Characterisation of the surface chemistry of magnesium exposed to the ambient atmosphere

The changes in the surface chemistry of the oxidised surface of evaporated magnesium metal stored in the ambient atmosphere are studied with water contact angle (WCA) measurement, polarisation‐modulation infrared reflection‐absorption spectroscopy (PM‐IRRAS) and X‐ray photoelectron spectroscopy (XPS). Upon exposure to the ambient atmosphere, the surface picks up volatile organic compounds (VOC), which cause a significant increase in the WCA values. The PM‐IRRAS and XPS analyses indicate that the adsorbates contain hydrocarbons, carboxylates and carbonate functionalities. After long ambient storage times, the composition of the carbon‐containing functionalities on the surface changes significantly. This change could be caused by the build‐up and/or surface‐catalysed oxidation of adsorbed organic species. Thickening of the air‐formed oxide/hydroxide layer was also noted, ascribed to the reaction of adsorbed atmospheric moisture with the magnesium surface. Copyright © 2006 John Wiley & Sons, Ltd.     

A model for combinatorial organic chemistry

The set of coset representations, CR's, of a group G, [G(/G1), G(/G2), ..., G(/Gs)], where G1 = [I], Gs = G, the marks, m(ij) of subgroup Gj on a given G(/Gi), 1 < or = i < or = s, and the subduction of G(/Gi) by Gj, j < or = i, G(/Gi) precipitates to Gj, are essential tools for the enumeration of stereoisomers and their classification according to their subgroup symmetry (Fujita, S. Symmetry and Combinatorial Enumeration in Chemistry; Springer-Verlag: Berlin, 1991). In this paper, each G(/Gi) is modeled by a set of colored equivalent configurations (called homomers), H = h1, h2, ..., hr, r = G/Gi, such that a given homomer, h(k), remains invariant only under all g epsilon Gi, where g is an element of symmetry. The resulting homomers generate the corresponding set of marks almost by inspection. The symmetry relations among a set H can be conveniently stored in a Cayley-like diagram (Chartrand, G. Graphs as Mathematical Models; Prindle, Weber and Schmidt Incorporated: Boston, MA, 1977; Chapter 10), which is a complete digraph on r vertices so that an arc from vertex v(i) to vertex v(j) is colored with the set Sij of symmetry elements such that h(i)[g(if)]-->h(j),g(ij) epsilon Sij. In addition, each vertex, v(i), is associated with a loop that is colored with a set Sii so that g(ii) epsilon Sii stabilizes h(i). A Cayley-like diagram of a given CR, G[G(/Gi)], leads to graphical generation of G(/Gi) precipitates to Gj for all values of j and also to all m(ij)'s. Several group-theoretical results are rederived and/or became more envisagable through this modeling. The approach is examplified using C2, C3, D2, T, and D3 point groups and is applied to trishomocubane, a molecule that belongs to the D3 point group.     

A new bicomponent catalyst for NOx reduction in oxygen-rich atmosphere

The NO_x catalytic reduction in oxygen-rich atmosphere using propene as a reductant is studied. A Mn-perovskite-type catalyst containing rare earth in A site and copper in B site substitution is synthesized. Then, it is supported as clusters on a silicon carbide ceramic sponge, where alumina has previously been deposited. This catalyst is tested for NO_x reduction reaction and the influence of gas space velocity, oxygen and propene concentrations is investigated. NO_x conversion may reach 75 % under certain conditions. The activity of such a catalyst is explained by the formation of carbon on the surface of the alumina and its interaction with the perovskite.     

Recent advances in the chemistry of magnesium carbenoids

This tutorial review deals with recent advances in the chemistry and synthetic use of magnesium carbenoids. The reactivity of traditional carbenoids (alpha-haloalkyllithium species) was successfully reduced by using magnesium as the metal instead of lithium. Properties of these relatively stable carbenoids, magnesium carbenoids, were widely investigated and it was found that the magnesium carbenoids have very interesting reactivity toward several nucleophiles. The magnesium carbenoids, magnesium cyclopropylidenes, magnesium alkylidene carbenoids, and magnesium beta-oxido carbenoids are generated from alpha-chloroalkyl (or alpha-chloroalkenyl) aryl sulfoxides with a Grignard reagent at low temperature by sulfoxide-magnesium exchange reaction. The stability of the generated magnesium carbenoids and several new reactions based on the electrophilicity of the magnesium carbenoids, including 1,3-CH insertion, are reviewed. Magnesium carbenoids open up the new world of the chemistry of carbenoids.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

A new photocleavable linker in solid-phase chemistry for ether cleavage

We have designed a new linker (1) for the solid-phase synthesis that cleaves ether bonds photolytically. The linker was prepared in nine steps and anchored to the support via an amide bond. Photocleavage is a two-step process in which the immobilized alcohols are released by photolytic generation of a radical that undergoes a spontaneous beta-bond scission. The pivaloyl linker (1) was found to cleave off alcohols in high yields and purities. Only traces of acid (pH approximately 5.5) are necessary for an efficient cleavage.     

A restricted-open-shell complete-basis-set model chemistry

A restricted-open-shell model chemistry based on the complete basis set-quadratic Becke3 (CBS-QB3) model is formulated and denoted ROCBS-QB3. As the name implies, this method uses spin-restricted wave functions, both for the direct calculations of the various components of the electronic energy and for extrapolating the correlation energy to the complete-basis-set limit. These modifications eliminate the need for empirical corrections that are incorporated in standard CBS-QB3 to compensate for spin contamination when spin-unrestricted wave functions are used. We employ an initial test set of 19 severely spin-contaminated species including doublet radicals and both singlet and triplet biradicals. The mean absolute deviation (MAD) from experiment for the new ROCBS-QB3 model (3.6+/-1.5 kJ mol(-1)) is slightly smaller than that of the standard unrestricted CBS-QB3 version (4.8+/-1.5 kJ mol(-1)) and substantially smaller than the MAD for the unrestricted CBS-QB3 before inclusion of the spin correction (16.1+/-1.5 kJ mol(-1)). However, when applied to calculate the heats of formation at 298 K for the moderately spin-contaminated radicals in the G2/97 test set, ROCBS-QB3 does not perform quite as well as the standard unrestricted CBS-QB3, with a MAD from experiment of 3.8+/-1.6 kJ mol(-1) (compared with 2.9+/-1.6 kJ mol(-1) for standard CBS-QB3). ROCBS-QB3 performs marginally better than standard CBS-QB3 for the G2/97 set of ionization energies with a MAD of 4.1+/-0.1 kJ mol(-1) (compared with 4.4+/-0.1 kJ mol(-1)) and electron affinities with a MAD of 3.9+/-0.2 kJ mol(-1) (compared with 4.3+/-0.2 kJ mol(-1)), but the differences in MAD values are comparable to the experimental uncertainties. Our overall conclusion is that ROCBS-QB3 eliminates the spin correction in standard CBS-QB3 with no loss in accuracy.     

A kinetic study of Ca-containing ions reacting with O, O2, CO2 and H2O: implications for calcium ion chemistry in the upper atmosphere

A series of gas-phase reactions involving molecular Ca-containing ions was studied by the pulsed laser ablation of a calcite target to produce Ca(+) in a fast flow of He, followed by the addition of reagents downstream and detection of ions by quadrupole mass spectrometry. Most of the reactions that were studied are important for describing the chemistry of meteor-ablated calcium in the earth's upper atmosphere. The following rate coefficients were measured: k(CaO(+) + O --> Ca(+) + O(2)) = (4.2 +/- 2.8) x 10(-11) at 197 K and (6.3 +/- 3.0) x 10(-11) at 294 K; k(CaO(+) + CO --> Ca(+) + CO(2), 294 K) = (2.8 +/- 1.5) x 10(-10); k(Ca(+).CO(2) + O(2) --> CaO(2)(+) + CO(2), 294 K) = (1.2 +/- 0.5) x10(-10); k(Ca(+).CO(2) + H(2)O --> Ca(+).H(2)O + CO(2)) = (13.0 +/- 4.0) x 10(-10); and k(Ca(+).H(2)O + O(2) --> CaO(2)(+) + H(2)O, 294 K) = (4.0 +/- 2.5) x 10(-10) cm(3) molecule(-1) s(-1). The quoted uncertainties are a combination of the 1sigma standard errors in the kinetic data and the systematic errors in the models used to extract the rate coefficients. Rate coefficients were also obtained for the following recombination (also termed association) reactions in He bath gas: k(Ca(+).CO(2) + CO(2) --> Ca(+).(CO(2))(2), 294 K) = (2.6 +/- 1.0) x 10(-29); k(Ca(+).H(2)O + H(2)O --> Ca(+).(H(2)O)(2)) = (1.6 +/- 1.1) x 10(-27); and k(CaO(2)(+) + O(2) --> CaO(2)(+).O(2)) < 1 x 10(-31) cm(6) molecule(-2) s(-1). These recombination rate coefficients, as well as those for the ligand-switching reactions listed above, were then interpreted using a combination of high level quantum chemistry calculations and RRKM theory using an inverse Laplace transform solution of the master equation. The surprisingly slow reaction between CaO(+) and O was explained using quantum chemistry calculations on the lowest (2)A', (2)A' and (4)A' potential energy surfaces. These calculations indicate that reaction mostly occurs on the (2)A' surface, leading to production of Ca(+)((2)S) + O(2)((1)Delta(g)). The importance of this reaction for controlling the lifetime of Ca(+) in the upper mesosphere and lower thermosphere is then discussed.     

The photodissociation of vibrationally excited ozone in the upper atmosphere

The photodissociation of vibrationally excited O₃ in the sunlit mesosphere is investigated. Dissociation rate coefficients for specific vibrationally excited states of O₃ are calculated for the conditions of an overhead sun. Possible vibrational enhancements of the O(¹D) and O₂(a¹Δ_g) production rates are assessed. It is shown that such enhancements should make only minor contributions to the daytime production of these species in the mesosphere and lower thermosphere.     

A graph-based toy model of chemistry

Large scale chemical reaction networks are a ubiquitous phenomenon, from the metabolism of living cells to processes in planetary atmospheres and chemical technology. At least some of these networks exhibit distinctive global features such as the "small world" behavior. The systematic study of such properties, however, suffers from substantial sampling biases in the few networks that are known in detail. A computational model for generating them is therefore required. Here we present a Toy Model that provides a consistent framework in which generic properties of extensive chemical reaction networks can be explored in detail and that at the same time preserves the "look-and-feel" of chemistry: Molecules are represented as labeled graphs, i.e., by their structural formulas; their basic properties are derived by a caricature version of the Extended Hückel MO theory that operates directly on the graphs; chemical reaction mechanisms are implemented as graph rewriting rules acting on the structural formulas; reactivities and selectivities are modeled by a variant of the Frontier Molecular Orbital Theory based on the Extended Hückel scheme. The approach is illustrated for two types of reaction networks: Diels-Alder reactions and the formose reaction implicated in prebiotic sugar synthesis.     

A computational investigation of HCN2+ isomeric structures: implications for the chemistry of Titan's atmosphere.

The structure and stability of various HCN2+ isomeric structures have been investigated at the complete active space SCF (CASSCF) and multireference-configuration interaction [MR-Cl-SD(Q)] levels of theory with the 6-31G(d) and 6-311G(d,p) basis sets. The investigated species include the singlet (S) and triplet (T) open-chain H-N-C-N+ ions 1S, 1S', and 1T, the open-chain H-C-N-N+ ions 2S, 2S', and 2T, the HC-N2+ cyclic structures 3S and 3T, and the HN-CN+ cyclic structures 4S and 4T. All these species have been identified as true energy minima on the CASSCF(8,7)/6-31G(d) potential energy surface, and their optimised geometries, refined at the CASSCF(8,8)/6-31G(d) level of theory, have been used to perform single point calculations at the [MR-Cl-SD(Q]/6-311G(d,p) computational level. The most stable structure was the H-N-C-N+ ion 1T, whose absolute enthalpy of formation at 298.15 K has been estimated as 333.9 +/- 2 kcalmol(-1) using the Gaussian-3 (G3) procedure. The two species closest in energy to 1T are the triplet H-C-N-N+ ion 2T and the singlet diazirinyl cation 3S, whose G3 enthalpies of formation at 298.15 K are 343.5 +/- 2 and 340.6 +/- 2 kcalmol(-1), respectively. Finally, we have discussed the implications of our calculations for the detailed structure of the HCN2+ ions formed in the reaction between N3+ and HCN, experimentally observed by flowing after-glow-selected ion flow/drift tube mass spectrometry and possibly occurring in Titan's atmosphere.     

Advances in organoelement chemistry for the development of new materials

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A new model for the molecular conformation in polyoxymethylene crystals

Nonintegral indices are found for the extra meridional reflections in the x-ray diffraction of drawn polyoxymethylene. The index of one of the extra reflections is estimated at 00l: l = 2.830 ± 0.003. A new model for the molecular conformation which accounts for the nonintegral indices is proposed. The new model is a helix with defects at constant intervals along the helix axis. The defect is an unwinding of the helix by 20° around the helix axis. This defect is believed to be characteristic of crystals of helical polymers. The interval between the defects is estimated as 18 monomers. The crystal structure with these defects is consistent with precise measurements of the layer line positions.     

A computer model of struvite solution chemistry

A computer model is developed to describe struvite solution chemistry; this includes the electroneutrality equation and allows greater variability in the input components. Relationships between the major ions are retained without approximation. The model results fit data derived both in our laboratory and from the literature. Equilibrium constants which markedly affect the output are identified and solubility constants are derived. Applications of the model include studies of waste-water treatment and of the formation of kidney stones.     

A triazene-based new photolabile linker in solid phase chemistry

An extension of the T2 linker methodology by showing its applicability as a photocleavable linker is reported. Photocleavage was carried out with 355 nm UV laser irradiation (3ω Nd-YAG) in methanol/diethyl ether and is suitable for protected amino acid derivatives, as well as simple small organic molecules including resin-bound biotin. The linker is stable under a wide range of conditions with the exception of strongly acidic media.