Methane cluster ions

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. 

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. 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

Photodissociation of the transition-metal cluster ion VFe provides an indirect measure of the absorption spectrum of the lon. Matrix photolysis of M(Tj -CgH,)2(H)CO and M(n -C,H,)2H2 (M-Nb, Ta) yields the monohydrides M(ti -C5Hs)H, uld irradiation of (ti -C.Hs)2V(CH,)2 results in formation of methane and ethame. The amount of ethaine formed is dependent on the solvent, concentration of substrate, time aind temperature. ... 

In the middle and upper stratosphere, where increasing water vapor mixing ratios are observed, the formation of water vapor results from methane oxidation via the mechanisms discussed in Section 5.3. In the mesosphere, the presence of water vapor leads to the formation of hydrated cluster ions, such as H+(H20)n, which have been commonly observed in the D-region (see Chapter 7). H2O photolyzes in the thermosphere and upper mesosphere by absorption of shortwave... 

It is noteworthy that while ground state rhodium atoms insert into methane molecules [9a], there is no evidence of chemical reaction for the ground state of rhenium with CH4 up to a temperature of 548 K [9b], Reactions of metal cluster ions, for example, MgFe [10a], Pt [10b], Nb ", and Rh [10c] with saturated and aromatic hydrocarbons in the gas phase have been recently described. 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

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. 

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)". ... 

In the thermal ion/molecule reactions of the singlet platinum methylidene clusters [Pt (CH)] ( = 1, 2) with NHg, the dominant path corresponds to proton transfer to generate [NH ] [11]. In addition, for the mononuclear precursor, the couple [Pt(CH)] /NH3 gives rise to the formation of [CH2NH2] concomitant with the loss of atomic platinum clearly, transfer to and insertion of the electrophilic CH unit in a N-H bond of ammonia provides the methane iminium ion [CH2NH2]. ... 

Figure 2.17 Comparison of the isotopic clusters of the molecular ion for (a) methane, (b) ethane, (c) chloromethane and (d) decane...

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