The Localized Orbitals of a CH2 Group

The appropriate valence atomic orbitals which must be considered are 2s, 2pj., 2p, on carbon, and the Is orbitals lsH and Ish. The orbital 2py is clearly nonbonding (n) relative to the carbon-hydrogen interactions and need not be considered further. The hydrogen orbitals can be combined into a (lsH + Ish) combination of a symmetry and a (lsH — ls ) combination of 7r symmetry. (Fig. 5). Although there are three available basic cr orbitals, only two of these belong to the C1H2 group proper. We can first eliminate the ( out ) 

Two 7r-type CH2 group orbitals can be constructed in a similar manner (Fig. 7). The two electrons in the bonding 7tCH2 combination bring the total number of CH2 electrons to four, corresponding to the two electron pairs in the CH bonds. The reader will find 7rCH2 

The procedure for determining the six localized orbitals of a CII3 group (Fig. 9), is similar to that for determining the four localized orbitals of a CH2 group. This time the basic valence orbitals can be divided into three sets a orbitals with either cylindrical or pseudo-cylindrical symmetry about the x axis a ttv orbitals, which 

The reader will find far more elegant drawings of these orbitals in Section III, for instance by referring to the orbitals (II1.0) of the pyramidal methyl radical. Similar orbitals exist, of course, for ammonia (II 1.8). Again, as in the CH2 case, the energy ordering is 

Now that we have established the major types of localized orbitals, we must inquire as to how they combine in a molecule to form delocalized combihations. 

---

It may be proper at this stage to lead the reader back to the stage where we constructed the localized orbitals of a CH2 group. At that time two valence orbitals were set aside—the 2pv orbital, and the outer (2s, 2pr) hybrid. Both of these orbitals lie in the. r, y plane. Now in our description of cyclopropane, we used bond orbitals to describe the CC bonding these bond orbitals are derived from in-plane (xy y) hybrids on each carbon. The two hybrids which are required on each carbon atom—in ordet to participate in two bond orbitals—are built precisely from the 2py orbital and the (2s, 2pj.) out combination on each CH2 group. 

---