Orbital interactions on a surface

H2, N2, or CO dissociates on a surface, we need to take two orbitals of the molecule into account, the highest occupied and the lowest unoccupied molecular orbital (the HOMO and LUMO of the so-called frontier orbital concept). Let us take a simple case to start with the molecule A2 with occupied bonding level a and unoccupied anti-bonding level a. We use jellium as the substrate metal and discuss the chemisorption of A2 in the resonant level model. What happens is that the two levels broaden because of the rather weak interaction with the free electron cloud of the metal. 

When an atom or molecule is adsorbed on a surface new electronic states are formed due to the bonding to the surface. The nature of the surface chemical bond will determine the properties and reactivity of the adsorbed molecule. In the case of physisorption, the bond is rather weak, of the order of 0.3 eV. The overlap of the wave functions of the molecule and the substrate is rather small and no major change in the electronic structure is usually observed. On the contrary, when the interaction energy is substantially higher, there are rearrangements of the valence levels of the molecule, a process often denoted chemisorption. The discrete molecular orbitals interact with the substrate to produce a new set of electronic levels, which are usually broadened and shifted with respect to the gas phase species. In some cases completely new electronic levels emerge which have no resemblance to the original orbitals of the free molecule. 

The p orbital stabilizing interaction appears to be quite common. It also lowers the activation energy for the O and CO recombination reaction to form linear CO2. On a surface that interacts strongly with CO, e.g. a transition metal with partially filled d valence electron orbitals, the reaction proceeds as sketched in Fig. 4.47. 

Fig. 2. A pictorial representation of the possible interactions on spacecraft surfaces in low Earth orbit that involve atomic-oxygen.

Fig. 4. Schematic representation of the two-electron two-orbitals interaction between a donor sp orbital on a four coordinated 04c site of the MgO surface and the accepting orbitals of SO2. Reproduced from ref. [96]. Copyright 1994 Elsevier.

One-dimensional systems, 3, 125-126 Orbital interactions, 65-68 on a surface, 68-71, 107-111 two orbital... 

In the adsorption of ammonia on the surface, the NH3 2p lone-pair molecular orbital interacts with a transition-metal smface atom to form bonding and antibonding orbital fragments. The resulting 4d 2 electron distributions are also shown in Fig. 3.3b. 

Figure 3.6. OPDOS of the carbon atomic orbital adsorbed on a Ru surface with Ru d-valence electrons. Also the orbitals with maximum density are shown, (a) CO adsorbed atop (b) CO adsorbed three-fold. —, CT-Symmetric orbital interactions . .., tr-symmetric orbital interactions. The Fermi level is at -4.8 eV.

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

In diatomic molecules such as N2, O2, and CO the valence electrons are located on the 5cr, Ijt and 2jt orbitals, as shown by Fig. 6.6. [Note that the 5cr level is below the Ijt level due to interaction with the 4cr level, which was not included in the figure.] In general, the Ijt level is filled and sufficiently low in energy that the interaction with a metal surface is primarily though the 5cr and 2jt orbitals. Note that the former is bonding and the latter antibonding for the molecule. We discuss the adsorption of CO on d metals. CO is the favorite test molecule of surface scientists, as it is stable and shows a rich chemistry upon adsorption that is conveniently tracked by vibrational spectroscopy. 

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