Ionization potentials substrates

Ayala, M. Robledo, N. R. Lopez-Munguia, A., and Vazquez-Duhalt, R., Substrate specificity and ionization potential in chloroperoxidase-catalyzed oxidation of diesel fuel. Environmental Science Technology, 2000. 34(13) pp. 2804-2809. 

Fig. 10 Electrochemical energy level model for orbital mediated tunneling. Ap and Ac are the gas-and crystalline-phase electron affinities, 1/2(SCE) is the electrochemical potential referenced to the saturated calomel electrode, and provides the solution-phase electron affinity. Ev, is the Fermi level of the substrate (Au here). The corresponding positions in the OMT spectrum are shown by Ar and A0 and correspond to the electron affinity and ionization potential of the adsorbate film modified by interaction with the supporting metal, At. The spectrum is that of nickel(II) tetraphenyl-porphyrin on Au (111). (Reprinted with permission from [26])...

Fig. 31 Substrate-oxygen charge transfer band onsets as a function of the ionization potentials of the substrates.

The fact that evaporated potassium arrives at the surface as a neutral atom, whereas in real life it is applied as KOH, is not a real drawback, because atomically dispersed potassium is almost a K+ ion. The reason is that alkali metals have a low ionization potential (see Table A.3). Consequently, they tend to charge positively on many metal surfaces, as explained in the Appendix. A density-of-state calculation of a potassium atom adsorbed on the model metal jellium (see Appendix) reveals that the 4s orbital of adsorbed K, occupied with one electron in the free atom, falls largely above the Fermi level of the metal, such that it is about 80% empty. Thus adsorbed potassium is present as K, with 8close to one [35]. Calculations with a more realistic substrate such as nickel show a similar result. The K 4s orbital shifts largely above the Fermi level of the substrate and potassium becomes positive [36], Table 9.2 shows the charge of K on several metals. 

A good starting point for discussing chemisorption is the resonant level model. The substrate metal is jellium, implying that we look at metals without d-electrons, and the adsorbate is an atom. We focus on only two electron levels of the atom. Level 1 is occupied and has ionization potential / level 2 is empty and... 

It is tempting to relate the thermodynamics of electron-transfer between metal atoms or ions and organic substrates directly to the relevant ionization potentials and electron affinities. These quantities certainly play a role in ET-thermo-dynamics but the dominant factor in inner sphere processes in which the product of electron transfer is an ion pair is the electrostatic interaction between the product ions. Model calculations on the reduction of ethylene by alkali metal atoms, for instance [69], showed that the energy difference between the M C2H4 ground state and the electron-transfer state can be... 

Let us compare M-ZSM-5 zeolites with M = H+, Li+, Na, K+, Rb, Cs, AF+, on one hand, and organic electron donors of variable ionization potentials, on the other. Zeolite H-ZSM-5 generates cation-radicals from substrates with an oxidation potential of up to 1.65 V (Ramamurthy et al. 1991). The naphthalene sorption by Al-ZSM-5 zeolites calcified in an atmosphere of oxygen or argon leads to the appearance of two occluded particles—the naphthalene cation-radical and isolated electron. Both particles were fixed by ESR method. Back reaction between the oppositely charged particles proceeds in an extremely slow manner and both the signals persist over several weeks at room temperature (Moissette et al. 2003). 

Some Srj I reactions can take place in the dark withont a catalyst. For example, the interaction of freons with nncleophiles in DMF at 20°C proceeds withont photoirradiation. The chain process begins when the system pressnre reaches 2 atm, in other words, when the concentration of the gaseons reagent becomes snfficient (Waksehnan and Tordenx 1984 Scheme 7.69). There is a favorable difference between the ionization potential of the nncleophile (PhS ) and EA of the substrate (CFjBr) the expressed bromide fugacity is also a favorable factor. 

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. 

Photoelectron spectroscopy (PES) is also carried out in the gas phase photons of known energy (E/,v), for example, the He(I) line (21.21 eV), ionize a substrate the kinetic energy (Eyn) of the emitted electrons is measured and the vertical ionization potentials (/y) derived (Eq. 13). The PES provides information on the energies of occupied molecular orbitals (MOs) " the highest occupied molecular orbital (HOMO) of the parent reveals the bond(s) likely to be weakened or broken upon ionization. The PES data reflect the geometries of the parent molecule and need not have any bearing on the equilibrium structure of the radical cation. 

One method used to position the energy levels of the dyes relative to those of the silver halide depends on the measurement of ionization potentials of the dyes adsorbed by a substrate. Nelson (251,252) measured ionization potentials of several sensitizing dyes coated on a variety of substrates ... 

TABLE 2. Oxidation potential and ionization potential of disilirane and digermirane Substrate Eox (V) vs SCE° 7p (eV)b... 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

This mechanism seems to imply that if some substitution occurs at the classical position of attack (the first a complex), then such a substitution should show the deuterium isotope effect for proton loss from this position. The deuterium effect is absent in the majority of nitration cases, except for nitration in sterically shielded positions (Schoffield 1980). Perhaps a systematic investigation of kinetic isotopic effects would be useful in a much wider range of substrates in comparison with their ionization potentials and nitration conditions. 

Examples of the multiplicity of nitration mechanisms that depend on the ionization potentials of substrates are the nitration of naphthalene (NaphH + NO) scheme) and of perylene (PerH + NO) scheme) (Scheme 4-25). 

There is a favorable difference between the ionization potential of the nucleophile (PhS ) and the electron affinity of the substrate (CF3Br) the expressed bromide fugacity is also a favorable factor. 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

The alpha effect in the SN2 reactions of methyl substrates with three different nucleophiles was shown to correlate with Koopman s theorem ionization potentials for the leaving group.128 This was taken as evidence that (1) the size of the alpha effect in Sn2 reactions depends on the ability of the nucleophile and the leaving group to donate an electron to the methyl group and (2) these transition states have some SET character. The results support the Hoz model129 for the alpha effect. 

Among the methods that have found the most widespread application in the study of radical cations, ultraviolet photoelectron spectroscopy (UV-PES) has a special place, because it provides a wealth of detailed information concerning the orbital energies of organic molecules [57], In this experiment, a substrate is ionized by ultraviolet radiation with photons of known energy (Ehv), e.g. the He(I), line (21.21 eV), and the kinetic energy (Ekin) of the emitted electrons is measured. The vertical ionization potential (Iv) can then be calculated from Ehv and Ekin (Eq. 13). 

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