Electron Spin Resonance Study of Nitroxide Radical Adsorption at Cupric Ions in the Metal−Organic Framework Compound Cu₃(btc)₂

The Cu(II) pairs in the paddle-wheel building blocks of the metal−organic framework compound Cu₃(btc)₂ give rise to an antiferromagnetic spin state with an electron spin resonance (ESR)-silent <i>S</i> = 0 ground state. However, the adsorption of di-<i>tert</i>-butyl nitroxide (DTBN) radicals leads to the formation of an unusual nitroxide ESR spectrum and later, upon thermal treatment of the samples, to distinct paramagnetic Cu(II) centers, whose ESR signals can be observed at temperatures below 70 K. Various scenarios for the suppression of the antiferromagnetic coupling of the Cu(II) ions in the paddle-wheel units by interaction with the nitroxide and the subsequent formation of these <i>S</i> = 1/2 copper centers are discussed

X-Nuclei NMR Self-Diffusion Studies in Mesoporous Silica Foam and Microporous MOF CuBTC

A standard X-observe NMR probe was equipped with a z-gradient coil to enable high-sensitivity pulsed field gradient NMR diffusion studies of Li+ and Cs+ cations of aqueous salt solutions in a high-porosity mesocellular silica foam (MCF) and of CO2 adsorbed in metal-organic frameworks (MOF). The coil design and the necessary probe modifications, which yield pulsed field gradients of up to ±16.2Tm−1, are introduced. The system was calibrated at 2H resonance frequency and successfully applied for diffusion studies at 7Li, 23Na, 13C and 133Cs frequencies. Significant reductions of the diffusivities of the cations in LiClac and CsClac solution introduced into MCFs are observed. By comparison of the diffusion behavior with the bulk solutions, a tortuosity of the silica foam of 4.5 ± 0.6 was derived. Single component self-diffusion of CO2 and CH4 (measured by 1H NMR) as well as self-diffusion of the individual components in CO2/CH4 mixtures was studied in the MOF CuBTC. The experimental results confirm high mobilities of the adsorbed gases and trends for diffusion separation factors predicted by MD simulations

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework- 8 (ZIF-8) were performed by 1H and 13C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible framework

Hull of H.M.A.S. Australia with three workers beneath, Cockatoo Island Dock, Sydney, 4 June 1930 [picture].

Title devised from accompanying information where available.; Part of the: Fairfax archive of glass plate negatives.; Fairfax number: 6945.; Also available online at: http://nla.gov.au/nla.pic-vn6266612; Acquired from Fairfax Media, 2012