Spectroscopic Signature of Dynamical Instability of the Aqueous Complex in the Brown‐Ring Nitrate Test

The chemistry of the brown‐ring test has been investigated for nearly a century. Though recent studies have focused on solid state structure determination and measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. We have studied structural fluctuations of the brown‐ring complex in aqueous solution with ab‐initio molecular dynamics simulations, from which we identified that the classically established pseudo‐octahedral [Fe(H(2)O)(5)(NO)](2+) complex is present along with a square‐pyramidal [Fe(H(2)O)(4)(NO)](2+) complex. Based on the inability in multi‐reference calculations to reproduce the experimental UV‐vis spectra in aqueous solution by inclusion of thermal fluctuations of the [Fe(H(2)O)(5)(NO)](2+) complex alone, we propose the existence of an equilibrium between pseudo‐octahedral and square‐pyramidal complexes. Despite challenges in constructing models reproducing the solid‐state UV‐vis spectrum, the advanced spectrum simulation tool motivates us to challenge the established picture of a sole pseudo‐octahedral complex in solution.