Ionization potentials of radicals

The ionization being accompanied by a vibrational excitation, the fine structure of bands can be exploited for determination of vibrational levels of an ionized system in the ground and excited states. Of course, the first (0-0) and the strongest vibrational bands are the most important because they determine adiabatic and vertical ionization potentials of radicals. 

Here R+ is the carbenium ion from equation (1), AHS6 are solvation enthalpies, (R ) is the ionization potential of radical R, and Z includes all electro-energetic terms which do not depend upon the nature of R. If, to a first approximation, TAS0 and AHS(R ) are taken to be independent of the nature of R and are incorporated into Z, then ... 

The Koopmans estimates of the vertical ionization potentials of radicals can be calculated from the energy of the lowest unoccupied molecular orbital (LUMO) of the nonradical cations in the geometry of the radicals [145]. Employing this method and using the ROHF/6-31G and ROHF/D95v//6-31G basis sets, ionization potentials were calculated. These results were then scaled to results found for experimentally known ionization potentials of several model compounds. The final results gave the estimates of vertical ionization potentials of the DNA adducts (shown in Figure 6). 

Figure 2. Enhancement of total butene yields from 100-torr ethylene with ionization potential of additive present in 10% concentration, 3 torr oxygen added when necessary to inhibit free radical reactions. The letter symbols indicate ionization potentials from Ref. 58 in parenthesis values in e.v.

The competing pathways to radical or carbenium ion derived products are determined, apart from experimental factors (see chap. 2), by the ionization potential of the radical. From product ratios and ionization potentials of the intermediate radicals, the conclusion could be drawn that such radicals with ionization potentials above 8 eV lead preferentially to coupling products, whilst those with ionization potentials below 8 eV are further oxidized to carbenium ions [8 c]. 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Formerly, we used for < the value of 11.22 eV, which is commonly employed in closed-shell calculations, but a correct interpretation of ionization potentials requires (34) that Ic be equated to the ionization potential of methyl radical, 9.84 eV. This change, however, does not affect the values of transition energies. 

Until now, applications of semiempirical all-valence-electron methods have been rare, although the experimental data for a series of alkyl radicals are available (108,109). In Figure 9, we present the theoretical values of ionization potentials calculated (68) for formyl radical by the CNDO version of Del Bene and Jaffe (110), which is superior to the standard CNDO/2 method in estimation of ionization potentials of closed-shell systems (111). The first ionization potential is seen, in Figure 9, to agree fairly well with the experimental value. Similarly, good results were also obtained (113) with some other radicals (Table VII). 

Figure 9. Determination of the first electron affinity, and the first and higher ionization potentials of formyl radical from the SCF orbital energies and electronic repulsion integrals, and K,j (cf. eqs. (90), (92), and (93)). The experimental value (112), 9.88 eV, for the first ionization potential corresponds to the theoretical value I . All entries are given in eV. With A and I a lower index stands for MO the upper one indicates the state multiplicity after ionization.

Calculated and Experimental First Ionization Potentials of Small Radicals (113)... 

In silane discharges, one observes the following when the discharge is off, the mass spectrometric signal at m/e = 31 amu/e (SiH ) as a function of electron energy is due to dissociative ionization of SiHa in the ionizer of the QMS, with an ionization potential of 12.2 eV [312]. The signal with the discharge on is due to ionization of the radical SiHa plus the contribution from dissociative ionization... 

The difference in stabilities of cation radicals located on G, GG, and GGG sequences was initially investigated by Sugiyama and Saito [14], who employed ab initio methods to calculate the gas phase ionization potentials of nucleobases stacked in B-DNA geometries. Their results indicated large differences in potential for holes on G vs GG (0.47 eV) and GGG (0.68 eV) sequences. A similar G vs GG difference was calculated by Prat et al. [62]. These values suggest that GG and GGG are, in fact, deep hole traps and they have been widely cited as evidence to that effect [54, 63]. 

Fortunately, for this solvent, the electron-capture centres give very broad e.s.r. features at 77 K, and hence the spectra for S + cations are readily distinguished. We know of no instance in which S + cations are not formed provided the ionization potential of S is less than that of the solvent. There are two complicating factors, one is unimolecular break-down or rearrangement of the radical cations, and the other is weak complexation with a solvent molecule. The latter is readily detected because specific interaction with one chlorine or one fluorine nucleus occurs, and the resulting hyperfine features are usually well-defined. 

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). 

Ionization Potential of PAH and Charge Localization in Radical Cations... 

Figure 3. Direct relationship between the ionization potentials of alkyl radicals (R ) with ID of the alkylmetals RHgMe (O) and R2SnMe2 (%),...

Figure 4. Correlation of the ionization potentials of alkylmetal donors with the electron-transfer rate constant (log kFe) for Fe(phen)s3+ (%), Fe(bpy)s3+ (O), and Fe(Cl-phen)s3+ ((D), (left). The figure on the right is the same as the left figure for Fe(phen)s3+ except for the inclusion of electron-transfer rates for some alkyl radicals as identified, (Note the expanded scale,)...

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 proposed subsequent reaction fits the fragmentation patterns observed in mass spectrometry where, even at 20 eV, group 14 centered radicals form in increasing order Sibasic data of this kind can provide estimates of kinetic behavior of such reactions, where M—M bonds are cleaved by electrophiles and which depend on the ionization potentials of the former as well as the electron affinity of the latter. 

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. 

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... 

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