Methane clusters structure

Pulse electron-beam mass spectrometry was applied by Kebarle, Hiraoka, and co-workers766,772 to study the existence and structure of CH5+(CH4) cluster ions in the gas phase. These CH5+(CH4) clusters were previously observed by mass spectrometry by Field and Beggs.773 The enthalpy and free energy changes measured are compatible with the Cs symmetrical structure. Electron ionization mass spectrometry has been recently used by Jung and co-workers774 to explore ion-molecule reactions within ionized methane clusters. The most abundant CH5+(CH4) cluster is supposed to be the product of the intracluster ion-molecule reaction depicted in Eq. (3.120) involving the methane dimer ion 424. 

FIGURE 1.223 (a) Model of porous silica nanoparticle (diameter 3.8 nm, pore 1.3 nm in diameter) with different adsorbates (b) nitrogen water at the (c, d) initial surface and (e, f) with trimethylsilyl groups and (g) in a mixture with methane (d, f) cluster structure of adsorbed water (h) individual nanodomain of water of 4.5 nm in diameter (density 1.01 g/cm ) calculations were carried out by the PM6 method (MOPAC2009). 

Copper clusters containing two to four atoms have been formed (94) in argon and methane, whereas large, colloidal-copper particles resulted in dodecane matrices (94). The authors suggested that the "birth of the band structure of copper is clearly visible on passing from the dimer to the tetramer, with CU4 already possessing many of the features of the bulk metal (94). 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

Perrhenate and related building blocks are constituents of several cluster compounds where they act as terminal groups in organometallic rhenium oxides such as in [(cp Re)3(//2-0)3(/U3-0)3Re03]+ (49)21 Qj. jjj heterometallic clusters such as the structurally related [(Re)3(//f dppm)3(/u -0)3Re03]+ (dppm = bis(diphenylphosphino)methane) and Pt4 P(C6H 11)3)4 (//-C0)2(Re04)2]. A series of platinum-rhenium and platinum-rhenium-mercury clusters with Pt-Re multiple bonds has been isolated from reactions of Pt3 precursors with Rc207 or perrhenate. " ... 

Heavy-atom derivation of an object as large as a ribosomal particle requires the use of extremely dense and ultraheavy compounds. Examples of such compounds are a) tetrakis(acetoxy-mercuri)methane (TAMM) which was the key heavy atom derivative in the structure determination of nucleosomes and the membrane reaction center and b) an undecagold cluster in which the gold core has a diameter of 8.2 A (Fig. 14 and in and ). Several variations of this cluster, modified with different ligands, have been prepared The cluster compounds, in which all the moieties R (Fig. 14) are amine or alcohol, are soluble in the crystallization solution of SOS subunits from H. marismortui. Thus, they could be used for soaking. Crystallographic data (to 18 A resolution) show isomorphous unit cell constants with observable differences in the intensity (Fig. 15). 

From simulation studies [23] it arises that the isosteric heat of adsorption increases from zero coverage up to 0 2/3. This increase is mainly due to attractive interactions between neighboring methane. This attraction favours very much the adsorption of pair of methane molecules as small clusters (dimers). The structure of this quasi-one-dimensional phase is essentially determined by the local minima in the gas-solid potential. 

The synthetic methods used involve reaction of a cluster anion with [AuCIL], elimination of methane between a cluster hydride and [AuMeL] or addition of LAu+ units to metal-metal bonds. The emphasis here will be on structure and reactions of the complexes. Some examples of mixed gold clusters are given in Table 15, where it can be seen that most work has been on derivatives of clusters of iron, ruthenium and osmium. 

In these compounds, an alternative structure to the cubane cluster is found, for example in [Ph3PAgI]4.106 This is the chair or step type of arrangement, illustrated in Figure 2, in which two of the four halogen atoms retain a coordination number of three. This arrangement has also been found in some copper(I) halide complexes with bis(diphenylphosphino)methane.1O7 10S The pyramidal coordination of the halogen atom is similar to that in the cubane structure, although often more distorted. 

Reaction of the bis(diphenylphosphino)methane (dppm) complex PdPt-(yi-dppm)2Cl2 (132) with carbonylate anions affords several tri- and tetranuclear clusters. Reactions are outlined in Scheme 7. Treatment of 132 with 2 mol equivalents of [Fe(CO)3(NO)] (796,797), [Co(CO)4] (196,198), or [Mn(CO)5] (196,197) results in the tetranuclear spiked triangular clusters 133,134, or 135. Crystal structures on the dipalladium analogues of 134 and 135 have been reported (797,799). Grossel et al. (136) and Braunstein et al. (200) have independently reported that reaction of 132 with 1 mol equivalent of [Fe(CO)4]2 at ambient temperatures leads to an inseparable 1 1 mixture... 

It seems probable that other redox centres contain this binuclear iron structure, but that this has not yet been recognized. For example, a non-heme iron protein of the methane monooxygenase from Methylococcus capsulatus (Bath), which functions as the oxygenase in equation (28), has been described as having a novel iron centre which is not an iron-sulfur cluster. This may well be an oxo-bridged system. Analysis suggests 2.3 Fe per molecule of protein. 

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