Benzene metal surfaces

Figure Bl.6.10 Energy-loss spectrum of 3.5 eV electrons specularly reflected from benzene absorbed on the rheniiun(l 11) surface [H]. Excitation of C-H vibrational modes appears at 100, 140 and 372 meV. Only modes with a changing electric dipole perpendicular to the surface are allowed for excitation in specular reflection. The great intensity of the out-of-plane C-H bending mode at 100 meV confimis that the plane of the molecule is parallel to the metal surface. Transitions at 43, 68 and 176 meV are associated with Rli-C and C-C vibrations.

However, many adsorbates caimot reach a coverage of 1 ML as defined in this way this occurs most clearly when the adsorbate is too large to fit in one unit cell of the surface. For example, benzene molecules nonnally lie flat on a metal surface, but the size of the benzene molecule is much larger than typical unit cell areas on many metal surfaces. Thus, such an adsorbate will saturate the surface at a lower coverage than 1 ML deposition beyond this coverage can only be achieved by starting the growdi of a second layer on top of the first layer. 

In the case of ions, the repulsive interaction can be altered to an attractive interaction if an ion of opposite charge is simultaneously adsorbed. In a solution containing inhibitive anions and cations the adsorption of both ions may be enhanced and the inhibitive efficiency greatly increased compared to solutions of the individual ions. Thus, synergistic inhibitive effects occur in such mixtures of anionic and cationic inhibitors . These synergistic effects are particularly well defined in solutions containing halide ions, I. Br , Cl", with other inhibitors such as quaternary ammonium cations , alkyl benzene pyridinium cations , and various types of amines . It seems likely that co-ordinate-bond interactions also play some part in these synergistic effects, particularly in the interaction of the halide ions with the metal surfaces and with some amines . 

The accuracy of LDF calculations in the prediction of surface geometries not only holds for clean metal surfaces such as the W(001) surface discussed above, but is also found for adsorbates such as H (27), O (28), and S (29) on Ni(OOl) surfaces. Rather than going into detail on clean and adsorbate covered surfaces, we will now focus on the description of the C-C bond by LDF theory. To this end, we first discuss a layer of condensed benzene rings, i.e. a graphite monolayer, and then focus our attention on the ethylene molecule. 

To summarize, the use of heavy water as a deuterium source has provided a wealth of experimental information. Evidence for the associative ir-adsorption of benzene [species (I) J is secure (2). Evidence for hydrogen exchange in the benzene ring by an abstraction-addition mechanism is less well established, partly because of uncertainties that surround the mode of chemisorption and reaction of water at metal surfaces. Nevertheless, it would be wrong to deny that Scheme 6 is consistent with a large body of experimental work. 

In the two previous sections, evidence has been presented concerning the chemisorbed states formed when benzene interacts with metal surfaces. It is not the intention in this Section to discuss benzene hydrogenation in detail, but rather to enquire whether studies of this hydrogen-addition reaction provide information about the chemisorbed state of benzene. 

Fig. 6. Schematic representation of the range of chemisorbed species formed from benzene and the reactions that benzene undergoes at a transition metal surface.

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

Magnetization, in chemisorption of benzene on metal surfaces, 23 129 Magnetocatalytic effect, 27 23, 26 8 Magnetocrystalline anisotropy, 26 146, 147 Magnetogyric ratio, 42 122 Mahan-Nozi6res-De Dominicis theory, 34 ... 

Highly nitrated derivatives of benzene readily react with water to form phenols. 1,2,3,5-Tetranitrobenzene (54) is readily converted to picric acid on reaction with hot water. This type of reaction has practical concerns if such an explosive is used in a military context -picric acid forms dangerous picrates if allowed to come into contact with a metal surface i.e. the inside of a munition s shell. Other explosives like 2,3,4,6-tetranitrophenol (121) and... 

Another type of reaction which occurs at a metal surface and which is most conveniently discussed in this section is catalytic hydrogenation. Pyridine is reduced rather more easily than benzene, and quinoline and isoquinoline give the 1,2,3,4-tetrahydro derivatives. 

While the adsorption of benzene molecules before the maximum was reached increased the sensitivity, the molecules condensed on the platinum surface after the maximum had been reached decreased the sensitivity (C, D, and F in Fig. 28). The excess of the benzene molecules, however, can be desorbed in about 30 min. if no further molecules strike the surface E and G in Fig. 28). The work function was lowered by the adsorption of the optimum benzene layer from 4.54 volts (in J.) to 4.11 volts (in JS). The ir electrons were therefore displaced to the metal surface by the adsorption. 

The electronic interaction between benzene and the metal surface may be made up of two effects the polarization of the molecule, which may be concluded from the above-described research, and the shifting of the v electrons to the metal surface to become part of the metal electron gas, which has been hypothesized by Polanyi (77). The first effect has been shown in Fig. 28, the second apparently can be seen from the research (18) illustrated by Fig. 29, in which the change of resistance of a transparent nickel film was studied during the adsorption of benzene molecules. As the temperature of the benzene capsule was 90°K., the evaporation velocity was so low that only a small number of benzene molecules struck the surface in unit time. The resistance therefore diminished only... 

There is general agreement, based on measurements of vibrational spectra taken at room temperature or below, that benzene adsorbs nondissocia-tively on single-crystal metal surfaces with the C6 ring oriented parallel or near-parallel to the surface. Furthermore, there is a strong general resem-... 

Primet et al. (250, 251) also showed that aromatic rCH absorptions characteristic of benzene-derived species on Pt/Si02 caused a marked lowering (from 2065 to 2030 cm"1) of the strong bands characteristic of coadsorbed CO. The authors interpreted the vCO shift as arising from donation of electrons from the benzene 77-orbitals to the metal surface. Palazov (245) drew similar conclusions concerning benzene and CO coadsorption on Ni/Si02. 

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