Correlating DFT-calculated energy barriers to experiments in nonheme octahedral Fe(IV)O species

The experimentally measured bimolecular reaction rate constant, k(2), should in principle correlate with the theoretically calculated rate-limiting free energy barrier, ΔG(≠), through the Eyring equation, but it fails quite often to do so due to the inability of current computational methods to account in a precise manner for all the factors contributing to ΔG(≠). This is further aggravated by the exponential sensitivity of the Eyring equation to these factors. We have taken herein a pragmatic approach for C-H activation reactions of 1,4-cyclohexadiene with a variety of octahedral nonheme Fe(IV)O complexes. The approach consists of empirically determining two constants that would aid in predicting experimental k(2) values uniformly from theoretically calculated electronic energy (ΔE(≠)) values. Shown in this study is the predictive power as well as insights into energy relationships in Fe(IV)O C-H activation reactions. We also find that the difference between ΔG(≠) and ΔE(≠) converges at slow reactions, in a manner suggestive of changes in the importance of the triplet spin state weight in the overall reaction.