Quantitative Account of the Bonding Properties of a Rubredoxin Model Complex [Fe(SCH3)4]q, q = -2, -1, +2, +3

Iron-sulfur clusters play important roles in biology as parts of electron-transfer chains and catalytic cofactors. Here, we report a detailed computational analysis of a structural model of the simplest natural iron-sulfur cluster of rubredoxin and its cationic counterparts. Specifically, we investigated adiabatic reduction energies, dissociation energies, and bonding properties of the low-lying electronic states of the complexes [Fe(SCH3)4]2-/1-/2+/3+ using multireference (CASSCF, MRCISD), and coupled cluster [CCSD(T)] methodologies. We show that the nature of the Fe-S chemical bond and the magnitude of the ionization potentials in the anionic and cationic [Fe(SCH3)4] complexes offer a physical rationale for the relative stabilization, structure, and speciation of these complexes. Anionic and cationic complexes present different types of chemical bonds: prevalently ionic in [Fe(SCH3)4]2-/1- complexes and covalent in [Fe(SCH3)4]2+/3+ complexes. The ionic bonds result in an energy gain for the transition [Fe(SCH3)4]2- → [Fe(SCH3)4]- (i.e., FeII → FeIII) of 1.5 eV, while the covalent bonds result in an energy loss for the transition [Fe(SCH3)4]2+ → [Fe(SCH3)4]3+ of 16.6 eV, almost half of the ionization potential of Fe2+. The ionic versus covalent bond character influences the Fe-S bond strength and length, that is, ionic Fe-S bonds are longer than covalent ones by about 0.2 Å (for FeII) and 0.04 Å (for FeII). Finally, the average Fe-S heterolytic bond strength is 6.7 eV (FeII) and 14.6 eV (FeIII) at the RCCSD(T) level of theory. © 2021 American Chemical Society. All rights reserved

Theoretical investigation of the ground X3Σ- state of nitrogen bromide

The spectroscopic constants and the dissociation energy of the ground X3Σ- state of NBr have been computed using correlated wave functions and correlation-consistent orbital basis sets up to quintuple ζ quality. Our best estimates for the equilibrium separation (re) is 1.780 Å and for the dissociation energy (De) 49.5 kcal/ mol, suggesting that previous estimates obtained from indirect experimental measurements are in error by as much as 15 kcal/mol. The spectroscopic constants computed at the RCCSD(T)/aug-cc-pV5Z level of theory are ωe = 694.5 cm-1, ωeχe = 4.14 cm-1, and αe = 0.0039 cm-1. The linear variation of De for the NX species (X = F, Cl, Br) supports a prediction of about 35 kcal/mol for the dissociation energy of NI

A first principles study of the acetylene-water interaction

We present an extensive study of the stationary points on the acetylene-water (AW) ground-state potential energy surface (PES) aimed in establishing accurate energetics for the two different bonding scenarios that are considered. Those include arrangements in which water acts either as a proton acceptor from one of the acetylene hydrogen atoms or a proton donor to the triple bond. We used a hierarchy of theoretical methods to account for electron correlation [MP2 (second-order Moller-Plesset), MP4 (fourth-order Moller-Plesset), and CCSD(T) (coupled-cluster single double triple)] coupled with a series of increasing size augmented correlation consistent basis sets (aug-cc-pVnZ, n = 2,3,4). We furthermore examined the effect of corrections due to basis set superposition error (BSSE). We found that those have a large effect in altering the qualitative features of the PES of the complex. They are responsible for producing a structure of higher (C2v) symmetry for the global minimum. Zero-point energy (ZPE) corrections were found to increase the stability of the C2v arrangement. For the global (water acceptor) minimum of C2v symmetry our best estimates are ΔEe = -2.87 kcal/mol (ΔE0= -2.04 kcal/mol) and a van der Waals distance of Re= 2.190 Å. The water donor arrangement lies 0.3 kcal/mol (0.5 kcal/mol including ZPE corrections) above the global minimum. The barrier for its isomerization to the global minimum is Ee = 0.18 kcal/mol; however, inclusion of BSSE- and ZPE-corrections destabilize the water donor arrangement suggesting that it can readily convert to the global minimum. We therefore conclude that there exists only one minimum on the PES in accordance with previous experimental observations. To this end, vibrational averaging and to a lesser extend proper description of intermolecular interactions (BSSE) were found to have a large effect in altering the qualitative features of the ground-state PES of the acetylene-water complex. © 2000 American Institute of Physics

The dissociation energies of NF(X 3Σ-) and NCl(X 3Σ-)

We have computed potential energy functions for the ground states (X 3Σ-) of NF and NCl using a series of correlation consistent basis sets ranging from double to sextuple zeta quality and including core-valence correlation effects in conjunction with coupled-cluster single and double excitations with perturbative treatment of triple excitations [CCSD(T)] and large internally contracted multireference configuration interaction (icMRCI) wave functions. The best estimates for the dissociation energies (De's) are 76.6±1.3 kcal/mol for NF and 64.6±1.3 kcal/mol for NCl, respectively. Our results suggest that previous experimental estimates for the dissociation energy of NCl are in error by as much as 15 kcal/mol. The calculated spectroscopic constants for NF and NCl are in good agreement with the measured constants. © 1997 American Institute of Physics

The Effect of Geometry, Spin, and Orbital Optimization in Achieving Accurate, Correlated Results for Iron-Sulfur Cubanes

Iron-sulfur clusters comprise an important functional motif in the catalytic centers of biological systems, capable of enabling important chemical transformations at ambient conditions. This remarkable capability derives from a notoriously complex electronic structure that is characterized by a high density of states that is sensitive to geometric changes. The spectral sensitivity to subtle geometric changes has received little attention from correlated, large active space calculations, owing partly to the exceptional computational complexity for treating these large and correlated systems accurately. To provide insight into this aspect, we report the first Complete Active Space Self Consistent Field (CASSCF) calculations for different geometries of the [Fe(II/III)4S4(SMe)4]-2 clusters using two complementary, correlated solvers: spin-pure Adaptive Sampling Configuration Interaction (ASCI) and Density Matrix Renormalization Group (DMRG). We find that the previously established picture of a double-exchange driven magnetic structure, with minute energy gaps (<1 mHa) between consecutive spin states, has a weak dependence on the underlying geometry. However, the spin gap between the singlet and the spin state 2S + 1 = 19, corresponding to a maximal number of Fe-d electrons being unpaired and of parallel spin, is strongly geometry dependent, changing by a factor of 3 upon slight deformations that are still within biologically relevant parameters. The CASSCF orbital optimization procedure, using active spaces as large as 86 electrons in 52 orbitals, was found to reduce this gap compared to typical mean-field orbital approaches. Our results show the need for performing large active space calculations to unveil the challenging electronic structure of these complex catalytic centers and should serve as accurate starting points for fully correlated treatments upon inclusion of dynamical correlation outside the active space.