Methane ionization potential

W. A. Chupka and J. Berkowitz, Photoionization of methane ionization potential and proton affinity of CH4, J. Chem. Phys. 54, 4256-4259 (1971). 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

Energy Levels for Hole Injection. For the hole conductor TPD (6), measurements are available from different groups that allow a direct comparison of different experimental setups. The ionization potential that corresponds to the HOMO level under the assumptions mentioned above was measured by photoelectron spectroscopy to be 5.34 eV [230]. Anderson et al. [231] identified the onset of the photoelectron spectrum with the ionization potential and the first peak with the HOMO energy, and reported separate values of 5.38 and 5.73 eV, respectively. The cyclovoltammetric data reveal a first oxidation wave at 0.34 V vs. Fc/Fc+ in acetonitrile [232], and 0.48 V vs. Ag/0.01 Ag+ in dichloro-methane [102], respectively. The oxidation proceeds by two successive one-electron oxidations, the second one being located at 0.47 V vs. Fc/Fc+. 

Fig. 23. Adiabatic ionization potentials for methane, ethane, propane, and butane arranged as an energy level diagram.

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

Sander applied DFT (B3LYP) theory to carbenic philicity, computing the electron affinities (EA) and ionization potentials (IP) of the carbenes." " The EA tracks the carbene s electrophilicity (its ability to accept electron density), whereas the IP represents the carbene s nucleophilicity (its ability to donate electron density). This approach parallels the differential orbital energy treatment. Both EA and IP can be calculated for any carbene, so Sander was able to analyze the reactivity of super electrophilic carbenes such as difluorovinylidene (9)" which is sufficiently electrophilic to insert into the C—H bond of methane. It even reacts with the H—H bond of dihydrogen at temperamres as low as 40 K, Scheme 7.2) ... 

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... 

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 question of equivalence of bonds or lone pairs is relevant here. Methane has two valence shell ionization potentials. Clearly, having four equivalent bonds does not imply four equal-energy bond orbitals, that is, a single ionization potential. In the same vein, ammonia has three, water has four, and HF has three valence shell ionization potentials. 

I11 AN without supporting electrolyte, the positive end of the potential window reaches +6 V vs Ag/Ag+, and the electrolytic oxidations of substances with high ionization potentials, including methane, butane, pentane, heptane, rare gases (Ar, Kr, Xe) and oxygen, can be observed [69 a]. In SO2 at -70°C and without supporting electrolyte, Cs+, Rb+, K+ and Na+ were oxidized at a platinum UME (10 pm diameter)... 

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>. 

The slow rate of the methane reaction could then be ascribed to the exceptionally high ionization potential of the methyl radical (10.0 ev.) In turn, the high rate of the Si-H reaction might be explained by the... 

Methane, n-butane, isobutane, and cyclohexane have strong C—H bonds (I>c—h > 390 kJ mol and high oxidation potentials (IP > 9.8 ev), and vigorous conditions are required to oxidize these hydrocarbons with inorganic oxidants. On the other hand, toluene and o-, m-, and p-xylenes have weaker C—H bonds ( >c h 355 kJ mol ) and lower ionization potentials (IP < 8.8 eV), and can be oxidized more readily. 

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