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The use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterization difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, [CuUO2(OH)4], is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidizing groundwater containing dissolved copper. Consequently, an attempt has been made to characterize two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates, and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Furthermore, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity. [ABSTRACT FROM AUTHOR]

Much effort has been devoted to the investigation of the reactivity of glycine, the smallest amino acid, in different environments in the interstellar medium (ISM). While the formation paths are expected to follow a gas-solid mechanism, the full picture of glycine survival in the ISM remains yet unrevealed. In this work, we have adopted density functional theory under periodic boundary conditions to simulate mechanisms for the decarboxylation of glycine on a water-rich surface and on a glycine ice. We have performed calculations at the PBE-D3/USPP level, from which several adsorption modes of glycine on each surface were investigated and decomposition mechanisms into CO2 and CH3NH2 on the different interfaces were suggested. Most favourable adsorption sites of glycine have adsorption energies of −106.54 and −98.52 kJ mol−1 on the water ice and glycine ice, respectively. Glycine decomposes into CO2 and CH3NH2 through a two-step mechanism on the water ice and four-step mechanism on the glycine surface, from which the barrier heights of the determinant steps were of 288.98 and 111.58 kJ mol−1, respectively. At temperatures of 50 K, decomposition of glycine into CO2 and CH3NH2 is an exergonic reaction, pointing to a thermodynamically controlled reaction in specific interstellar regions, such as hot-cores. Compared with reported glycine gas-phase reaction, much lower barrier heights for glycine formation were found in the surface models studied here. [ABSTRACT FROM AUTHOR]

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi 2 Te 3 . Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi 2 Te 3 . Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor. Water molecular motion on surfaces underpins a range of phenomena in nature. The authors resolve the nanoscale-nanosecond motion of water at a topological insulator's surface by helium spin-echo spectroscopy and computations, reporting hopping among sites and repulsion between water molecules. [ABSTRACT FROM AUTHOR]

The monolayer crystal structure of phenazine adsorbed on graphite is determined by a combination of synchrotron X-ray diffraction and DFT calculations. The molecules adopt a rectangular unit cell with lattice parametersa= 13.55 Å andb= 10.55 Å, which contains 2 molecules. The plane group of the unit cell isp2gg, and each molecule is essentially flat to the plane of the surface, with only a small amount of out-of-plane tilt. Density functional theory (DFT) calculations find a minimum energy structure with a unit cell which agrees within 7.5% with that deduced by diffraction. DFT including dispersion force corrections (DFT+D) calculations help to identify the nature of the intermolecular bonding. The overlayer interactions are principally van der Waals, with a smaller contribution from weak C-H···N hydrogen bonds. This behaviour is compared with that of 4,4′-bipyridyl. [ABSTRACT FROM AUTHOR]

Thiswork describes the combined use of synchrotron X-ray diffraction anddensity functional theory (DFT) calculations to understand the cocrystalformation or phase separation in 2D monolayers capable of halogenbonding. The solid monolayer structure of 1,4-diiodobenzene (DIB)has been determined by X-ray synchrotron diffraction. The mixing behaviorof DIB with 4,4′-bipyridyl (BPY) has also been studied andinterestingly is found to phase-separate rather than form a cocrystal,as observed in the bulk. DFT calculations are used to establish theunderlying origin of this interesting behavior. The DFT calculationsare demonstrated to agree well with the recently proposed monolayerstructure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene(DITFB) (the perfluorinated analogue of DIB), where halogen bondinghas also been identified by diffraction. Here we have calculated anestimate of the halogen bond strength by DFT calculations for theDITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol.Computationally, we find that the nonfluorinated DIB and BPY are notexpected to form a halogen-bonded cocrystal in a 2D layer; for thispair of species, phase separation of the components is calculatedto be lower energy, in good agreement with the diffraction results. [ABSTRACT FROM AUTHOR]

[Display omitted] The dehydrogenation of alkane feedstock to produce alkenes is a significant and energy intensive industrial process, generally occurring on metals and metal oxides. Here, we investigate a catalytic mechanism for the dehydrogenation of butane on single-layer, metal-free graphene using a combination of ab initio quantum chemical calculations and adsorption microcalorimetry. Dispersion-corrected Density Functional Theory (DFT) is employed to calculate transition states and energy minima that describe the reaction pathways connecting butane to the two possible products, but-1-ene and but-2-ene. The deprotonations occur with moderate energy barriers in the 0.54 eV-0.69 eV range. A strong agreement is observed between the results of the adsorption energies calculated by DFT (0.40 eV) and the measured differential heat of adsorption of n -butane on a graphitic overlayer. We conclude that the active-site for this catalytic reaction is a metal-free graphene vacancy, created by removing a carbon atom from a single-layer graphene sheet. [ABSTRACT FROM AUTHOR]