Highest occupied molecular orbital, unpaired electron

Dealing with a molecular ion it is necessary to identify its ground state, that is to remove an electron from the highest occupied molecular orbital (HOMO). The most favorable sites for the charge and unpaired electron localization may be established by taking away an electron with minimal ionization energy. The energy requirements in this case are similar to these known in UV-spectroscopy for the electron transitions a < tt< n. 

The unusual properties of nitric oxide result from an unpaired electron in the highest occupied molecular orbital (HOMO). Nitrogen contains five valence electrons (electrons in the outermost shell with the greatest influence on bonding), while oxygen contains six valence electrons (Fig. 1). Therefore, nitric oxide contains a total of eleven valence electrons. Because orbitals can hold only two electrons with each electron possessing an opposite spin, there must be a single... 

Orbital is the highest occupied molecular orbital (HOMO) in the ground state. It corresponds to the structural formula of the molecule, with double bonds between and C2, and between C3 and C4. Orbital is likewise the lowest unoccupied molecular orbital (LUMO) in the ground state and corresponds to a biradical structure of the molecule with unpaired electrons on Gj and C4. Such a biradical structure can be a very simple but sometimes useful representation of the excited molecule (in states Si or T. ... 

In the donor-acceptor interaction, the acceptor provides its lowest unoccupied molecular orbital (LUMO) and the donor participates at the expense of its highest occupied molecular orbital (HOMO). These orbitals are frontier orbitals. In the corresponding ion radicals, the distribution of an unpaired electron proceeds, naturally, under frontier-orbital control. This definitely is reflected in ion radical reactivity. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion radicals and cation radicals. For the cyano vinylferrocene anion radical, the strong delocalization of an unpaired electron was observed see Section 1.2.2. This is accompanied by effective cis —> trans conversion (the barrier of rotation around the —C=C— bond is lowered). As for the cation radicals of the vinylferrocene series, a single electron remains in the highest molecular orbital formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the highest occupied molecular orbital is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without participation of the ethylenic fragment. The situation is quite different from the arylethylene radical cations, in which all ir-orbitals overlap. 

F. Fenske. We demonstrate for transition metal complexes that the non-empirical Fenske-Hall (FH) approach provides qualitative results that are quite similar to the more rigorous treatment given by density functional theory (DFT) and are quite different from Hartree-Fock-Roothaan (HFR) calculations which have no electron correlation. For example, the highest occupied molecular orbital of ferrocene is metal based for both DFT and FH while it is ligand (cyclopentadienyl) based for HFR. In the doublet (S = 1/2) cluster, Cp2Ni2(pi-S)2(MnCO)3, the unpaired electron is delocalized over the complex in agreement with the DFT and FH results, but localized on Mn in the HFR calculation. A brief description of the theory of FH calculations is used to rationalize the origin of its similarity to DFT. 

The term species may mean a discrete molecule, a simple or complex ion or even a solid exhibiting non-molecularity in one or more dimensions (graphite as an example). Free atoms seldom act as Lewis acids and bases. They usually have one or more unpaired electrons and flieir reactions are more accurately classified as free radical. The donor orbital is usually die highest occupied molecular orbital HOMO, and the acceptor orbital is usually the lowest unoccupied molecular orbital or LUMO. The molecular orbital definitions have a number of important consequences ... 

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