Aromatics, adsorbed state

Photochemically induced Z,E-isomerization has been observed in 4-diethyl-aminoazobenzene and 4-diethylamino-4 -methoxyazobenzene and in the mesoionic azo-compounds (6). The photoisomerization of aromatic azo-compounds on hydrated Si02 and AI2O3 has made available in the adsorbed state Z-isomers which were previously inaccessible. Z-2,3,4,5,6-Pentafluoro-phenylazobenzene, obtained by irradiation of the corresponding E-isomer, is unusually stable in the dark. ... 

Traces of octalins were always present during the hydrogenation of tetralin or naphthalene (3). Observed concentrations of octalins are listed in Table IV. The amounts were particularly small with palladium catalysts. One of the unique characteristics of palladium is its ability to adsorb and saturate olefins in the presence of aromatics or, conversely, its relative inability to adsorb and saturate aromatics in the presence of olefins. By way of contrast, some other metals, particularly ruthenium and rhodium, are more able to adsorb and saturate aromatics in the presence of olefins. Whatever the nature of the adsorbed state of naphthalene that leads to hydrogenation, one could imagine the possibility of two isomeric forms—one of which behaved more like an adsorbed olefin and the other more like an adsorbed aromatic ... 

It is now firmly believed that the hydrogenation of aromatic substances has certain steps in common with the hydrogenation of cyclic olefins 16, 23), and hence a word concerning the adsorbed state of the aromatic ring and of possible partially hydrogenated intermediates is in... 

The shifts in wavenumbers indicate a change in the electronic structure of aromatics adsorbed compared to the liquid or gas phase. Such modiflcadons in adsorbates have been rather extensively studied in the case of benzene using diffuse reflectance UV spectroscopy (77), UV-visible transmission microscope spectrophotometry (78) or laser raman spectroscopy (79). All the results suggest the formation of a complex between the cation and the it electrons of the aromatic ring. Hindrance in mobility compared to that of benzene in the solid state have been observed (78). 

Styryl and vinyl groups are well known as photochemically and thermally reactive functional groups. Especially, 2-vinylstilbene, ( , )-1,2-distyrylbenzene (3w), and the naphthalene analog (3x) having these reactive groups at orf/to-position of aromatic ring are highly reactive for photochemical dimerization and valence isomerization in solution, even in solid state, and in adsorbed state on silica gel. However, little is known about the cation radical of 3w, probably because the cation radical of 3w is too reactive to afford the identifiable products. 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

Thus, transition metal cations in the lower valence state may also act as Lewis bases. Factors that affect the reactions promoted by Lewis acidity are listed in Table I. Lewis acid sites reversibly adsorb water (6s 9, 42), which may thus strongly compete with organic compounds that have weaker Lewis base properties, such as aromatic hydrocarbons. Lewis acidity depends on the degree of hydration and is strongest under desiccating conditions. Examples of reactions that are promoted by Lewis acidity are summarized in Table II. Other examples have been reviewed by Solomon and Howthorne (37). 

In the dissociative mechanism, the tt complex adsorbed aromatic reacts with a metal radical (active site) by a substitution process. During this reaction [Eq. (9)] the molecule rotates through 90°, and changes from its horizontally tt complex adsorbed position to a vertically cr-bonded chemisorbed state ... 

If we start with states of tt-symmetry (dashed lines) we find three distinct peaks in the XES spectra reflecting the occupied states. The 1 a2u and lelgTr-like orbitals are essentially intact from the gas phase, while the third state, labeled e2u, is not seen for the free molecule. Based on symmetry-selection rules, it can be shown that this state is derived from the lowest unoccupied molecular orbital (LUMO) e2utt -orbital that becomes slightly occupied upon adsorption. We anticipate a similar bonding mechanism as discussed in the previous section for adsorbed ethylene with the exception of a weaker rehybridization due to the extra stability in the -system from the aromatic character. 

The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

The object of this work was to study the influence of pretreated, decationized NH4-zeolites on adsorbed A,iV-dimethylaniline molecules. Such influence is caused by, proton-donating and electron-deficient active sites in decationized zeolites. Interaction of an aromatic amine molecule (M) with the proton-donating site leads to the formation of the MH+ molecule ion interaction with the electron-deficient site results in the M+ cation radical. Stabilization of these states by adsorption leads to the... 

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