Orbitals antibonding molecular orbital

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... 

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. 

The molecular orbital description of the bonding in NO is similar to that in N2 or CO (p. 927) but with an extra electron in one of the tt antibonding orbitals. This effectively reduces the bond order from 3 to 2.5 and accounts for the fact that the interatomic N 0 distance (115 pm) is intermediate between that in the triple-bonded NO+ (106 pm) and values typical of double-bonded NO species ( 120 pm). It also interprets the very low ionization energy of the molecule (9.25 eV, compared with 15.6 eV for N2, 14.0 eV for CO, and 12.1 eV for O2). Similarly, the notable reluctance of NO to dimerize can be related both to the geometrical distribution of the unpaired electron over the entire molecule and to the fact that dimerization to 0=N—N=0 leaves the total bond order unchanged (2 x 2.5 = 5). When NO condenses to a liquid, partial dimerization occurs, the cis-form being more stable than the trans-. The pure liquid is colourless, not blue as sometimes stated blue samples owe their colour to traces of the intensely coloured N2O3.6O ) Crystalline nitric oxide is also colourless (not blue) when pure, ° and X-ray diffraction data are best interpreted in terms of weak association into... 

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