Once a carbocation is formed, it can rearrange its pattern of carbon-carbon and carbon-hydrogen bonding through a series of hydride and methyl shifts (Figure 4.12). These shifts proceed via stable ``bridged'' intermediates that are more stable than the starting carbocation. Consider the ethyl cation . The bridged intermediate for a [1,2]-hydride shift has a hydride lying above the molecular plane, in between the two carbons of ethyl cation; from this intermediate, a cation can be formed on either carbon. We find that eFF overestimates the extent of the hydride, making the bonds to it too long, and the complex becomes less stable rather than more stable than the carbocation ( = 10.1 kcal/mol vs -8.1 kcal/mol CCSD(T) ).
A similar phenomenon is observed for propyl cation, where a methyl carbanion can transfer from one carbon to another in a [1,2] methyl shift. We find that eFF makes the carbanion electrons large and unstable, and the complex is uphill rather than downhill in energy ( = 91.0 kcal/mol vs. kcal/mol MP4/6-311g** ). In propyl cation, a [1,3] hydride shift is possible as well. The carbon-hydride bond is overestimated by nearly the same amount as in the ethyl case (1.7 vs 1.4 exact), and the difference between estimated and exact energies are nearly the same as well (18 kcal/mol in both cases).