Radical-surface interactions analysis

The present analysis relies on - and extends - the comprehensive theoretical study of Refs. [23,24] on the multi-state interactions in the manifold of the X — E states of Bz+. Like this recent work, it utilizes an ab initio quantum-dynamical approach. In Refs. [23,24] we have, in addition, identified strong coupling effects between the B — C and B — D electronic states, caused by additional conical intersections between their potential energy surfaces. A whole sequence of stepwise femtosecond internal conversion processes results [24]. Such sequential internal conversion processes are of general importance as is evidenced indirectly by the fluorescence and fragmentation dynamics of organic closed-shell molecules and radical cations [49,50]. It is therefore to be expected that the present approach and results may be of relevance for many other medium-sized molecular systems. 

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

The detailed studies of the surface of CdS nanocrystallites prepared in N,N-dimethylformamide (CdS-DMF) by means of emission measurements, in-situ Cd K-edge EXAFS analysis, and theoretical MO calculations reveal the correlation of the photocatalysis of CdS-DMF and the formation of sulfur vacancies on its surface. It has been experimentally proved that CO2 interacts with the sulfur vacancies and is converted into its radical anion under irradiation as an intermediate in the photocatalysis. The knowledge on the photocatalysis obtained above has led to the achievement of the photofixation of CO2 into benzophenone, acetophenone and benzyl halides under visible light irradiation in the presence of TEA as an electron donor. 

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

The above analysis shows that the formation of free radicals in the interaction of alkane molecules with the surface of oxides may prove to be energetically preferable as compared to any other mechanisms of their activation. Furthermore, this process requires only one type of single active centers and it proceeds in a single step. The combination of these factors may render this process the most favorable. This conclusion is experimentally confirmed by... 

ENDOR measurements of glassy, polycrystalline or amorphous samples usually aim at deducing structural properties from an analysis of hyperfine- and nuclear quadrupole interactions that are too small to be resolved by ESR. The subject has been summarized in recent textbooks [3,4,12], in reviews about radicals on surfaces [38], about radical ions in frozen matrices [39], and about paramagnetic species in biological systems [40 4]. 

FIG. 7 shows the pH dependence of the non-dispersion interactions Wg (determined by the two-liqid-method with liquid 1, cyclohexane and liquid 2, water) for coated Hercules carbon fibres. One of the carbon fibres was coated in a benzene/air plasma, the other in a benzene/ammonia plasma. Both reactive gas components added during the plasma polymerization process influence the term Ws in a similar way. This may be caused by the consecutive reactions between the plasma polymers and atmospheric oxygen due to the high concentration of free radicals on the film surface. This explanation is in accordance with the results of IR analysis. 

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