Strong Electron Correlation in Nitrogenase Cofactor, FeMoco

Abstract

FeMoco, MoFeSC, has been shown to be the active catalytic site for the reduction of nitrogen to ammonia in the nitrogenase protein. An understanding of its electronic structure including strong electron correlation is key to designing mimic catalysts capable of ambient nitrogen fixation. Active spaces ranging from [54, 54] to [65, 57] have been predicted for a quantitative description of FeMoco's electronic structure. However, a wave function approach for a singlet state using a [54, 54] active space would require 10 variables. In this work, we systematically explore the active-space size necessary to qualitatively capture strong correlation in FeMoco and two related moieties, MoFeS and FeS. Using CASSCF and 2-RDM methods, we consider active-space sizes up to [14, 14] and [30, 30], respectively, with STO-3G, 3-21G, and DZP basis sets and use fractional natural-orbital occupation numbers to assess the level of multireference electron correlation, an examination of which reveals a competition between single-reference and multireference solutions to the electronic Schrödinger equation for smaller active spaces and a consistent multireference solution for larger active spaces.

Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds

“…Instead, we employed variational 2-electron reduced density matrix (V2RDM) techniques, 32 which have previously been demonstrated to successfully describe the electronic structure of a variety of strongly correlated large molecules. 33 V2RDM calculations were carried out as implemented in the Maple Quantum Chemistry Package. 34 The phenyl ligands were replaced with methyl groups and [18,20] active space V2RDM calculations with the 3-21G basis set were performed for both geometries providing the data shown in Table S7.…”

Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N 2 reduction

“…The energy of the pure spin states can be estimated using the Heisenberg spin ladder framework (14,15,(48)(49)(50), which might be problematic for the FeMo-co because of the large exchange coupling constants (14,15). A few recent pioneering studies point the way beyond DFT in addressing the nitrogenase problem (51)(52)(53). Conclusions drawn below are tempered by this recognition.…”

Computational Investigations of the Chemical Mechanism of the Enzyme Nitrogenase

“…A cluster of eight transition metals must have a complex electronic structure, a topic that has been discussed many times . The resting state of FeMo‐co has net S =3/2.…”