Antibonding molecular

This is the area of greatest interest to quantitative biochemistry and is dependent upon the electronic structure of carbon compounds. Absorption of radiation in this region of the spectrum causes transitions of electrons from molecular bonding orbitals to the higher energy molecular antibonding orbitals. 

Empirical correlations have been established between pre-edge band intensity and coordination charge (derived from MO valence-bond concepts) (67, 224, 312) or the composition of molecular antibonding orbitals, corresponding to bound states (243). Local orbital symmetry considerations explain simply the intensity of the K pre-edge of first-row transition-metal complexes (28,101,256,261,270) and the detailed edges in a variety of other organometallic complexes (112, 258). 

The triplet excited state of H2 is obtained by promoting an electron to a higher-energy molecular orbital. This higher-energy (antibonding) orbital is written and can be considered to arise from two Is orbitals as follows ... 

Valence Atomic Orbitals on Neighboring Atoms Combine to Form Bonding, Non-Bonding and Antibonding Molecular Orbitals... 

The bonding n molecular orbital pair (with m = +1 and -1) is of Tty symmetry whereas the corresponding antibonding orbital is of Tig symmetry. Examples of such molecular orbital symmetries are shown above. 

Unfortunately, these methods require more technical sophistication on the part of the user. This is because there is no completely automated way to choose which configurations are in the calculation (called the active space). The user must determine which molecular orbitals to use. In choosing which orbitals to include, the user should ensure that the bonding and corresponding antibonding orbitals are correlated. The orbitals that will yield the most correlation... 

A molecular orbital description of benzene has three tt orbitals that are bonding and three that are antibonding Each of the bonding orbitals is fully occupied (two electrons each) and the antibonding orbitals are vacant... 

This displays the LUMO of ethylene This IS an unoccupied antibonding molecular orbital... 

The remaining AOs are the four H 1, two C 1, and four C 2p orbitals. All lie in the molecular plane. Only two combinations of the C 2s and H U orbitals meet the molecular symmetry requirements. One of these, nearest-neighbor atoms. No other combination corresponds to the symmetry of the ethylene molecule. 

As is tr-ue for all orbitals, a tt orbital may contain a maximum of two electrons. Ethylene has two tt electrons, and these occupy the bonding tt molecular- orbital, which is the HOMO. The antibonding tt molecular orbital is vacant, and is the LUMO. 

In ethylene, both the HOMO and LUMO are formed primarily from p orbitals from the two carbons. The carbons lie in the YZ-plane, and so the p,j orbitals lie above and below the C-C bond. In the HOMO, the orbitals have like signs, and so they combine to form a bonding n molecular orbital. In contrast, in the LUMO, they have opposite signs, indicating that they combine to form an antibonding Tt molecular orbital. 

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