From here, we next sought to gain insight into the course of the reaction (Figure ). We first computed the association complexes between FeCl3 and other components in the reaction. All attempts to locate a structure for "HFeCl4" through coordination of HCl to the iron center of FeCl3 led to dissociation of the HCl upon optimization. This indicates that HFeCl4 is not a well-defined minimum on the potential energy surface at this level of theory. In addition, the formation of the HCl---FeCl3 association complex is uphill (ΔG°/ΔH° = 5.5/-1.8 kcal•mol -1 ). We found that the most stable 1:1 complex is between t-BuOH and FeCl3 (t-BuOH + FeCl3 → t-BuOH-FeCl3) where ΔG°/ΔH° = -10.4/-21.7 kcal•mol -1 . Direct ionization from this complex to form the tert-butyl cation is significantly thermodynamically uphill (ΔG°/ΔH° = 26.2/39.9 kcal•mol -1 ), which is consistent with how FeCl3 has not been successful in catalyzing transformations with unactivated tert-alkanols. It is also not clear what role HCl would play in this process. Alternatively, HCl association with t-BuOH to form a hydrogen bonded complex is slightly unfavored (t-BuOH + HCl → t-BuOH-HCl) where (ΔG°/ΔH° = 2.0/-5.4 kcal•mol -1 ). However, putting FeCl3 near the HCl and optimizing the geometry results in spontaneous protonation of the alcohol to form the t-BuOH2 + ---[FeCl4] -ion pair. The ion pair is lower in free energy than the hydrogen bonded complex by 12.8 kcal•mol -1 , indicating that FeCl3facilitated protonation of the alcohol is competitive with direct coordination of FeCl3 to t-BuOH. From the ion pair, ionization to the tert-butyl cation is only 7.7 kcal•mol -1 uphill. Thus, the combination of FeCl3 and HCl provides a low energy pathway to the formation of the reactive tert-butyl cation.