States antibonding unoccupied

Figure Bl.6.11 Electron transmission spectrum of 1,3-cyclohexadiene presented as the derivative of transmitted electron current as a fiinction of the incident electron energy [17]. The prominent resonances correspond to electron capture into the two unoccupied, antibonding a -orbitals. The negative ion state is sufficiently long lived that discrete vibronic components can be resolved.

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

The other two n molecular orbitals are antibonding molecular orbitals, i) In the ground state these orbitals are unoccupied. 

Hybridization. The simplest way to think about bonding in [1.1.1 Jpropellane is to characterize the state of hybridization at the two types of carbon atoms and then combine the hybrids pairwise into localized bond orbitals. In ordinary molecules, these have electron occupancy close to two, and the corresponding antibonds are nearly unoccupied. Bond delocalization can then be introduced by considering interactions of the occupied localized bond orbitals with the unoccupied localized antibond orbitals. 

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