Benchmarking Plane Waves Quantum Mechanical Calculations of Iron(II) Tris(2,2′-bipyridine) Complex by X-ray Absorption Spectroscopy

Nico Sanna,Maurizio Benfatto

doi:10.3390/condmat7010016

Abstract

In this work, we used, for the first time, a computational Self-Consistent Field procedure based on plane waves to describe the low and high spin conformational states of the complex [Fe(bpy)3]2+. The results obtained in the study of the minimum energy structures of this complex, a prototype of a wide class of compounds called Spin Cross Over, show how the plane wave calculations are in line with the most recent studies based on gaussian basis set functions and, above all, reproduce within acceptable errors the experimental spectra of X-ray absorption near-edge structure spectroscopy (XANES). This preliminary study shows the capabilities of plane wave methods to correctly describe the molecular structures of metal-organic complexes of this type and paves the way for future even complex computational simulations based on the energy gradient, such as Nudge Elastic Band or ab-initio Born-Oppenheimer molecular dynamics.

Highlights

X-Ray Absorption spectroscopy (XAS) is a powerful tool to investigate both the electronic and geometrical structure around to a well-defined absorbing atom belonging to any type of material, from biological samples to condensed matter
We have recently published [13] a revision of the minimum energy structures of the low spin (LS) and high spin (HS) states of the complex [Fe(bpy)3 ]2+ which confirms, for the singlet state (LS), the symmetric structure belonging to the D3 group of symmetry, while, for the transient quintet state, we have discovered a non-symmetric structure (C1 ) which, hypothesized by Quantum Mechanical (QM) calculations based on density functional using a PBE/6-311+G** method, has been further confirmed by the analysis of X-ray absorption nearedge structure spectroscopy (XANES) spectra, validating the distorted structure, and by ultrafast spectroscopic experiments assigning the Fe-N stretching frequencies observed experimentally with an asymmetrical environment around the central Fe2+ atom of the complex
In this work, we present a first step toward the dynamical study of this intriguing molecular system by setting up a computational procedure using plane wave Self-Consistent Field (SCF) codes (Quantum ESPRESSO–PWscf [16])

Summary

Introduction

X-Ray Absorption spectroscopy (XAS) is a powerful tool to investigate both the electronic and geometrical structure around to a well-defined absorbing atom belonging to any type of material, from biological samples to condensed matter. XAS spectroscopy is based on the measurement of absorption coefficient μ(E) as a function of energy, where μ(E) can be written as μ(E) = na σ(E), where na is the density of the absorbing medium. The quantity σ(E) is the total absorption cross section for the excitation of an electron from the core level (typically s and p orbitals) to continuum states. This can be calculated on the basis of the so-called Fermi “golden rule”: with regard to jurisdictional claims in published maps and institutional affiliations.

Methods

Findings

Discussion

Conclusion