Orbitals, antibonding hydrogen—like

The signs have been chosen to make the orbitals antibonding for positive values of the constants. This choice is based on the assumption of nodeless radial functions for all orbitals. If hydrogen-like radial functions are used, not all constants will be positive. 

The new sp hybrids are generally shaped like the sp hybrids constructed earlier. They are directed orbitals—there is a fat lobe and a thin lobe, so they lead to quite efficient overlap with the hydrogen Is orbitals (Fig. 3.4). The directed sp hybrid orbital overlaps with the spherical Is orbital on hydrogen to produce a carbon-hydrogen o bond. The antibonding orbital (o ) is not shown in Figure 3.4. 

Methylation of the conformationally rigid exocyclic enolate (46) derived from 4-r-butylcyclohexyl methyl ketone has been shown to yield an 85 15 mixture of products (47) and (48) derived from equatorial and axial attack of the electrophile, respectively (Scheme 21). In species such as (46) geometric factors associated with a stereoelectronically controlled approach to either ir-face of the enolate are very similar. It has been suggested that the interaction of the filled p-orbital at C-1 with the vacant symmetrical antibonding orbital of the C(2)—C(3) and C(5)—C(6) bonds causes an increase in electron density on the equatorial ir-face of the enolate and favors approach of the electrophile from that side. However, the more generally accepted argument is that in a reactant-like transition state the axial hydrogen atoms at C-3 and C-5 hinder axial attack, while equatorial attack is relatively unencumbered. ... 

Now, the probability density between the two hydrogen atoms is very small, almost zero. So, the electron is most likely to be found somewhere other than between the nuclei. This orbital does not lead to a stable bond and we expect dissociation to H + H+. This is an antibonding orbital. We label the antibonding orbital a s and the bonding orbital a a. 

---