Ionization potentials of hydrocarbons

Opeida [46] compared the values of the rate constants of peroxyl radical reactions with hydrocarbons with the BDE of the oxidized hydrocarbon, electron affinity of peroxyl radical, EA(R02 ) ionization potential of hydrocarbon (/Rn), and steric hindrance of a-substituent R(Fr). They had drawn out the following empirical equation ... 

The anions originate from the attachment of an electron to whatever electron acceptors are available in the system in bulk hydrocarbon monomer this results in the formation of radical anions. Because the electron affinities of alkenes are much lower than the ionization potentials of hydrocarbon radicals, the neutralization reaction between the cations and the anions, one possible version of which is... 

These energy values are calculated from thermochemical tables (11) and the ionization potentials of hydrocarbons obtained by Stevenson (15) using mass spectrometric methods. The union of an olefin and a proton from an acid catalyst leads to the formation of a positively charged radical, called a carbonium ion. The two shown above are sec-propyl and fer -butyl, respectively. [For addition to the other side of the double bond, A 298 = —151.5 and —146 kg.-cal. per mole, respectively. For comparison, reference is made to the older (4) values of Evans and Polanyi, which show differences of —7 and —21 kg.-cal. per mole between the resultant n- and s-propyl and iso-and tert-butyl ions, respectively, against —29.5 and —49 kg.-cal. per mole here. These energy differences control the carbonium ion isomerization reactions discussed below.]... 

Loosely bound aggregates (chemical effects) are formed with the hydrocarbons acting as electron donors (Lewis base) and the solvents acting as electron acceptors (Lewis acid). The hydrocarbon that forms the most stable complex with the solvent experiences a decrease in volatility. Electron donors are rated by ionization potential, and electron acceptors are rated by their electron affinities. The selectivity will be higher, the larger the difference in ionization potential between the hydrocarbons and the larger the electron affinity of the solvent (9). While data on ionization potentials of hydrocarbons can be found (15, 16), electron affinities data are rare because of difficulties in their experimental determination. Prausnitz and Anderson (8) recommend that the sigma scale, proposed by Hammett (17), be used to determine approximately the solvents relative ability to form complexes with the two hydrocarbons. Attempts by this author, however, to use this scale were not conclusive. Prausnitz and Anderson (8) should be consulted to understand better the physical and chemical effects. 

Hall correlated the ionization potentials of hydrocarbons, ethane to decane, assuming xmchanged localized orbitals and energies c, and y. ... 

Fig. 2. Relationship between the ionization potential of hydrocarbons and the peak wavenumber of the CT absorption and CT fluorescence bands in PMDA-hydrocarbons CT complexes in nonpolar solvents.

Due to the symmetry, observed in the classical L.C.A.O. molecular orbital calculations, of the h.o.m.o. (related to the ionization potential) and of the l.e.m.o. (related to the ease of acceptance of an electron in reduction) in certain types of compounds, e.g., conjugated hydrocarbons (complete symmetry when overlap is neglected, less complete when it is included), any property related, i.e., proportional, to one will be proportional to the other (this will not hold in more refined treatments). Thus, the theoretical r-ionization potentials of hydrocarbons are linearly proportional to the values for diphenyl, naphthalene, phenanthrene and anthracene the point for butadiene deviates slightly from the straig[ht line relation, while that for st5nrene deviates markedly. Such incidental correlations, due primarily to mathematical S3munetry rather than to fundamental physical significance, must be watched for carefully. The same remark is valid for a proposed correlation" between absorption spectra and half-wave potentials (cf. reference 21). 

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

Whether or not our representation of the non-ionic chain-carrier as an ester is correct, the balance between the ionic and non-ionic forms for the system styrene—perchloric acid—methylene dichloride seems to be very delicate. Since the enthalpy terms affecting this balance must be small, and the entropy terms are likely to be important, it is not possible at present to analyse the situation in detail. However it is predictable that the factors which would favour the ionic form, as against the ester, are lower ionization potential of the hydrocarbon radical, weaker ester bond, more polar solvent, and lower temperature. 

Among oxo-metals, osmium tetroxide is a particularly intriguing oxidant since it is known to oxidize various types of alkenes rapidly, but it nonetheless eschews the electron-rich aromatic hydrocarbons like benzene and naphthalene (Criegee et al., 1942 Schroder, 1980). Such selectivities do not obviously derive from differences in the donor properties of the hydrocarbons since the oxidation (ionization) potentials of arenes are actually less than those of alkenes. The similarity in the electronic interactions of arenes and alkenes towards osmium tetroxide relates to the series of electron donor-acceptor (EDA) complexes formed with both types of hydrocarbons (26). Common to both arenes and alkenes is the immediate appearance of similar colours that are diagnostic of charge-transfer absorp-... 

The second ionization potentials (e.v.) of aliphatic amines compared with first ionization potentials of the isoelectronic hydrocarbons... 

Honig, R.E. Ionization Potentials of Some Hydrocarbon Series. J. Chem. Phys. 1948, 16, 105-112. 

Krishna, B. and Gupta. co-Type calculations on n-electron systems with incltrsion of overlap charges. I. Ionization potential of some alternant hydrocarbons, J. Am. Chem. Soc., 92(25) 7247-7248, 1970. 

Ru" (0)(N40)]"+ oxidizes a variety of organic substrates such as alcohols, alkenes, THE, and saturated hydrocarbons. " In all cases [Ru (0)(N40)] " is reduced to [Ru (N40)(0H2)] ". The C— H deuterium isotope effects for the oxidation of cyclohexane, tetrahydrofuran, 2-propanol, and benzyl alcohol are 5.3, 6.0, 5.3, and 5.9 respectively, indicating the importance of C— H cleavage in the transitions state. For the oxidation of alcohols, a linear correlation is observed between log(rate constant) and the ionization potential of the alcohols. [Ru (0)(N40)] is also able to function as an electrocatalyst for the oxidation of alcohols. Using rotating disk voltammetry, the rate constant for the oxidation of benzyl alcohol by [Ru (0)(N40)] is found to be The Ru electrocatalyst remains active when immobilized inside Nafion films. 

Obenland and Schmidt120) measured the PES of benzene, naphthalene, phen-anthrene and all other orthoannelated hydrocarbons up to [14]. In Table 19 the first two bands of the vertical n-ionization potential of helicenes are given. 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

For the nitration of aromatic hydrocarbons with acetylnitrate, there is a clearly linear correlation between the ionization potentials of these hydrocarbons and the rate constants relative to benzene (Pedersen and others 1973). Table 4-4 juxtaposes spin densities of the cation radicals and the partial rate factors of ring attacks in the case of the nitration of isomeric xylenes by means of the (nitric acid-acetic anhydride) mixture. 

For compounds composed solely of carbon and hydrogen atoms, AU is primarily the energy required to overcome the induced dipole-induced dipole, or dispersion interactions. The polarizability and ionization potential of the molecules determines the magnitude of dispersion interactions, and therefore of AHm for hydrocarbons. 

This expression is generally used as such with or without neglect of the second term and using as Ip the negative of the valence-state ionization potential of atom P. Although such a simplification may be of little importance in hydrocarbons, where it seems to influence merely the absolute values of the molecular ionization potentials and where the atomic -electron populations are close to unity, this is not the case for heteromolecules. Here, an empirical choice of Up values in place of the first two terms in Eq. (24) is better.84... 

The [Ruv(N40)(0)]2+ complex is shown to oxidize a variety of organic substrates such as alcohols, alkenes, THF, and saturated hydrocarbons, which follows a second-order kinetics with rate = MRu(V)][substrate] (142). The oxidation reaction is accompanied by a concomitant reduction of [Ruv(N40)(0)]2+ to [RuIII(N40)(0H2)]2+. The mechanism of C—H bond oxidation by this Ru(V) complex has also been investigated. The C—H bond kinetic isotope effects for the oxidation of cyclohexane, tetrahydrofuran, propan-2-ol, and benzyl alcohol are 5.3 0.6, 6.0 0.7, 5.3 0.5, and 5.9 0.5, respectively. A mechanism involving a linear [Ru=0"H"-R] transition state has been suggested for the oxidation of C—H bonds. Since a linear free-energy relationship between log(rate constant) and the ionization potential of alcohols is observed, facilitation by charge transfer from the C—H bond to the Ru=0 moiety is suggested for the oxidation. 

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

The relative rates of these processes are dependent on several factors (i) the ionization potential of the hydrocarbon, (ii) the oxidation potential of the anion in which the relative ease of oxidation is in the order Br > CT AcO, and (iii) the temperature. The different results discussed in the foregoing for the... 

The unsubstituted thiopyrylium ion (2) has been found to form CT complexes in CHjCN with both olefins and aromatic hydrocarbons (72CL17 75BCJ1519). Two CT absorption bands have been observed in the former case, and one in the latter. The slope obtained by the plot of the CT transition energies vs the ionization potentials of donors is 0.27 for the olefin complexes and 1.04 for the aromatic hydrocarbon complexes. These slopes suggest that 2 interacts with olefins more strongly than with aromatic hydrocarbons. Strong interactions in the olefin complexes would manifest themselves also in the appearance of two CT bands. These have been ascribed to electronic transitions from the HOMO of the olefin donor to the lowest and the second lowest vacant orbital of 2. The CT absorption frequencies of the complexes of 2 with olefins and aromatic hydrocarbons have been used to calculate their heat of formation by an empirical relation (81MI4). 

In aromatic hydrocarbons, some substituted alkenes, dienes, substituted acetylenes and ketones, one half of the n orbitals are empty and an electron can easily be placed in these antibonding orbitals. The capture of an electron by the acceptor molecule is an exothermic process because the energy of the antibonding orbitals lies below the level of the ionization potential of the acceptor radical anion. Many radical anions formed from unsaturated molecules are themselves stable they do not decompose and may exist indefinitely under suitable experimental conditions [182a],On the other hand, they react easily with other molecules. 

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