Site effects, matrix isolation

Recently, alternate approaches to PAH-analysis have been proposed, which could possibly eliminate tedious separations in the future. These luminescence - based techniques make use of the Shpol skii effect, matrix isolation or fluorescence line narrowing spectrometry, combined with site selective laser excitationi Also, the refined mass spectrometric identification technique MS/MS was applied successfully to the analysis of PAH in synthetic fuels. 

The extent to which site effects manifest themselves in the spectra depends also on the way a matrix is made. It has been reported that pulsed deposition leads to a simpler spectral site structure and sharper lines than slow, continuous deposition. But then this depends on the backing pressure and pulse duration, as well as on the temperature of the matrix gas and the speed with which extra gas is removed, so no general rules can be given. Every practicioner of matrix isolation has to find a combination of these above variables that leads to the best spectra under their laboratory conditions. The search for these conditions should, however, not be guided by purely aesthetic spectral criteria, but by the need to acquire a maximum of useful information with minimal effort. 

A few methods produce reactive molecules by reactions in solid matrices. The most widely used consists of irradiating already isolated precursors with UV light (including vacuum UV light at A < 200 nm), y- or X-rays. In this case, the fragments which are formed as products of the precursor s dissociation must not recombine in the matrix site. To achieve this effect, one of the fragments should be either chemically inactive [e.g. N2, CO2 see (la)] or able to diffuse easily from the site [e.g. hydrogen atoms as in (lb)]. 

In the ONIOM(QM MM) scheme as described in Section 2.2, the protein is divided into two subsystems. The QM region (or model system ) contains the active-site selection and is treated by quantum mechanics (here most commonly the density functional B3LYP [31-34]). The MM region (referred to as the real system ) is treated with an empirical force field (here most commonly Amber 96 [35]). The real system contains the surrounding protein (or selected parts of it) and some solvent molecules. To analyze the effects of the protein on the catalytic reactions, we have in general compared the results from ONIOM QM MM models with active-site QM-only calculations. Such comparisons make it possible to isolate catalytic effects originating from e.g. the metal center itself from effects of the surrounding protein matrix. 

Site isolation effects catalytically active centres such as metal complexes can be dispersed over the zeolite matrix. In this way, reaction between adjacent metal centres can be impeded for steric reasons (7). In other well-documented cases, intramolecular reactions are preferred over intermolecular reactions due to the spatial isolation of molecules adsorbed in low concentrations (8)... 

Discussion. Copper in Krypton. The absorption spectra of copper atoms Isolated in rare gas matrices have been extensively studied (15-25) and the triplet of bands at 310nm attributed to a number of different causes. These include (1) spin orbit splitting and static axial site distortion (17), (2) multiple matrix sites (18), (3) exciplex formation between the metal and a single matrix atom (19), (4) long range metal-metal interactions (2 ), and (5) Jahn-Teller (JT) effect resulting from matrix cage atom vibrations on the excited metal (21,22,23). The MCD of Cu atoms in the rare gas matrices has recently been reported (24,25) and the results interpreted as consistent with either the distorted site or JT hypotheses (39). 

The ESR data shows that both the number of centers and the local structure of active sites associated with Cu isolated ions are not changed noticeably at T < 500°C as a result of cobalt introduction. At the same time, catalytic testing shows a 3-fold rise in oxidative activity of bi-cationic sample (Fig. 1) demonstrating an increase either in the number of sites or in the intrinsic activity of catalytic centers. The effect can be explained only in assumption of the high dispersion of cobalt ions in microporous matrix it is difficult to imagine a considerable contribution from the big particles of cobalt oxide on the outer surface of zeolitic crystals. 

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