Lower free orbital level

In the case of bindings, the potential energy of molecules and surfaces varies. When the molecule approaches the surface, free electrons of the molecule orbital facilitate the loss of electric charge. In opposite, the free orbital of the surface facilitates the loss of electric charge of the molecule. According to Fukui [10], when molecules A and B interact, it stabilizes when the highest level of occupied orbital (HOMO) of A interacts with the lower free orbital level (unoccupied) (LUMO) of B and vice versa. 

In the free ion, the five 3c/ orbitals all have the same energy. In a crystal, these levels are split for example, if the ion occupied an octahedral hole, the 3c/levels would be split into a lower, triply degenerate level and a higher, doubly degenerate (e level. This is depicted in Figure 8.2. 

For the catalytic reaction, it was assumed that adsorbed acrolein reacts on free dihydrop)T an, as suggested by the thermodynamical results. In fact, the complexation of acrolein with a Lewis acid reduces the LUMO-HOMO gap to 6.9 eV and thus stabilizes the transition state of the reaction. The acrolein protonation lowers again the orbital level of the diene and in this case the gap is lowered to 2.2 eV. In all three cases (uncatalyzed, Lewis and Bronsted acid-catalyzed) the major orbital interaction leading to the transition state occurs between the LUMO of acrolein and the HOMO of dihydropyran. Moreover the maximum overlap principle predicts the same major regioisomer, in agreement with experimental results. 

The first Hnnd s rule states that the lower energy level should have the higher spin mnltiphcity. This implies that a maximnm nnmber of electrons have to be impaired. Therefore, electrons first fill individnal orbitals and only when no more free orbitals are available do the additional electrons... 

Fig. XVIII-16. A four-electron two-orbital interaction that a) has no net bonding in the free molecule but can be bonding to a metal surface if (b) the Fermi level is below the antibonding level. In the lower part of the figure, a zero-electron two-orbital situation (c) has no bonding but there can be bonding to a metal surface as in (d) if the Fermi level is above the bonding level. (From Ref. 160.)...

The first interaction, between CO 5c and Rh d,2, gives two new molecular orbitals. The bonding orbital has mostly 5<7 character, and it is customary to call it 5c. However, the level is lower than in free CO. UPS spectra reveal this shift immediately (see the spectrum of CO/Fe in Fig. 3.20) and indicate that its energy is close to that of the CO 1ft level. The antibonding chemisorption orbital has mainly d,2 character and is shifted upwards in energy. If the latter falls below the Fermi level, the 5c - d,2 interaction is entirely repulsive. For CO/Rh(100) the calculations indicate that the dz2 level falls across the Fermi level, such that the repulsion is partially relieved. 

The splitting of the free-ion term in octahedral symmetry Oh symmetry) reduces the degeneracy of the five d orbitals. Three orbitals have energy lower than the other two. This means that if the orbitals are populated by one electron, three degenerate states are possible, according to the three possible positions for the electron in the low-energy levels (T symmetry) ... 

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