Benzene, adsorbed positive ions

The radical cation of benzene has been produced by y irradiation of benzene adsorbed on silica gel (62, 63). The seven-line spectrum shown in Fig. 24 is expected for a molecule having six equivalent protons. An experimental coupling constant of 4.4 G, compared to a value of 3.75 G for the negative ion, gives strong support for attributing the spectrum to a positive ion. The g value is also consistent with this assignment. 

McBride and Wesselink (1988) studied IR spectra of catechol adsorbed onto the oxide surface and found evidence that the compound was chemically altered, indicating that chemisorption was the dominant mechanism. In addition to catechols, phenols are known to adsorb onto metal oxide surfaces. This adsorption is dependent on the number and position of hydroxy substitutions on the benzene ring. Diphenolic compounds adsorb to a greater extent than monophenolic compounds, suggesting the formation of a bidentate bond with the metal oxide. This bidentate bond is formed when the two phenolic ligands coordinate with one or two surface metal ions (McBride and Wesselink, 1988). 

The same picture holds for physical adsorption on metal surfaces. The polarization of the adsorbed molecules causes dipoles pointing with their positive ends away from the metal surface. The work function of the metal will be lowered by this effect, and it seems as if the increase of the normal nonselective photoelectric emission of metals by the adsorption of water molecules (122) or molecules of organic substances such as pyridine, propionic acid, and benzene (123) or alcohol, diethyl ether, and acetone (124) is caused by this effect. The explanation, which, many years ago, was given by the author (125), viz., polarization by positive hydrogen ions which should still be present, may seem to be unnecessary and obsolete. 

Unlike the mobility of imsaturated hydrocarbons, the mobihty of saturated hydrocarbons in Na-X is, in fact, essentially unaffected by the presence of sodium cations. This has been confirmed by PFG NMR diffusion studies with benzene and -heptane in zeohte Na-X and La-X [103,151]. Since the triva-lent lanthanum ions are predominantly localized at positions in the hexagonal prisms and sodalite units, the molecules adsorbed in lanthanum-exchanged zeolites are essentially without contact to the cations. As a consequence of the specific interaction between the cations and the unsaturated hydrocarbon, the benzene mobility in Na-X was found to be two orders of magnitude smaller than in La-X, while the -heptane diffusivities were the same. 

Diffraction studies have been used in a small number of cases to determine the minimum energy positions of adsorbed moleeules within cationic zeolites. These difficult and relatively expensive experiments have been used to establish computational simulation as a reliable method to predict such interactions without recourse to experimentation. Crystallographic X-ray studies of the adsorption of small moleeules sueh as CO and NO on cationic forms of zeolite are of particular relevance to study of eation-molecule interactions, as are neutron powder diffraction studies of the location of pyridine, coordinatively bound to potassium ions in zeolite K-L, and of benzene, bound to sodium ions in zeolite Na-X. ... 

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