Sites electron-acceptor

Molecular orbitals are useful tools for identifying reactive sites m a molecule For exam pie the positive charge m allyl cation is delocalized over the two terminal carbon atoms and both atoms can act as electron acceptors This is normally shown using two reso nance structures but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron acceptor orbital) Allyl cation s LUMO appears as four surfaces Two surfaces are positioned near each of the terminal carbon atoms and they identify allyl cation s electron acceptor sites... 

LUMO of thionyl chloride reveals the best electron-acceptor sites. 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

The presence of the electron acceptor site adjacent to the donor site creates an electronic perturbation. Application of time dependent perturbation theory to the system in Figure 1 gives a general result for the transition rate between the states D,A and D+,A. The rate constant is the product of three terms 1) 27rv2/fi where V is the electronic resonance energy arising from the perturbation. 2) The vibrational overlap term. 3) The density of states in the product vibrational energy manifold. 

It is now well established that when a surface presents electron donor or electron acceptor sites, it is possible to ionize molecules of relatively high electron affinity (> 2 eV) or low ionization potential values, resulting in paramagnetic radical ions. For instance anthracene and perylene are easily positively ionized on alumina (7 ) (IP = 7.2 and 6.8 eV respectively). The adsorption at room temperature of benzenic solution of perylene, anthracene and napthalene on H-ZSM-5 and H-ZSM-11 samples heated up to 800°C prior to adsorption did not give rise to the formation of the corresponding radical cation. For samples outgassed at high... 

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

The acid sites strength can be determined by measuring the heats of adsorption of basic probe molecules. The basic probes most commonly used are NH3 (pTTa = 9.24, proton affinity in gas-phase = 857.7 kJ/mol) and pyridine (pTTa = 5.19, proton affinity in gas-phase = 922.2 kJ/mol). The center of basicity of these probes is the electron lone pair on the nitrogen. When chemisorbed on a surface possessing acid properties, these probes can interact with acidic protons, electron acceptor sites, and hydrogen from neutral or weakly acidic hydroxyls. 

Some properties of palladium deposited on different amorphous or zeolitic supports were determined, including catalytic activity per surface metal atom (N) for benzene hydrogenation, number of electron-acceptor sites, and infrared spectra of chemisorbed CO. An increase of the value of N and a shift of CO vibration toward higher frequencies were observed on the supports which possessed electron-acceptor sites. The results are interpreted in terms of the existence of an interaction between the metal and oxidizing sites modifying the electronic state of palladium. 

Determination of the Number of Electron-Acceptor Sites. This determination was done in a few cases using perylene and phenothiazine as reactants. The catalysts were reduced for 2 hours at 400°C under H2. Since we presume that silica-alumina may be modified by the reaction medium during palladium exchange, we used as our blank a support treated under conditions similar to that of the catalysts. The results (Table V) show a clear decrease of the number of oxidizing sites after palladium deposition. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

Vanadyl porphyrin interaction with the surface is a function of the catalyst. Adsorption through electron acceptor sites dominates on the oxide surface, whereas the sulfided catalyst interacts through electron donor sites (see Section IV,B,5). Heats of adsorption have been estimated to be 8 to 12 kcal/mole. Values in this range are indicative of weak adsorption interactions that are of reduced importance at hydroprocessing conditions. 

Diffuse reflectance spectroscopy (DRS) of VO-porphyrins on reduced and sulfided catalysts exhibit shifts in the porphyrinic electronic spectra (Soret, a, (3 bands) to higher frequencies. Adsorption results in modification of the delocalized electronic resonance structure not observed on the oxide form of the catalyst. X-ray photoelectron spectroscopy reveals shifts to higher Mo binding energies on reduced and sulfided catalysts following VO-porphyrin adsorption, consistent with transfer of electrons from Mo electron donor sites to the V02+ ion. Interaction at the electron donor sites is stronger than interaction at electron acceptor sites typical of the oxide catalyst. This gives rise to the possibility of lower VO-porphyrin diffusion rates on sulfided catalysts, but this effect has not been experimentally demonstrated. 

The main feature of electron tunneling is that it can provide the occurrence of both primary and secondary reactions of PET between remote electron donor and electron acceptor sites, at distances sometimes as great as several tens of Angstroms. 

Electron-acceptor sites [A]s Polynuclear aromatic hydrocarbons Phenothiazine [PhTh] PhTh + [A]s [PhTh+- A"]s IV. I... 

There exists still some controversy as to the nature of the electron-acceptor sites on oxide surfaces that lead to the formation of radical cations. Various... 

In the case of some paramagnetic molecules, the formation of a charge-transfer complex can be determined by EPR either directly, when the electron of the donor molecule interacts with the nuclear spin of the electron-acceptor site, or indirectly by observation of the electron delocalisation in the probe molecule. Both methods can yield quantitative information about the distrubution of electron donor or electron acceptor sites when a range of probe molecules having different electron affinities or ionization energies are used. 

When an electron donor and an electron acceptor site are present in the same molecule, intermolecular association may occur with formation of cychc species. This process is called self-assembly. Self-assembly is defined as a spontaneous association of molecules under equilibrium conditions into stable aggregates held together by noncovalentforces The resulting species is a supermolecule (see above). 

It seemed of some interest to test the ability of a series of REY zeolites to ionize polynuclear aromatics since the oxidizing properties of zeolites were pointed out (8, 16), but the nature of the electron acceptor site is still under discussion. Hall et al. (5), studying dehydroxylated HY zeolites, presumed it to be molecular oxygen trapped in an anion vacancy, while Hirschler (7) asserted that the protons may be the oxidative centers. In a previous work, as stated by Turkevich et al. (16), we concluded that the active sites are Lewis centers, while the chemisorbed oxygen increases their electron affinity (27). In a recent work, Richardson (14) related spin concentration to the electron affinity of the cation, presuming that the electron transfer took place from the anthracene to the cupric ion, but he could not observe any variation of the Cu peak intensities. 

C. Naccache Figure 1 shows that rehydration results in a large decrease of spin concentration. For example, as can be determined in this figure (dotted line corresponds to sample activated at 450° and then rehydrated), LaY zeolite dehydrated at 450° gives about 1.5 X 10 positive radical ions, while after rehydration the number of radical ions is only 1.1 X 10 . The decrease in electron-acceptor sites resulting from rehydration of the zeolite is demonstrated. 

The second significant feature of the spectra is that the new bands position is practically the same for the different cationic and decationated forms of zeolites. According to the simple Mulliken s CTC theory [4] this observation allows us to suggest that the electron-accepting sites are the same for of all forms of the zeolites under investigation. Therefore the second conclusion is that the exchangeable alkali and alkali-earth cations cannot be considered as electron-acceptor sites. 

It is interesting, that recently published ref [2] showed that the cation-radical formation upon perylene and tetracene adsorption on NaY was significantly higher than on NaX. If the cations were the acceptor sites, the effect would be just opposite. Hence we believe that this observation supports our assumption about the acid-type nature of the electron-acceptor sites in the alkali forms of zeolites. 

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