Methane clusters

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. 

Adsorption position Adsorption energy (kcal/mol) AQ (methane —> cluster) Activation energy (kcal/mol) Vc-Hdis (cm 1)... 

The gas emitted from coal wall inrush into topcarving area continuously. So the process can be seen as a process of diffusion of gas and air. For the low air-speed, gas can not be driven out from top-carving area, which methane cluster formed. Finally, the distribution and concentration of gas and air is interaction of field of air and gas concentration. For top-carving area is place gas accumulated with high concentration, which the existence of gas changed the physical mechanical property of air flow, the concentration of mixed air, kinematic viscosity and flow-speed should be the weighted mean of two kind. 

FIGURE 3.41 Clathrates (a) 5 /CH4 and (b) 5 6 /CH4, water/methane clusters (c) free or bound in (d) carbon or polymeric nanopores AC. (Taken from Appl. Surf. Sci., 255, Turov, V.V., Turova, A.A., Goncharuk, E.V., and Gim ko, V.M., Adsorption of methane with the presence of water on oxide, polymer, carbon adsorbents studied using NMR spectroscopy at low temperatures, 3310-3317,2008b. Copyright 2008, with permission from Elsevier.)... 

This process is observed, for example, in the use of methane when the C2H5 ion (m/z 29) contained in the methane cluster abstracts hydride ions from alkyl chains. 

Frequently, an (M -F R)" ion is not immediately recognized, but can give information which is as valuable as that from the quasimolecular ion formed by protonation (see Methane). Cluster ions of this type, nevertheless sometimes, make interpretation of spectra more difficult, particularly when the transition complex does not lose immediately recognizable neutral species. 

The unique adduct ions (M -1- C2H5) = (M -1- 29) and (M -I- CjHj)" = (M -I- 41) formed by the methane cluster confirm the molecular mass interpretation. These adduct ions are easily seen as a mass difference of 28 resp. 40 from the protonated molecular ion (M+H)". ... 

Covalent. Formed by most of the non-metals and transition metals. This class includes such diverse compounds as methane, CH4 and iron carbonyl hydride, H2Fe(CO)4. In many compounds the hydrogen atoms act as bridges. Where there are more than one hydride sites there is often hydrogen exchange between the sites. Hydrogens may be inside metal clusters. 

Gas hydrates are an ice-like material which is constituted of methane molecules encaged in a cluster of water molecules and held together by hydrogen bonds. This material occurs in large underground deposits found beneath the ocean floor on continental margins and in places north of the arctic circle such as Siberia. It is estimated that gas hydrate deposits contain twice as much carbon as all other fossil fuels on earth. This source, if proven feasible for recovery, could be a future energy as well as chemical source for petrochemicals. 

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 the next section we derive the Taylor expansion of the coupled cluster cubic response function in its frequency arguments and the equations for the required expansions of the cluster amplitude and Lagrangian multiplier responses. For the experimentally important isotropic averages 7, 7i and yx we give explicit expressions for the A and higher-order coefficients in terms of the coefficients of the Taylor series. In Sec. 4 we present an application of the developed approach to the second hyperpolarizability of the methane molecule. We test the convergence of the hyperpolarizabilities with respect to the order of the expansion and investigate the sensitivity of the coefficients to basis sets and correlation treatment. The results are compared with dispersion coefficients derived by least square fits to experimental hyperpolarizability data or to pointwise calculated hyperpolarizabilities of other ab inito studies. 

If we compare results obtained with the same basis sets with the three coupled cluster models CCS, CC2 and CCSD, we find similar trends as observed in Refs. [22,45] The CCS model underestimates strongly the static hyperpolarizabilities and their dispersion. The results are usually of similar quality as those obtained with SCF. For methane, the CCS static hyperpolarizabilities are intermediate between the SCF and the CCSD values obtained in the same basis set. In Ref. [45] the CCS percentage dispersion contribution to the third harmonic generation (THG) hyperpolarizability of methane was found to be slightly smaller than for SCF, both underestimating significantly the dispersion obtained with the correlated coupled cluster models CC2 and CCSD. Accordingly the CCS dispersion coeflBcients listed in Table 3 are substantially smaller than the respective CCSD results obtained in the same basis sets. 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

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 ( ). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Methyl coenzyme M reductase plays a key role in the production of methane in archaea. It catalyzes the reduction of methyl-coenzyme M with coenzyme B to produce methane and the heterodisulfide (Figure 3.35). The enzyme is an a2P2Y2 hexamer, embedded between two molecules of the nickel-porphinoid F jg and the reaction sequence has been delineated (Ermler et al. 1997). The heterodisulfide is reduced to the sulfides HS-CoB and HS-CoM by a reductase that has been characterized in Methanosarcina thermoph-ila, and involves low-potential hemes, [Fe4S4] clusters, and a membrane-bound metha-nophenazine that contains an isoprenoid chain linked by an ether bond to phenazine (Murakami et al. 2001). 

McAnulla C, CA Woodall, IR McDonald, A Studer, S Vuilleumier, T Leisinger, JC Murrell (2001a) Chloro-methane utilization gene cluster from Hyphomicrobium chloromethanicum strain CM2 and development of functional gene prohes to detect halomethane degrading bacteria. Appl Environ Microbiol 67 307-316. 

It has been established that methane is produced on rice roots by reduction of CO2. This was examined in rice roots using a combination of 16S rRNA sequencing and density gradient fractionation of C-labeled DNA after incubation with C02. The major groups of archaea detected were Methanosarcinaceae that decreased with time to be replaced by the hitherto uncultured Rice Cluster I, although the former subsequently dominated (Lu et al. 2005). 

Multidentate thioethers are readily introduced into the coordination sphere of low-valent cobalt. As an example, the tridentate thioethers 1,3,5-trithiacyclohexane and tris(methylthio)methane both replace three facially arranged carbonyl ligands in Co3(CO)9(/i-CPh), leaving the cluster otherwise intact.169... 

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