Molecular orbitals charge-transfer spectra

We should finally briefly discuss the calculation of spectra for the surface adsorbates which we will use to verify the theoretical models and to assign peaks in the spectra. The calculation of XES spectra has been discussed extensively previously [3]. Briefly we have shown that the ground state orbitals provide a balanced description of initial and final state and calculate the spectrum as the dipole transition between the valence orbitals and the selected Is core level [21]. The success of this approach relies on similar charge transfer screening in the core-ionized initial (or intermediate) state as for the valence-ionized levels. XES thus reflects the ground state molecular orbitals. 

The radical anion of molecular oxygen (O ) has been prepared and trapped in a range of alcohols, water and benzene but not in aliphatic hydrocarbons (Bennett et al., 1968a). In contrast to COg the e.s.r. spectrum shows that 0 interacts strongly with its immediate environment. This interaction which alters the separation of the upper molecular orbitals of the anion is strongly dependent on the nature of the matrix. Previously, the Oj" radical ion has been stabilized only in ionic materials such as the alkali halides thus it is of particular interest to find that this anion can be trapped successfully in a non-polar matrix (benzene). There is some evidence (Evans, 1961), from optical spectroscopic studies that molecular oxygen can form a weak charge transfer complex with the 77-electron system in benzene and it seems probable that O2 is stabilized in benzene by the formation of a similar complex. 

It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have e values of 20 L moF cm or less) molar absorp-tivities of 50,000 L mole cm or greater are not uncommon for these bands. Such absorption bands involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa). For example, consider an octahedral d complex with cr-donor ligands. The ligand electron pairs are stabilized, as shown in Figure 11-15. 

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