bonding in Te8(2+) is similar, but with a stronger transannular E3-E7 (E = Te) bonding. The bonding in E8(2+) (E = S, Se, Te) can also be understood in terms of a sigma-bonded E8 framework with additional bonding and charge delocalization occurring by a combination of transannular n pi *-n pi * (n = 3, 4, 5), and np2-->n sigma * bonding. The classically bonded S8(2+) (Se8(2+) dication containing a short transannular S(+)-S+ (Se(+)-Se+) bond of 2.20 (2.57) A is 29 (6) kJ/mol higher i...