Reassessment of the Mechanisms of Thermal C−H Bond Activation of Methane by Cationic Magnesium Oxides: A Critical Evaluation of the Suitability of Different Density Functionals

Abstract

The mechanisms of the thermal reactions of the two iconic magnesium oxide cations MgO.+ and Mg2O2.+ with methane have been re‐evaluated at the CCSD(T)/CBS//CCSD/def2‐TZVP level of theory. For the reaction of MgO.+ with CH4, only the classical hydrogen‐atom transfer (HAT) was found; in contrast, for the Mg2O2.+/CH4 couple, both HAT and proton‐coupled electron‐transfer (PCET) exist as mechanistic variants. In order to evaluate the suitability of density functional theory (DFT) methods, the reactions were computed by using 27 density functionals. The results obtained demonstrate that the various DFT methods often deliver rather different results for both geometric and energetic features. As to the prediction of the apparent barriers, pure functionals give the largest mean absolute errors. BMK, ωB97XD, and the double‐hybrid functional mPW2PLYP were confirmed to come closest to the results provided by CCSD(T)/CBS. Thus, mechanistic conclusions based on a single DFT method should be viewed with great caution. In summary, this study may assist in the selection of a suitable quantum chemical method to unravel the mechanistic details of C−H bond activation by charged metal oxides.

Old School Techniques with Modern Capabilities: Kinetics Determination of Dynamical Information Such as Barriers, Multiple Entrance Channel Complexes, Product States, Spin Crossings, and Size Effects in Metallic Ion–Molecule Reactions

“…B3LYP/TZVP. t Reference . Various functionals, MP2, B2PLYP with 6-311+G­(d,p) and def2-TZVP. u Reference . v N/A is not available. …”

“…The agreement is good indicating that the Schwarz and co-workers calculated the energy of the ratelimiting transition state in the MgO + + CH 4 system using a variety of DFT functionals with the def2-TZVP basis set. 86 We can compare those calculations to our statistical model value of −37 ± 10 kJ mol −1 in a similar fashion. For this system, the unsigned average deviation is −5 kJ mol −1 , the MAD is 14 kJ mol −1 and the RMSE is 19 kJ mol −1 , indicating a 2σ uncertainty in the calculated values of about 35 kJ mol −1 , half that of the transition metal systems.…”

Thermal activation of methane by MgO+: temperature dependent kinetics, reactive molecular dynamics simulations and statistical modeling

“…19,20 Reaction 1 has been previously investigated at room temperature using an ion trap apparatus and through ab initio calculations 9 and computationally from density functional theory and higher-level correlated calculations. 21 Although oxygen atom transfer from the oxide cation forming methanol is energetically preferred, it was observed for < 2% of reactions.…”

Revisiting the Intriguing Electronic Features of the BeOBeC Carbyne and Some Isomers: A Quantum‐Chemical Assessment

“…Thus, the CCSD(T) results may not be sufficiently reliable. Moreover, M06‐2X, widely used for Be‐containing systems, predicts even a reversed trend of stability, as does the ωB97 functional, which demonstrated excellent performance for MgO + and Mg 2 O 2 + . Furthermore, out of 45 tested DFT functionals, 11 predict that 6 prefers a triplet ground state; for details, see Table S2.…”

“…Moreover, M06-2X, widely used for Be-containing systems, [21] predicts even a reversed trend of stability, as does the wB97 functional, which demonstrated excellent performance for MgO + and Mg 2 O 2 + . [22] Furthermore, out of 45 tested DFT functionals, 11 predict that 6 prefers a triplet ground state; for details, see Table S2. Since the IR spectroscopic features of 3 6 and 5 6 are practically indistinguishable (see Tables S3 and S4), an experimental differentiation and unambiguous assignment of these ferroand antiferromagnetically coupled states would require an approach based on the different magnetic properties of the two multiplicities.…”