Ionization potentials , cation

Values for these coefficients, a, b, c, of Eq. (12) can be obtained from the ionization potentials and electron affinities of the neutral, the cationic, and the anionic states of an orbital. 

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

Trioxanes bond angles, 3, 949 bond lengths, 3, 949 H NMR, 3, 952 ionization potential, 3, 959 IR spectra, 3, 956 photoelectron spectroscopy, 3, 959 radical cations... 

The ionization potential is defined as the amount of energy required to remove an electron from a molecule, computed as the energy difference between the cation and the neutral molecule. For example, the ionization potential of PH2 may be computed as -E(PH2)... 

The most evident of these is the marked stability of radical cations formed in an aprotic medium by the oxidation of compounds where the first ionization potential (in the sense of photoelectron spectroscopy) is for the removal of an electron from a non-bonding orbital, e. g. thianthrene... 

Their reasoning is based on the difference in energy between a bent secondary vinyl cation such as 84 and a linear secondary vinyl cation such as 85. The authors, based on a third of the difference in ground-state ionization potentials for a carbon 2s and 2p orbital, estimate this difference to be 77 kcal/mole in favor of the linear ion 85 yet despite this large difference, there are significant amounts of 6-membered cyclic products, which, in the authors opinion, rule out distinct bent and linear vinyl cations such as 84 and 85 (82). 

For radical cations a quantity, AH , can be defined, the meaning of which is close to the heat of atomization. It is obtained by subtraction of the first ionization potential, I, of a parent hydrocarbon from the heat of atomization, AHa. of that hydrocarbon ... 

Ionization Potentials and Vibration Frequencies of Formaldehyde and its Radical Cation [after Turner (103)]... 

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

Cation Ionization potential / Species Acidity-basicity... 

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. 

Although the work discussed thus far has covered primarily neutral organic radicals, there are many types of cation and anion radicals that are stabilized on the surface. Some of these ion radicals are formed through photochemical processes however, many others are spontaneously generated on a surface. The type of radical ion that is formed depends on the oxidizing or reducing character of particular sites on the surface, as well as on the ionization potential and the electronegativity of the adsorbed molecule. 

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

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). 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

PAH radical cations are also involved in the metabolic conversion of PAH to PAH diones. Carcinogenicity studies of PAH in rat mammary gland indicate that only PAH with ionization potential low enough for activation by one-electron oxidation induce tumors in this target organ. These results and others indicate that one-electron oxidation of PAH is involved in their tumor initiation process. 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

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

Figure 16a shows the progressive bathochromic shift in the CT absorption bands (hvct) obtained from PyN02+ with aromatic donors with increasing donor strength (or decreasing ionization potential). A similar red shift is observed in the CT absorption bands (hvCj) of hexamethylbenzene complexes with various para-substituted JV-nitropyridinium cations (X-PyNO ) as shown in Fig. 16b. Such a trend in the hvct is in accord with the increasing acceptor strength of X-PyNO in the order X = OMe < Me < H < Cl < C02Me < CN.

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