Lowest energy occupied orbitals

The lowest energy molecular orbital of singlet methylene looks like a Is atomic orbital on carbon. The electrons occupying this orbital restrict their motion to the immediate region of the carbon nucleus and do not significantly affect bonding. Because of this restriction, and because the orbital s energy is very low (-11 au), this orbital is referred to as a core orbital and its electrons are referred to as core electrons. 

Singlet methylene also possesses unoccupied molecular orbitals. The unoccupied orbitals have higher (more positive) energies than the occupied orbitals, and these orbitals, because they are unoccupied, do not describe the electron distribution in singlet methylene. Nevertheless, the shapes of unoccupied orbitals, in particular, the few lowest energy unoccupied orbitals, are worth considering because they provide valuable insight into the methylene s chemical reactivity. 

The CC and ECC energies are in millihartree relative to the corresponding fiill Cl energy values. The full Cl energies E are reported as - ( + 107) in hartree. The lowest two occupied orbitals were frozen in the correlated calculations. 

Configuration interaction (Cl) is conceptually the simplest procedure for improving on the Hartree-Fock approximation. Consider the determinant formed from the n lowest-energy occupied spin orbitals this determinant is o) and represents the appropriate SCF reference state. In addition, consider the determinants formed by promoting one electron from an orbital k to an orbital v that is unoccupied in these are the singly excited determinants ). Similarly, consider doubly excited (k, v,t) determinants and so on up to n-tuply excited determinants. Then use these many-electron wavefimctions in an expansion describing the Cl many-electron wavefunction [Pg.13]

The reduction of DMSO catalyzed by molybdenum is an important step in the process of anaerobic respiration carried out by a number of bacteria (169). Much like sulfite oxidase, early MCD studies of DMSO reductase were complicated by the presence of heme iron (173). The discovery of two enzymes that do not include an iron center led to the measurement of MCD spectra of Rhodobacter sphaeroides DMSO reductase that could be assigned exclusively in terms of transitions of the Mo site (Fig. 10b) (174). The six major peaks are assigned as LMCT transitions from the three highest energy occupied orbitals to the two lowest unoccupied orbitals (174). 

The nitro group in the parent nitrobenzene evidently acts as ir-acceptor, which pulls the electron density out of the aromatic ring. An unpaired electron will obviously occupy the first vacant it-orbital of the nitro fragment (i.e., the lowest-energy-fragment orbital). Interaction between occupied and vacant orbitals is the most favorable. In the nitrobenzene... 

To construct the HF determinant we used only occupied MOs four electrons require only two spatial component MOs, pi and i//2, and for each of these there are two spin orbitals, created by multiplying ijj by one of the spin functions a or jl the resulting four spin orbitals (i/qa, pi/31p2x, i//2/ ) are used four times, once with each electron. The determinant the HF wavefunction, thus consists of the four lowest-energy spin orbitals it is the simplest representation of the total wavefunction that is antisymmetric and satisfies the Pauli exclusion principle (Section 5.2.2), but as we shall see it is not a complete representation of the total wavefunction. 

Butadiene has two Jt bonds and so four electrons in the n system. Which molecular orbitals are these electrons in Since each molecular orbital can hold two electrons, only the two molecular orbitals lowest in energy are filled. Let s have a closer look at these orbitals. In, the lowest-energy bonding orbital, the electrons are spread out over all four carbon atoms (above and below the plane) in one continuous orbital. There is bonding between all the atoms. The other two electrons are in 2. This orbital has bonding interactions between carbon atoms 1 and 2, and also between 3 and 4 but an antibonding interaction between carbons 2 and 3. Overall, in both the occupied Jt orbitals there are... 

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