the lower energy spin state (doublet, D, or quartet, Q) of the pre-catalyst 1a-Co was
first addressed. The energy difference between these two spin states of 1a-Co was
calculated using a range of functionals with different amount of HF exchange. For
comparison purposes post-Hartre-Fock methods were also used. Figure 12 displays
the computed quartet-doublet energy differences.
As already mentioned, higher spin states are stabilized with respect to lower spin
states by increasing the amount of the exact exchange in the exchange-correlation
functional [73]. This trend is also found for complex 1a-Co, with the doublet state
higher in energy than the quartet state. Pure functionals give rise to an energy
difference close to 10 kcal mol
À1 . Hybrid functionals range from 20 to
30 kcalÁmol
À1 , depending on the exact exchange introduced. Post-Hartree-Fock
methods give the highest energy differences, with values of more than 50 kcalÁmol
À1 .
Overall, the energy difference between doublet and quartet spin states are highly
Scheme 6 Cobalt-catalyzed cyclohydroamination [60]
Fig. 12 Energy difference between the quartet (Q) and doublet (D) spin states of 1a-Co computed
with different methods. In all cases, the quartet state is more stable [60]
What Makes a Good (Computed) Energy Profile?
21