O-antibonding orbitals

An electronic transition in which an n orbital electron is promoted to a o- antibonding orbital. See Absorption Spectroscopy... 

It follows from this analysis that any substituent, whether donor or acceptor, weakens the bond to which it is attached. Consider an acceptor substituent. Let M be the bond and N the substituent. The HOMO(M) is the o bonding orbital, which is depopulated by the M- N interaction. A similar reasoning shows that a donor will also weaken the bond because it will partially populate the o antibonding orbital. 

Figure 6. Interaction between a C-Si bonding orbital and a C-O antibonding orbital.

Complex molecules may not possess any symmetry elements, or if they do, the localizations of the electrons can so distort the electron cloud that its symmetry bears little relation to the molecular symmetry. In such cases it may be best to revert to a description of states in terms of the individual orbitals. As an example, we will consider formaldehyde, although a molecule as simple as this is probably best described by the group-theoretical term symbol of the last paragraph. The last filled orbitals in H2CO can easily be shown to be. ..(jtco)2 (no)2, where no represents the nonbonding orbital on the O atom and the two electrons in it are the lone pair. The first unfilled orbitals in formaldehyde are the tt 0 and rr o antibonding orbitals. Promotion of one... 

A theoretical study of the anomeric effect in 1,3-dioxole 9 involving interaction of the oxygen lone pairs with the C-O antibonding orbitals has provided an explanation for the unexpected nonplanarity observed for this compound <1996JA9850>. A combined theoretical and calorimetric study of the four benzodioxole compounds 10-13 has been used to obtain fundamental thermodynamic data for them <20040BC908>. The conformational preferences of 4,5-disubstituted-l,3-dioxolanes 14 have been analyzed computationally <1996T8275>. 

Both donates electron density from a bonding orbital on CO) and tt acceptance (which places electron density in C—O antibonding orbitals) would be expected to weaken the C—O bond and to decrease the energy necessary to stretch that bond. 

In an E2 elimination, the new n bond is formed by overlap of the C-H a bond with the C-X o antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

T-nonbonding, and two ir-nonbonding orbitals. Besides, there is a o-antibonding orbital that will play a decisive role in the A-H- X andA interactions. The anion is isoelectronic to FH and has the same electronic... 

Would you expect these two to go into the n antibonding orbital, 2p7Cg (lJtg), or the o antibonding orbital, 2pou (3ou) ... 

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