Ionization potentials of alkanes

Fig. 5. Rate of H—D exchange versus ionization potential of alkanes and aromatic compounds 1 = methane 2 = ethane 3 = propane 4 = n-butane 5 = n-pentane 6 = n-hexane 7 = cyclopentane 8 = cyclohexane 9 = benzene 10 = naphthalene 11 = phenanthrene 12 = 2,2-dimethylbutane (see text) 13 = 1,1-dimethylpropy I benzene (see text) 14 = 2-methylpropane 15 = 2-methylbutane 16 = 2,2-dimethylpropane 17 = 2-methylpentane 18 = 3-methylpentane 19 = 2,3-dimethylbutane 20 = 2,2-dimethylbutane.

The process (4) seems to be improbable because it requires the existence of the hole centers with an electron aflSnity comparable with ionization potentials of alkane molecules. 

In this section, we present an overview of the photoabsorption cross section (o ) and the photoionization quantum yields (rh) for normal alkanes, C H2 +2 ( = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range, and of the number of carbon atoms in the alkane molecule, because normal alkanes are typical polyatomic molecules of chemical interest. In Fig. 5, the vertical ionization potentials of the valence electrons, which interact with the vacuum ultraviolet photons, in each of these alkane molecules are indicated to show how the outer- and inner-valence orbitals associated with carbon 2p and 2s orbitals, respectively, locate in energy [7]. 

The exchange of a considerable number of linear, branched-chain, and cyclic alkanes have been studied (5i). The exchange rate increases with increase in the carbon chain length forn-alkanes (methane to hexane). It is found that there is a linear correlation between the logarithm of the exchange rate and the ionization potential of the alkane (Fig. 5 n-alkanes are plotted as circles). This correlation extends to aromatic compounds (Fig. 5 aromatic compounds are plotted as squares) and is evidence that alkanes and aromatic compounds react by a common mechanism. Indeed, the least reactive aromatic, benzene, is only about... 

The best estimates have been obtained to date by using the MINDO and PNDO methods. In Tables 6 to 8 we show the ionization potential values obtained by each of these methods for alkanes and cycloalkanes, alkenes, acetylenes and aromatic compounds. Dewar and Klopman (PNDO) and Dewar et al. (MINDO/2) also compared their calculated inner orbital energies with experimental ionization potentials obtained from photoionization spectra. The ionization potentials of methane and ethane have also been calculated by the PNDO method along the more sophisticated procedure of minimizing separately the energy of the ion and that of the molecule. In these cases, the experimental value of the first ionization potential was reproduced accurately 48>. 

Some of the first applications of molecular-orbital theory in mass spectrometry were in calculations of ionization potentials of n-alkanes (see Streitwieser, 1961, for leading references). Strictly, these ionization potentials are a property of both the molecule and the ion produced, but often the effect of electron correlation in the ion is ignored (Koopmans, 1933) and resort is then made to adjustment of parameters to give good agreement between theory and practice. In the absence of experimental confirmation, such calculations must be viewed cautiously. 

Many bicycloalkanes are as unreactive as alkanes or unstrained cycloalkanes. They have very high values and their low electron affinities make them poor electron acceptors. The ionization potentials of unstrained bicycloalkanes are somewhat lower than those of unstrained cycloalkanes—bicyclo[2.2.2]octane IP 9.47 eV), bicyclo[4.3.0]nonane [IP 9.46 eV), or bicyclo[4.4.0]decane [IP 9.32 eV) lie below the 9.75-9.90 eV range characteristic of cycloalkanes [5]. As a result, they are somewhat easier to oxidize in solution or in solid matrices. Strained bicycloalkanes have even lower ionization potentials (bicyclo] 1.1. Ojbutane, IP 8.7 eV) the ring strain... 

The correlation of k or Egct with bond strengths are the most commonly used for estimation purposes but correlations with other properties of the molecules are possible. For example, it is possible to correlate the rate constant for OH + alkane reactions with the ionization potential of the alkane [10] and, for other types of reaction, it has been suggested that both electron affinities and ionization potentials of the species involved may correlate with the kinetics parameters [51]. However, because of the great sensitivity of the rate constant to small errors in Eact, correlations aimed at predicting Eact are very rarely used. 

Some properties do depend heavily on atom count but others depend not so much on the structure of the overall molecule as on the specific bonding in a local skeletal area or even on a single atom. Ionization potential of monosub-stituted alkanes is one example of such structure-property dependence. The Woodward rules for estimation of ultraviolet absorption in organic molecules is another example of such a structure-property scheme. ... 

The relative rate constants for linear alkanes C5-C9 correlate with the ionization potentials of these hydrocarbons. 

By the introduction of a molecule into the mass spectrometer it will be charged. In most cases we study the positive ions, which can be produced by various ionization methods. The El method is most commonly used for the analysis of alkanes. When the energy of the electron beam is greater than the ionization potential of the substrate, the following processes may occur ... 

Acetonitrile has a very anodic decomposition potential. Exchange of the perchlorate anion against tetrafluoroborate or hexafluorophosphate shifts the anodic decomposition potential (ImAcm" ) of acetonitrile from 2.48 to 2.91 or 3.02 V vs Ag/Ag, respectively. In acetonitrile/TEABF4 some half-wave potentials have been determined to be 3.0 V for i-pentane, 3.01 V for 2-methylpentane, 3.28 V for 2,2-dimethyl-butane and >3.4V for n-octane, n-heptane and n-hexane. The half-wave potentials of some alkanes have also been obtained by the use of ultramicroelectrodes in acetonitrile without any supporting electrolyte the values range from 3.87 V for methane to 3.5 V for n-heptane vs Ag/Ag. These values correlate well with the ionization potentials of the alkanes". Controlled potential electrolyses were carried out on saturated solutions of four alkanes in acetonitrile 0.1 M TBABF4 to yield acetamidoalkanes (Table 5). ... 

Figure 10.5 Effect of the number n of carbons, (a) On the ionization potential of C2-C4 hydrocarbons, = indicates olefin, (b) In the case of ODH, on the differenee of ionization potential AI between reactant and product indicates ODH of C alkane (e.g., C2 for ethane to ethene)...

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