A guided ion beam tandem mass spectrometer is used to study the reactions of atomic 187 Re + with CH 4 and CD 4 and collision-induced dissociation (CID) of ReCH 4 + with Xe. These studies examine the activation of methane by Re + in a low-pressure environment free of ligand supports or other reactive species. In the bimolecular reaction, ReCH 2 + is efficiently produced in a slightly endothermic process and is the only ionic product observed at low energies, whereas at higher energies, ReH + dominates the product spectrum. Other products observed include ReC + , ReCH + , and ReCH 3 + . Modeling of these endothermic reactions yields 0 K bond dissociation energies in eV of D 0 (Re + -C) ) 5.12 ( 0.04, D 0 (Re + -CH) ) 5.84 ( 0.06, D 0 (Re + -CH 2 ) ) 4.14 ( 0.06, D 0 (Re + -CH 3 ) ) 2.22 ( 0.13. Analysis of the behavior of the cross sections suggests that formation of ReH + , ReCH 2 + , and ReCH 3 + occurs via an H-Re + -CH 3 intermediate. CID of ReCH 4 + reveals a bond energy of 0.53 ( 0.15 eV for Re + -CH 4 . The experimental bond energies compare favorably with theoretical calculations at the B3LYP/HW+/6-311++G(3df,3p) level with the exception of the singly bonded species (ReH + , ReCH 3 + ), where the Becke-half-and-half-LYP functional performs much better. Theoretical calculations also elucidate the reaction pathways for each product and provide their electronic structures. Overall we find that the dehydrogenation reaction, which occurs with an efficiency of 86 ( 10%, must involve three facile spin changes (2s + 1 ) 7 f 5 f 3 f 5) indicating that little hint of spin conservation remains in this heavy-metal system. † Part of the special issue "Tomas Baer Festschrift".

While pyrolysis of a polysiloxane precursor in argon typically produces a black amorphous Si–O–C ceramic containing “free” carbon (sp2 carbon), pyrolyzing the same precursor in hydrogen leads to a white amorphous ceramic with a negligible amount of sp2 carbon and a considerable hydrogen content. 29Si magic‐angle‐spinning nuclear magnetic resonance (MAS NMR) spectroscopy confirms the existence of very similar bonding environments of Si atoms in the Si–O–C network for both samples. In addition, 1H NMR spectroscopic measurements on both samples reveal that the hydrogen atoms are bonded mainly to carbon. For the thermodynamic analysis, the enthalpies of formation with respect to the most stable components (SiO2, SiC, C) of the black‐and‐white Si–O–C samples obtained after the pyrolysis at 1100°C are determined using high‐temperature oxidative drop‐solution calorimetry in a molten oxide solvent. The white ceramic is 6 kJ/g‐atom more stable in enthalpy than the black one. Although the role of hydrogen in the thermodynamic stability of the white sample remains ambiguous, the thermodynamic findings and structural analysis suggest that the existence of sp2‐bonded carbon in the amorphous network of polymer derived Si–O–C ceramics does not provide additional thermodynamic stability to the ceramic.

Abstract

[1] We measured the density of iron-ringwoodite and its pressure and temperature dependence at conditions of the mantle transition zone using the laser-heated diamond anvil cell in conjunction with X-ray diffraction. Our new data combined with previous measurements constrain the thermoelastic properties of ringwoodite as a function of pressure and temperature throughout the transition zone. Our best fit Mie-Grüneisen-Debye equation of state parameters for Fe end-member ringwoodite are V 0 = 42.03 cm 3 /mol, K 0 = 202 (4) GPa, K′ = 4, g 0 = 1.08 (6), q = 2, and D = 685 K. This new equation of state revises calculated densities of the Fe end-member at transition zone conditions upwards by ∼0.6% compared with previous formulations. We combine our data with equation of state parameters across the Mg-Fe compositional range to quantify the effect of iron and temperature on the density and bulk sound velocity of ringwoodite at pressure and temperature conditions of the Earth's transition zone. The results show that variations in iron content and temperature have opposing effects on density and bulk sound velocity, suggesting that compositional (iron content) and temperature variations in the transition zone may be distinguished using seismic observables. Citation: Armentrout, M., and A. Kavner (2011), High pressure, high temperature equation of state for Fe 2 SiO 4 ringwoodite and implications for the Earth's transition zone, Geophys. Res. Lett., 38, L08309,