Resolving the Structures of AlM2O4+ (M = Fe, Co) through Multireference Methods

Area of Science:

• Computational chemistry
• Physical chemistry
• Spectroscopy

Background:

• Multicenter transition metal clusters present complex electronic structures.
• Ligand-field splitting and spin coupling lead to dense electronic states, complicating vibrational spectral assignments.
• Previous interpretations of AlM2O4+ (M = Fe, Co) infrared photodissociation (IRPD) spectra were hindered by sensitivity to isomer and electronic state.

Purpose of the Study:

• To resolve the challenging vibrational spectral assignments for AlM2O4+ (M = Fe, Co) clusters.
• To identify the low-energy structures and ground-state electronic configurations of these clusters.
• To elucidate the factors governing the energetics and vibrational spectra of binuclear complexes.

Main Methods:

• Employed multireference methods within a localized active space framework to determine low-energy structures and electronic configurations.
• Utilized Kohn-Sham density functional theory for vibrational analysis.
• Correlated computational results with experimental IRPD spectra, ion mobility distributions, and reactivity data.

Main Results:

• Identified the coexistence of two structural isomers, both in low-spin ground states, as the source of experimental IRPD spectra.
• Calculated vibrational spectra for the identified ground states accurately reproduced experimental features.
• Demonstrated that ligand-field effects are dominant over spin coupling in influencing cluster energetics and vibrational spectra.

Conclusions:

• The assignment of IRPD spectra for AlM2O4+ (M = Fe, Co) is settled by identifying two coexisting low-spin isomers.
• Multireference methods are powerful tools for analyzing complex electronic structures in large active spaces required for binuclear complexes.
• Ligand-field effects play a crucial role in determining the properties of these transition metal clusters.