Correlating DFT-calculated energy barriers to experiments in nonheme octahedral Fe^IVO 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^IVO complexes. The approach consists of empirically determining two constants that would aid in predicting experimental k₂ 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 ^IVO 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. Useful correlations: Seven different synthetic nonheme Fe^IVO species were investigated with both theoretical and experimental methods in order to obtain a statistical base from which useful predictions and insights can be made (see figure).