Surface stabilized radicals

As discussed earlier, surface trapped electrons are not only important as paramagnetic surface probes but, being localized at a solid-gas interface, they are extremely reactive and can act as powerful reducing agents leading to surface stabilized radicals. An account of these radicals has been given in the past volume of this series1 and we limit ourselves here to report on the 1702 radical stabilized on the surface of MgO for which the hyperfine tensor was fully... 

Some of the earliest ESR studies of surface stabilized radicals were performed by Russian workers. Parllskil, et al, (6) have studied CH radicals... 

The intriguing point is that the actual alkylation step may be the same at the anode and cathode, presumably by alkyl radicals which, in analogy to the Paneth reaction, alkylate the metal. The lifetime of the radical ion, reactivity of the radical ion or the radical towards the metal, stabilization of the radical by adsorption on the electrode surface, stabilization of each of the intermediates by solvation, their build-up in the double layer, the potential applied, all have an important contribution to the outcome. In certain cases the ET takes place catalytically, by a mediator or under the influence of surface effects17. It is therefore important to keep in mind the possible subtle differences between cases described below that otherwise appear similar. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

Calculate CH bond dissociation energies in propene and in toluene, leading to allyl and benzyl radicals, respectively. (The energy of hydrogen atom is given at right.) Is bond dissociation easier or more difficult in these systems relative to bond dissociation in 3-ethylpentane (methyl CH) Examine spin density surfaces for allyl and benzyl radicals. Draw Lewis structures that account for the electron distribution in each radical. Does spin delocalization appear to stabilize radicals in the same way charge delocalization stabilizes ions ... 

MSi and RSi). By involving these centers in the reactions with gas-phase molecules, one can conduct directional chemical modification of their structure (Table 7.3) and quantitatively convert them from one form to the other. This lies at the basis of the method for preparing the surface-stabilized PCs of the type (=Si -0)2Sia -r, where r — H(D), OH(OD), NH2, CH3(CD3), C2H5, etc. in silica [18,19]. This method amounts to generating low-molecular mobile radicals r in the reaction between the surface-stabilized paramagnetic center S and the gas-phase r1— r molecule ... 

Again, the activated gold surface stabilizes the carbon radical intermediate [ C(0)0H] and facilitates the second electron-transfer oxidation of HO- to HO- via coupling to the carbon radical. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

The study of surface-stabilized inorganic radicals by EPR has a long history. This partially arises from their ease of generation and their favorable stabihty on the ionic oxide surfaces. From a catalysis point of view, such radicals are fundamentally important, since they can act as intermediates or oxidants in the catalytic cycle. If isotopic substitution of the radical is facile, then a very thorough description of the electronic and geometric properties of the species can once again be obtained by analysis of the powder EPR pattern. 

In all such cases the process initiated by persulfate begins in the aqueous phase with the formation of water-soluble, surface-active polymeric radicals which, after growing to a certain critical size, precipitate to form particles subsequent polymerization proceeds mainly within these particles. The higher the solubility of the monomer in water, the more surface-active radicals and therefore primary particles are formed and the higher the stability and the concentration of the latex, A kinetic curve of the emulsifier-free polymerization of ethyl acrylate confirming this scheme is shown by Curve 1 in Fig. 3 (Yeliseyeva and Petrova, 1970). The process... 

In a very interesting report of Molle and Bauer [7] using adamantyl halides and the hindered ketones hexamethylacetone and adamantanone, they observed no alcohol products when the hindered ketones were mixed with preformed adamantyl lithium at low temperature ( —20 C) in diethyl ether. However, when the ketones and halide were mixed with lithium powder under the same low-temperature conditions, substantial yields of the expected carbinols were isolated. They postulated a surface-mediated radical ion or radical pair reaction with the ketone or the ketyl radical. The reaction of a ketyl with the halide was ruled out because earlier researchers [4] had shown that no carbinol was obtained from the reaction of an alkyl halide and benzophenone ketyl. The authors also did not find any carbinol product when the ketyl of adamantanone was reacted with adamantyl halide. By extending their studies to other homologues of adamantane, the authors concluded that the stability of the cagelike radical species determined the extent to which the radical pathway is favored over the organometallic pathway. 

The conversion of methane and the various product selectivities achieve approximately constant values for surface concentrations of lead equal to approximately 3 mol % which corresponds to a PlV(Pb + Ca) ratio of 0.02, suggesting that the changes produced by the introduction of Pb result primarily from its presence in the surface, rather than the bulk. Lead species in the surface tqjparently assist in the stabilization of methyl radicals, while the formation of ensembles of these species hold such stabilized radicals in sufficiently close proximity to permit lateral interaction and the formation of Cj hydrocarbons [13]. 

Several attempts have been made to relate the surface stability of the CH radical with its tumbling frequency as determined by the dependence of the linewidth on the nuclear spin quantum number (52). Thus Gardner and Casey... 

Garbutt (5) showed that the decay of surface stabilized CH radicals was complex and appeared to follow second order kinetics more closely than first order. A typical detailed decay curve for CH on PVG at 152 K (0 = 0.05 for CHjI) is shown in Fig. 5. A plot of - log Rate vs log C (where C is the concentration of CH as determined by peak-to-peak amplitude) gave a slope of about 10 (7 to 12) for the order of the reaction. 

The spectrum of the radical should contain a quartet splitting from the ortho and para protons and an additional doublet splitting owing to the /J-proton. The outermost lines are probably caused by cyclohexadienyl radicals which are firmly bound to the gel surface. Such radicals may be more stabilized and thus less reactive. The spectrum may be modified owing to a distorted conformation. 

Evidence for the formation of a surface stabilized organogermylene has also been presented. Lastly, both experimental and theoretical papers have appeared describing the formation of germylene radical anions. ... 

As the surface state orbitals are directed outward from the surface, the center of their electron density does not correspond with that of the Si nucleus. Their occupation with electrons gives rise to a surface dipole moment. The dangling bonds formed on the unreconstructed Si(lll) surface behave as electron radicals as long as they are half filled. In the presence of a gas this will lead to rapid reaction. In a vacuum the chemically unsaturated nature of the dangling bonds leads to reorganization of the surfaces, resulting in a decrease of surface reactivity. Because of this surface stabilization, the surface energy decreases. 

Apparently, pre-irradiating polyethylene containing a HALS with a low pressure Hg lamp imparts some surface stabilization toward sunlight exposure. Anthracene-HAS stabilizer molecules are found to be more effective stabilizers than the separate molecular moieties, while the nature of the polymer composition affects the distribution of nitroxyl radicals in stabilized materials. " Polyethylene with terminal HAS groups has been found to be very light stable, as were reactive acrylic coatings with acrylated HAS systems. A new epoxide amine based HAS has been found to be quite effective in... 

After FG addition to A2-MNP-1 micron-sized clusters do not form. So adsorption BSA layer formed in the presence of keeps stability. This stability can be explained by formation of covalent bonds between protein macromolecules [13] in adsorption layer as a result of free radicals generation on MNPs surface. Stability of BSA coating on MNPs was demonstrated for the samples Al-MNP-1 and A2-MNP-T incubated for more than 100 min before FG addition. Clusters are shown to appear if the incubation time is insufficient. 

As the decomposition product of the gemini inisurfs is a conventional surface-active radical with the ability to stabilize only a smaller surface area, the particle size distribution should be broad or bimodal if a certain number of... 

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