Highest occupied MO

A simple example would be in a study of a diatomic molecule that in a Hartree-Fock calculation has a bonded cr orbital as the highest occupied MO (HOMO) and a a lowest unoccupied MO (LUMO). A CASSCF calculation would then use the two a electrons and set up four CSFs with single and double excitations from the HOMO into the a orbital. This allows the bond dissociation to be described correctly, with different amounts of the neutral atoms, ion pair, and bonded pair controlled by the Cl coefficients, with the optimal shapes of the orbitals also being found. For more complicated systems... 

In the spirit of Koopmans theorem, the local ionization potential, IPi, at a point in space near a molecule is defined [46] as in Eq. (54), where HOMO is the highest occupied MO, p( is the electron density due to MO i at the point being considered, and ej is the eigenvalue of MO i. 

MR Molecular ReFraciivity llOMO Fnergy of Highest Occupied MO... 

According to one classification (15,16), symmetrical dinuclear PMDs can be divided into two classes, A and B, with respect to the symmetry of the frontier molecular orbital (MO). Thus, the lowest unoccupied MO (LUMO) of class-A dyes is antisymmetrical and the highest occupied MO (HOMO) is symmetrical, and the TT-system contains an odd number of TT-electron pairs. On the other hand, the frontier MO symmetry of class-B dyes is the opposite, and the molecule has an even number of TT-electron pairs. 

The He(Ia) photoelectron spectra of the parent heterocycles have been the subject of much study. Initially the assignment of the ionization energies to appropriate occupied molecular orbitals was confused by the unexpected reversal in the sequence of the two highest occupied MOs in tellurophene relative to the other heterocycles. The reported values are compared in Table 24. The assignments are based upon comparisons with the spectra... 

We will perform an AMI calculation on tetraphenylporphin (TPP), pictured at right, in order to examine the four important molecular orbitals for this molecule the second-highest and highest occupied MO s and the lowest and second-lowest unoccupied MO s (which we denote HOMO-1, HOMO, LUMO and LUMO-ll, respectively). 

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)). 

Several MO schemes are suggested, most with a single bond but differing to some extent on the ordering of the energy levels [78] (Figures 2.37 and 2.38). The most recent results indicate the highest occupied MO (HOMO) is of a-symmetry, consistent with ESR results on [Rh2(OCOR)4(PR3)2]+. 

The energies of the highest occupied MO (HOMO) and of the lowest unoccupied MO... 

On the other hand, this MO description matches a crystal field description of a Cu(II) ion insofar as the five highest occupied MO s are concerned four of these are doubly occupied and have a predominant copper 3 d character (65 % at least), whereas the fifth orbital contains the unpaired electron and still has 50% copper 3 d character. The very small MuUiken charge on the copper atom, which does not fit in the description of a Cu(II) ion, is caused by the partial occupation of the 4 p orbitals, which effect is neglected in the crystal field theory. 

Fig. 7.33. The symmetry of the highest occupied MO in the groundstate and the excited-state hexatriene molecules...

Wong and coworkers37 studied a series of both symmetrical and unsymmetrical tetraalkyltin compounds with different alkyl substituents, focusing their attention on the effect of these on the ionization energy of the highest occupied MOs. It is useful to recall the type of molecular orbitals deriving from the triply degenerate HOMO in the symmetrical R4M (local symmetry Td) upon substitution of one or more R ... 

In these compounds the highest occupied MOs are expected to be approximated as in-phase or out-of-phase combinations of the two nitrogen lone pairs nN and nN ... 

For polyenes in their ground state, the highest occupied MO (HOMO) will be symmetric with respect to m2 for 2, 6, 10. .. n electron systems and antisymmetric for 4, 8, 12. .. n electron systems. The lowest unoccupied MO (LUMO) has the symmetry opposite to that of HOMO. In the first excited state, the LUMO of the ground state will become singly occupied because of the promotion of an electron and it will thus become the new highest occupied orbital. Therefore in the circumstances the symmetry of the highest occupied orbital is opposite to that of the ground state. 

Fig. 5 a and b. Schematic representation of a highest occupied MO, b lowest unoccupied MO in hexa-triene... 

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