Resonance, HOMO-LUMO interaction

Fig. 1 (a) Resonance and (b) HOMO-LUMO interaction in simple amides. 

Although resonance structures generally give enough information about the partial pluses and minuses to explain a molecule s reactivity (on the basis of opposite charges attracting each other), we occasionally need to refer to HOMO-LUMO interactions for additional help. Few explanations in this text rely just on HOMO-LUMO arguments. 

Schematic diagram showing the electronic configuration of a neutral (a) and transient negative ion (TNI) (b, c). The interacting electron initially captures into the unoccupied MOs of the neutral molecule resulting in TNI formation via (a) shape resonance or (c) core-excited resonance. For a shape resonance, the electron can interact with any unoccupied MO. The SOMO was the empty LUMO before the LEE interaction. In core-excited resonance, on electron interaction an electronic transition takes place from an inner shell to the vacant MOs creating a "hole"(-i- charge) in the inner shell, shown by an arrow (c). The up and down arrows show the occupancy of the molecular orbitals (MOs) with electrons of ocand 3 spins. HOMO highest occupied molecular orbital, LUMO lowest unoccupied molecular orbital, SOMO singly occupied molecular orbital...

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. 

With this simplification in mind, the stabilization energy AE can be given by equation 15, homo and lumo being orbital energies, C A, C A and Cjy, Cp being the relevant orbital coefficients at the carbon centers to which the new bonds are being formed fi AD and f A,D, are the resonance integrals for the overlap at the sites of interaction. 

Another possibility concerns the resonance integrals /Sab which appear in the Klopman-Salera equation. In a Hiickel picture, these are independent of the orbital energy, but in a double-zeta or better description we would expect the more tightly-bound electrons to have more contracted orbitals, and the higher virtual orbitals to be more diffuse.131 It may be that the HOMO and LUMO have the optimum spatial distribution for strong interaction, and that interactions involving more contracted and more diffuse orbitals are weaker.122... 

Experimental log k2 values were correlated with Brown para-localization energies, Dewar reactivity numbers, Herndon structure count ratios, Hess-Schaad resonance energy differences, indices of free valence, and second-order perturbation stabilization energies. The latter are based on Fukui s frontier orbital theory [67] which classifies the Diels-Alder reaction of benzenoid hydrocarbons with maleic anhydride as mainly HOMO (aromatic hydrocarbon)-LUMO (maleic anhydride) controlled. However, the corresponding orbital interaction energy given by... 

Abbreviations BCC. body centered cubic DOS. density of states ESR. electron spin resonance HX.AI S, extended X-ray absorption fine structure F CC. face centered cubic (a crystal structure). FID, free induction decay FT, Fourier transform FWHM, full width at half maximum HCP, hexagonal close packed HOMO, highest occupied molecular orbital IR, Infrared or infrared spectroscopy LDOS, local density of states LUMO, lowest unoccupied molecular orbital MAS. magic angle spinning NMR. nuclear magnetic resonance PVP. poly(vinyl pyrrolidone) RF. Radiofrequency RT, room temperature SEDOR, spin echo double resonance Sf, sedor fraction SMSI, strong metal-support interaction TEM. transmission electron microscopy TOSS, total suppression of sidebands. 

---