In this work, we have examined the influence of scalar-relativistic effects, solvent effects and the nature of the exchange-correlation functional on the spin-state energy splittings of several transition metal complexes. In particular, we focus on the performance of OLYP and O3LYP. We also describe a new exchange-correlation density functional that provides the best agreement with the results of non-relativistic gas-phase Fixed-Node Diffusion Monte Carlo simulations on the spin-state splitting energies of four divalent iron complexes, [Fe(NCH)6]2+, [Fe(CO)6]2+, [Fe(H2O)6]2+ and [Fe(NH3)6]2+. For [Fe(NCH)6]2+, [Fe(CO)6]2+, [Fe(H2O)6]2+ and [Fe(NH3)6]2+, our new functional has a mean absolute error (MAE) of 0.18 eV relative to FN-DMC. Of the four data points, it is within 0.2 eV in three instances. Its performance is better than those of other DFT methods as well as CCSD(T). For a broader test, we examined the performance of DFT (OLYP, O3LYP and our new density functional as well as 10 other modern exchange-correlation functionals) for predicting the spin-state splitting energies of manganocene, [Fe(H2O)6]3+, [FeF6]3-, [V(H2O)6]2+, [Cr(H2O)6]3+, [Mn(H2O)6]2+, [Co(H2O)6]2+, [Ni(H2O)6]2+, [Ru(H2O)6]3+ and [Fe(phen)2(NCS)2]. We show that scalar-relativity, on average, stabilizes the low-spin state by about 3 kcal/mol. For the aquo complexes, second-coordination water ligands stabilize the low-spin state by up to 10-20 kcal/mol. OLYP and our new functional do very well for the aquo complexes and for [FeF6]3-. The most difficult cases studied in this work are manganocene and [Fe(phen)2(NCS)2]. For manganocene, M06L and MN15 are within 1 kcal/mol of the experimental estimate. For [Fe(phen)2(NCS)2], OLYP is within 1 kcal/mol of the experimental estimate. Unfortunately, we have not been able to come up with a DFT-based descriptor for determining the optimal amount of Hartree-Fock exchange needed for any given compound. We however discuss our findings in the context of reaction catalysis, specifically methane-to-methanol conversion in metal-exchanged zeolites.

Accurate computation of the spin state energetics of transition metal complexes. Role of exchange-correlation functional, scalar-relativistic effects and solvent effects: Application to methane-to-methanol conversion catalysis

256th National Meeting (2018)

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