Hydrocarbons, fragmentation

The above expressions are empirical approaches, with m and D. as parameters, for including an anliamionic correction in the RRKM rate constant. The utility of these equations is that they provide an analytic fomi for the anliamionic correction. Clearly, other analytic fomis are possible and may be more appropriate. For example, classical sums of states for Fl-C-C, F1-C=C, and F1-C=C hydrocarbon fragments with Morse stretching and bend-stretch coupling anhamionicity [M ] are fit accurately by the exponential... 

Figure Bl.25.2 shows the XPS spectra of two organoplatinum complexes which contain different amounts of chlorine. The spectrum shows the peaks of all elements expected from the compounds, the Pt 4f and 4d doublets (the 4f doublet is iimesolved due to the low energy resolution employed for broad energy range scans). Cl 2p and Cl 2s, N Is and C Is. Flowever, the C Is caimot be taken as characteristic for the complex only. All surfaces that have not been cleaned by sputtermg or oxidation in the XPS spectrometer contain carbon. The reason is that adsorbed hydrocarbons from the atmosphere give the optimum lowering of the surface free energy and hence, all surfaces are covered by hydrocarbon fragments [9].

Figure Bl.25.2. XPS scans between 0 and 450 eV of two organoplatiniiin complexes showing peaks due to Pt, Cl, N and C. The C Is signal represents not only carbon in the compound but also contaminant hydrocarbon fragments, as in any sample. The abbreviation Me in the structures stands for CH (courtesy of J C Muijsers, Eindlroven).

The Diels-Alder reactants as shown in Scheme 1.1 can consist of only hydrocarbon fragments (homo-Diels-Alder reaction) but can also contain one or more heteroatoms on any of the positions... 

Prompt NO Hydrocarbon fragments (such as C, CH, CH9) may react with atmospheric nitrogen under fuel-rich conditions to yield fixed nitrogen species such as NH, HCN, H9CN, and CN. These, in turn, can be oxidized to NO in the lean zone of the flame. In most flames, especially those from nitrogen-containing fuels, the prompt... 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Because mass-spectral fragmentation patterns are usually complex, it s often difficult to assign structures to fragment ions. Most hydrocarbons fragment in many ways, as the mass spectrum of hexane shown in Figure 12.4 demonstrates. The hexane spectrum shows a moderately abundant molecular ion at m/z = 86... 

Linear relations between the activation energies and heats of adsorption or heats of reaction have long been assumed to be valid. Such relations are called Bronsted-Evans-Polanyi relations [N. Bronsted, Chem. Rev. 5 (1928) 231 M.G. Evans and M. Polanyi, Trans. Faraday Soc. 34 (1938) 11]. In catalysis such relations have recently been found to hold for the dissociation reactions summarized in Pig. 6.42, and also for a number of reactions involving small hydrocarbon fragments such as the hydro-... 

Effects of Sulfur Coverage. The kinetic results reported in the previous section are from reactions performed on surfaces that are initially clean. The surfaces after reaction were examined by AES and shown to be covered with carbon and sulfur at coverages close to a monolayer. It is interesting to note that although this is the case even after reaction times on the order of minutes the reaction rate is constant for a period of approximately one hour. Either the reaction is occurring on top of this oarbon/sulfur layer or these species are present as sulfur containing hydrocarbon fragments that are intermediates in the desulfurization process. 

The mechanism for a non-surface-mediated hydrogen-tranfer from one hydrocarbon fragment to another cannot be identified in ambiguously from the current data alone. Possible reaction pathways include ... 

With catalysts such as nickel and rhodium for which it has been shown that 1-2 hydrogenolysis is seriously competitive with 1-3 hydrogenolysis, there is no need to assume that ir-olefin/allyl hydrogenolysis occurs (but neither can it with certainty be excluded). This conclusion is likely to be true for other catalysts such as cobalt and iron which also favor complete hydrocarbon fragmentation to methane. 

Thus, the hydrocarbon fragment blocks out the zinc half of the active site, and the ZnH band is not observed. 

Five bands are observed in the region near 3000 cm-1 corresponding to C—H stretching vibrations. (The band at 2947 cm-1, which appears as a shoulder in Fig. 15, is seen as a separate peak when the spectrum is observed on an expanded transmission scale.) Detailed identification of these bands will be deferred, but the weak band at 3055 cm-1 suggests that the hydrocarbon fragment is olefinic. 

It is clear that a complex hydrocarbon polymer chemistry must exist in the atmosphere of Titan involving polyyne species, polynitrile species and mixtures of the two, and additional routes to polyaromatic hydrocarbon formation. This presents a significant problem for the gas chromatography/mass spectrometry instruments on the Huygens probe. There should be hydrocarbon fragments, producing perhaps... 

It is apparent that in any hydrocarbon oxidation process CO is the primary product and forms in substantial amounts. However, substantial experimental evidence indicates that the oxidation of CO to C02 comes late in the reaction scheme [13]. The conversion to C02 is retarded until all the original fuel and intermediate hydrocarbon fragments have been consumed [4, 13]. When these species have disappeared, the hydroxyl concentration rises to high levels and converts CO to C02. Further examination of Fig. 3.6 reveals that the rate of reaction (3.44) does not begin to rise appreciably until the reaction reaches temperatures above 1100K. Thus, in practical hydrocarbon combustion systems whose temperatures are of the order of 1100K and below, the complete conversion of CO to C02 may not take place. 

The kinetic details for prompt NO formation must begin with the possible reactions between N2 and hydrocarbon fragments, as Fenimore [9] originally suggested. Hayhurst and Vance [17] suggest that two other likely candidate reactions may be added to those posited by Fenimore. The four candidate reactions would then be... 

For Example 30 Zidan et al. (102) postulated that NO oxidizes a surface site and produces atomic N propylene then reduces the site, and in a subsequent fast reaction the resulting adsorbed hydrocarbon fragment reacts with the adsorbed atomic N to produce acrylonitrile. According to the orders given, either Step 1 or 6 for NO, that is, NO oxidation of the surface, should be the rate-determining step. Since several questions are involved—which surface species there are, what the entropy of surface atomic N would be,... 

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