Surface orbitals

Apart from POs that virtually evolve on a single adiabatic potential-energy surface, there are numerous orbits that propagate on several or in between adiabatic surfaces. Orbit C (referred to as la in Table VI) is the shortest PO of this type, with a period of 39.2 fs. While the adiabatic population stays around Vad = 0.5, the PO oscillates between the diabatic states with a Rabi-type... 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

The division of the ( + 1) bonding AO s into two types, a unique radial orbital and n surface orbitals, incidentally is also a feature of a free-electron MO treatment (198) which has been applied to borane polyhedra, as distinct from the linear combination of atomic orbitals (LCAO) MO treatments mentioned above. 

No complete explanation has been advanced for the positive sign of the S.P., although some generalization has been made on the basis of the overlap conditions of the surface orbitals 86). With the exception of O2, these... 

When these details were first discussed by Gurney (a physicist), in 1931, it was not realized that the adiabatic reception of the electron inH30+ depended on a coupling of the motion of the H that was previously the proton in H30+ with the metal surface orbitals to which it must bond to become an adsorbed H—the intermediate radical of which has already been discussed. Hence, in Gurney s famous first publication, H had not, to use a phrase, come in from the cold it was left out of contact with the electrode, and lack of bonding to the metal led to improbably high values for the calculated heat of activation for the proton discharge reaction. 

Another characteristic property of the electron density of 1 is its relatively high value at the centre e of the ring (more than 80% of that at the CC bond critical point). Density is smeared out over the ring surface and concentrated at its centre because of the occupation of the w0 -orbital (MO 8, 3a(, Figure 6), which has the character of a surface orbital . Cremer and Kraka9, n 13 have termed this phenomenon surface delocalization of electrons, to be distinguished from ribbon delocalization and volume delocalization of electrons (Figure 12)12. 

Surface delocalization is not found for cyclobutane or larger cycloalkanes10. Furthermore, it does not appear for cyclotrisilane since in this case the overlap within the surface orbital is not sufficient to bring enough electron density into the centre of the ring71. 

The idea that a metal atom in the zero oxidation state is both a soft acid and soft base can be used to explain surface reactions of metals. Soft bases such as carbon monoxide and olefins are strongly adsorbed on surfaces of the transition metals. Bases containing P, As, Sb, Se, and Te in low oxidation states are strongly adsorbed, blocking the active sites (Pearson, 1966). The clean surfaces are incomplete solids, in that the surface atoms have no nearest neighbors in one of the three-dimensional coordinate system. This means that there are atomic orbitals, both filled and empty, which are not being used to form surface orbitals. 

Consider the interaction of methyl, CH3, with a surface, in on-top and bridging sites, 110.78 Let s assume low coverage. The important methyl orbital is obviously its nonbonding or radical orbital n, a hybrid pointing away from the CH3 group. It will have the greatest overlap with any surface orbitals. The position of the n orbital in energy is probably just below the bottom of the metal d band. How to analyze the interactions of metal and methyl ... 

For a second example, let s return to acetylene on Pt(lll), specifically in the twofold and fourfold geometries.29 In the twofold geometry, we saw earlier (from the decomposition of the DOS) that the most important acetylene orbitals were irff and it. These point toward the surface. Not surprisingly, their major interaction is with the surface z2 band. But t and 7r interact preferentially with different parts of the band, picking out those metal surface orbitals which have nodal patterns similar to those of the adsorbate. Diagram 116 shows this in the twofold geometry at hand the trff orbital interacts better with the bottom of the surface z2 band and the ir0 with the top of that band. 

Surface states can arise simply because the atomic bonding at a semiconductor surface is necessarily different from that in the bulk. For example, in a Si lattice, the bonds at the Si surface are not ftilly coordinatively saturated. To relieve this unsaturation, either a surface reconstruction can occur and/or bonds to the metallic material can be formed. This distinct type of surface bonding results in a localized electronic structure for the surface which is different from that in the bulk. The energies of these localized surface orbitals are not restricted to reside in the bands of the bulk material, and can often be located at energies that are inside the band gap of the semiconductor. Orbitals that reside in this forbidden gap region are particularly important, because they will require modifications of our ideal model of charge equilibration at semiconductor/metal interfaces. ... 

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