Correlating DFT-calculated energy barriers to experiments in nonheme octahedral FeIVO species

Abstract

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 FeIVO 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 FeIVO 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). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.