Simulations of the Aqueous “Brown-Ring” Complex Reveal Fluctuations in Electronic Character

[Image: see text] Ab initio molecular dynamics (AIMD) simulations of the aqueous [Fe(H(2)O)(5)(NO)](2+) “brown-ring” complex in different spin states, in combination with multiconfigurational quantum chemical calculations, show a structural dependence on the electronic character of the complex. Sampling in the quartet and sextet ground states show that the multiplicity is correlated with the Fe–N distance. This provides a motivation for a rigid Fe–N scan in the isolated “brown-ring” complex to investigate how the multiconfigurational wave function and the electron density change around the FeNO moiety. Our results show that subtle changes in the Fe–N distance produce a large response in the electronic configurations underlying the quartet wave function. However, while changes in spin density and potential energy are pronounced, variations in charge are negligible. These trends within the FeNO moiety are preserved in structural sampling of the AIMD simulations, despite distortions present in other degrees of freedom in the bulk solution.