Lowest unoccupied molecular orbital positions, measurement

The method involves the irradiation of a sample with polychromatic X-rays (synchrotron radiation) which inter alia promote electrons from the innermost Is level of the sulfur atom to the lowest unoccupied molecular orbitals. In the present case these are the S-S antibonding ct -MOs. The intensity of the absorption lines resulting from these electronic excitations are proportional to the number of such bonds in the molecule. Therefore, the spectra of sulfur compounds show significant differences in the positions and/or the relative intensities of the absorption lines [215, 220, 221]. In principle, solid, liquid and gaseous samples can be measured. 

FIGURE 3.4 Molecular level alignment diagrams constructed using the HOMO and vacuum levels measured using UPS. The lowest unoccupied molecular orbital LUMO positions are inferred assuming a HOMO/LUMO gap equal to the onset of optical absorption. The chemical structure of CuPc is shown. (From Hill, I.G. and Kahn, A., J. Appl. Phys., 86, 2116, 1991. With permission.)... 

For those electrophiles EV) that undergo direct electron-transfer reduction at an inert electrode (glassy carbon) [with the electron occupying the Lowest-Unoccupied-Molecular-Orbital (LUMO)], the reduction potential (fired) is a measure of their electron affinity and electrophilicity [relative to that for H O (—2.10 V vs NHE in aqueous media)] (the more positive the potential, the more electrophilic the molecule). 

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