4,140 research outputs found
The emplacement of an obsidian dyke through thin ice : Hrafntinnuhryggur, Krafla Iceland.
No full textAn eruption along a 2.5 km-long rhyolitic dyke at Krafla volcano, northern Iceland during the last glacial period formed a ridge of obsidian (Hrafntinnuhryggur). The ridge rises up to 80 m above the surrounding land and is composed of a number of small-volume lava bodies with minor fragmental material. The total volume is <0.05 km3. The lava bodies are flow- or dome-like in morphology and many display columnar-jointed sides typical of magma-ice interaction, quench-fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows. The fragmental material compromises poorly-sorted perlitic quench hyaloclastites and poorly-exposed pumiceous tuffs. Lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered. At the southern end of the ridge the feeder dyke is exposed at an elevation ~95 m beneath the ridge crest and flares upwards into a lava body. Using the distribution of lithofacies, we interpret that the eruption melted through ice only 35-55 m thick, which is likely to have been dominated by firn. Hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice/firn. The eruption breached the ice, leading to subaerial but ice/firn-contact lava effusion, and only minor explosive activity occurred. The ridge appears to have been well-drained during the eruption, aided by the high permeability of the thin ice/firn, which appears not to have greatly affected the eruption mechanisms. We estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jökulhlaup (<70 m3s-1)
A Study of the Atmospherically Important Reactions between Dimethyl Selenide (DMSe) and Molecular Halogens (X2 = Cl2, Br2, and I2) with ab initio Calculations
No full textThe atmospherically relevant reactions between dimethyl selenide
(DMSe) and the molecular halogens (X2 = Cl2, Br2, and I2) have been studied with
ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry
optimization calculations showed that the reactions proceed from the reagents to
the products (CH3SeCH2X + HX) via three minima, a van der Waals adduct
(DMSe:X2), a covalently bound intermediate (DMSeX2), and a product-like
complex (CH3SeCH2X:HX). The computed potential energy surfaces are used to
predict what molecular species are likely to be observed in spectroscopic
experiments such as gas-phase photoelectron spectroscopy and infrared matrix
isolation spectroscopy. It is concluded that, for the reactions of DMSe with Cl2
and Br2, the covalent intermediate should be seen in spectroscopic experiments,
whereas, in the DMSe + I2 reaction, the van der Waals adduct DMSe:I2 should be
observed. Comparison is made with previous related calculations and experiments
on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results
to atmospheric chemistry is discussed. The DMSeX2 and DMSe:X2 intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion
A theoretical study of the gas-phase chemiionization reaction between uranium and oxygen atoms
No full textThe gas-phase chemiionization reaction between samarium and oxygen atoms: a theoretical study
No full textComputational study on cesium azide trapped in a cyclopeptidic tubular structure
No full textThe structures and the electronic properties of host-guest complexes formed by a cyclopeptidic tubular aggregate and the species CsN3, Cs2(N3)2, and Cs2N6 have been investigated by means of density functional theory. Taking advantage of the azide property to act as a bridge ligand between two or more metal cations, it may be possible to trap ions inside a confined space. This could be important for the preparation of polynitrogen molecules Nn. Results show that there are significant attractive interactions between the azide ion and the cavity walls, which make the ion stay inside the inner empty space of the cyclopeptidic aggregate. The confinement of the species Cs2(N3)2 forces the azide moieties to get closer together. Further, the Cs2N6 molecule shows a remarkable interaction with the tubular host, which may indicate a stabilization of N6
Unexpected structural diversity in alkali metal azide-crown ether complexes: syntheses, X-ray structures, and quantum-chemical calculations
No full textA series of alkali metal azide-crown ether complexes, [Li([12]crown-4)(N-3)], [Na([15]crown-5)(N-3)], [Na([15]crown-5)(H2O)(2)]N-3, [K([18]crown-6)(N-3)(H2O)], [Rb([18]crown-6)(N-3)(H2O)], [Cs([18]crown-6)(N-3)](2), and [Cs([18]crown-6)(N-3)(H2O)(MeOH)], has been synthesised. In most cases, single crystals were obtained, which allowed X-ray crystal structures to be derived. The structures obtained have been compared with molecular structures computed by density functional theory (DFT) calculations. This has allowed the effects of the crystal lattice on the structures to be investigated. Also, a study of the M-N-terminal metalazide bond length and charge densities on the metal (M) and terminal nitrogen centre (N-terminal) in these complexes has allowed the nature of the metal-azide bond to be probed in each case. The bonding in these complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre and minimising ligand-ligand repulsions. The structures of the crown ether complexes determined in this work show the subtle interplay of such factors. The significant role of hydrogen bonding is also demonstrated, most clearly in the structures of the K and Rb dimers, but also in the chain structure of the hydrated Cs complex
Synthesis, structures and DFT calculations on alkaline-earth metal azide-crown ether complexes
No full textThe first examples of azide complexes of calcium, strontium or barium with crown ethers have been prepared and fully characterised, notably [Ba([18]crown-6)(N3)2(MeOH)], [Sr([15]crown-5)(N3)2(H2O)], [Ca([15]crown-5)(N3)2(H2O)] and [Sr([15]crown-5)(N3)(NO3)]. Crystal structures reveal the presence of a variety of coordination modes for the azide groups including 1-, -1,3- and linkages via H-bonded water molecules, in addition to azide ions. The [Ba([18]crown-6)(N3)2(MeOH)]1/3 MeOH contains dinuclear cations with three -1,3-NNN bridges, the first example of this type in main group chemistry. The structures obtained have been compared with molecular structures computed by density functional theory (DFT). This has allowed the effects of the crystal lattice to be investigated. A study of the MNterminal metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (Nterminal) in these complexes has allowed the nature of the metal-azide bond to be investigated in each case. As in our earlier work on alkali metal azide-crown ether complexes, the bonding in the alkaline-earth complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre, and minimising ligand-ligand repulsions
A study of the atmospherically important reactions of dimethyl sulphide(DMS) with I2 and ICl using matrix isolation spectroscopy and electronic structure calculations
No full textThe reactions of dimethylsulfide (DMS) with molecular iodine (I2) and iodine monochloride (ICl) have been studied by infrared matrix isolation spectroscopy by co-condensation of the reagents in an inert gas matrix. Molecular adducts of DMS + I2 and DMS + ICl have also been prepared using standard synthetic methods. The vapour above each of these adducts trapped in an inert gas matrix gave the same infrared spectrum as that recorded for the corresponding co-condensation reaction. In each case, the infrared spectrum has been interpreted in terms of a van der Waals adduct, DMS : I2 and DMS : ICl, with the aid of infrared spectra computed for their minimum energy structures at the MP2 level. Computed relative energies of minima and transition states on the potential energy surfaces of these reactions were used to understand why they do not proceed further than the reactant complexes DMS : I2 and DMS : ICl. The main findings of this research are compared with results obtained earlier for the DMS + Cl2 and DMS + Br2 reactions, and the atmospheric implications of the conclusions are also considered
The characterisation of molecular alkali-metal azides
No full textMatrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkali-metal azides MN3 (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN3, RbN3 and CsN3 are present as stable high-temperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN3 in particular, by the detection of the isotopomers CS((NNN)-N-14-N-15-N-14) and Cs((NNN)-N-15-N-14-N-14). The IR spectra are assigned to a "side-on" (C-2v) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN3, RbN3 and CsN3 isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2cm(-1), respectively, and correspond to the N-3 asymmetric stretch. The N-3 bending mode in CsN3 is identified at 629 cm(-1). An additional feature routinely observed in these experiments occurred at approximately 2323 cm(-1) and is assigned to molecular N-2, perturbed by the close proximity of an alkali-metal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition
An ab initio and DFT study of the fragmentation and isomerisation of MeP(O)(OMe)(+)
No full textThe fragmentation behaviour of the ion MeP(O)OMe+ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH2O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)(2)(+) followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH2O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable
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