Singlet-triplet gaps in large multireference systems: Spin-flip-driven alternatives for bioinorganic modeling

Abstract

The proper description of low-spin states of open-shell systems, which are commonly encountered in the field of bioinorganic chemistry, rigorously requires using multireference ab initio methodologies. Such approaches are unfortunately very CPU-time consuming as dynamic correlation effects also have to be taken into account. The broken-symmetry unrestricted (spin-polarized) density functional theory (DFT) technique has been widely employed up to now to bypass that drawback, but despite a number of relative successes in the determination of singlet-triplet gaps, this framework cannot be considered as entirely satisfactory. In this contribution, we investigate some alternative ways relying on the spin-flip time-dependent DFT approach [Y. Shao et al. J. Chem. Phys. 118, 4807 (2003)]. Taking a few well-documented copper-dioxygen adducts as examples, we show that spin-flip (SF)-DFT computed singlet-triplet gaps compare very favorably to either experimental results or large-scale CASMP2 computations. Moreover, it is shown that this approach can be used to optimize geometries at a DFT level including some multireference effects. Finally, a clear-cut added value of the SF-DFT computations is drawn: if pure ab initio data are required, then the electronic excitations revealed by SF-DFT can be considered in designing dramatically reduced zeroth-order variational spaces to be used in subsequent multireference configuration interaction or multireference perturbation treatments.

Theoretical Exploration of the Oxidative Properties of a [(trenMe1)CuO2]+ Adduct Relevant to Copper Monooxygenase Enzymes: Insights into Competitive Dehydrogenation versus Hydroxylation Reaction Pathways

“…[43] The same conclusion applies here. This is in agreement with the experimental data reported for the related [(tren TMG )CuO 2 ] + adduct, which has an O À O distance of 1.280 , [45] and was recently shown to have a triplet fundamental state.…”

“…It is shown that injection of a supplementary electron into the product resulting from initial C À H bond cleavage may be the key to achieving hydroxylation of an alkyl group. The computational details of the present study are described in the Supporting Information and in references [41][42][43].…”

“…[43] The triplet transition states are, however, located about 10 kcal mol À1 above the singlet transition states. Hence, this rules out any reactivity on the triplet surfaces.…”

Mononuclear Cu–O2 Complexes: Geometries, Spectroscopic Properties, Electronic Structures, and Reactivity

“…33,34 A protocol that we have found particularly effective is using DFT to obtain molecular structures and then to correct singlet energies relative to those of triplets (which as single determinants are well treated by DFT) based on multireference second-order perturbation theory (CASPT2), 35 the latter being designed to handle multideterminantal character in a rigorous fashion. The utility of this approach was first demonstrated in a mechanistic study of the activation of molecular oxygen to generate 3a.…”

Understanding Lead Chemistry from Topological Insights: The Transition between Holo‐ and Hemidirected Structures within the [Pb(CO)n]2+ Model Series

“…Since bond lengths are not impacted, the stretching vibration should also remain unaffected in our PP computations. [119][120][121][122][123] …”