Singly occupied highest molecular

Like its neutral parent molecule, the NFg ion is a regular pyramid with Cgv symmetry. A single vacancy 6a in the doubly occupied highest molecular orbital 6ai of neutral NFg gives the ground term A [4]. Some excited states with orbital vacancies 5e la 4e 5a and 3e" are known from photoelectron spectra of NFg, see p. 185. 

If the number of electrons, N, is even, you can have a closed shell (as shown) where the occupied orbitals each contain two electrons. For an odd number of electrons, at least one orbital must be singly occupied. In the example, three orbitals are occupied by electrons and two orbitals are unoccupied. The highest occupied molecular orbital (HOMO) is /3, and the lowest unoccupied molecular orbital (LUMO) is 11/4. The example above is a singlet, a state of total spin S=0. Exciting one electron from the HOMO to the LUMO orbital would give one of the following excited states ... 

For example, in a 4-electron, 6-orbital CAS—specified as CASSCF 4,6)—performed on a singlet system, the active space would consist of the two highest occupied molecular orbitals (where the four electrons reside) and the four lowest virtual orbitals. Similarly, for a 6-electron, 5-orbital CAS on a triplet system, the active space would consist of the four highest occupied MO s— two of which are doubly-occupied and two are singly-occupied—and the LUMO (the keyword is CASSCF(6,5)). 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

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. 

In Chapter 4 we talked about the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of organic molecules. CH3 (like all radicals) has an orbital containing one electron, which we call a Singly Occupied Molecular Orbital (SOMO). 

The results of the extended Hiickel calculation indicate that the highest occupied molecular orbital (HOMO) (which is singly occupied) was indeed a 7r-type orbital in the NF2 radical. This supports the validity of computing the ionization potential from the Pople-SCF -electron energies. Furthermore, the energy of the HOMO (11.52 e.v.) is quite close to the experimental value. 

DFT = density functional theory HMQC = heteronuclear multiple quantum coherence SOMO = singly occupied molecular orbital PES = photoelectron spectroscopy Cl = configuration interaction LUMO = lowest unoccupied molecular orbital HOMO = highest occupied molecular orbital TLC = thin layer chromatography. 

Figure 11.3 (o) The interaction of a singly occupied molecular orbital with the lowest unoccupied molecular orbital of a pi acceptor, (b) The interaction of a singly occupied molecular orbital with the highest occupied molecular orbital of a pi donor. 

Start from the two-highest singly occupied molecular orbitals triplet state of A-B. When A = B, and are given by [54] ... 

Figure 2.2 Orbital energy diagram for (a) a molecule and (b) a radical. HOMO = highest occupied molecular orbital LUMO = lowest unoccupied MO SOMO = singly occupied MO...

In this equation, u denotes the irreducible representation spanned by uA and pB. From each pair of magnetic orbitals pA and uB localized on the A and B fragments respectively, one can construct two molecular orbitals delocalized on the whole molecule A - B. Each of these molecular orbitals is singly occupied in the pair state of highest spin multiplicity. We denote A,v the energy gap between these molecular orbitals for this state and Dw the energy gap between the two magnetic orbitals as shown below. 

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