Hydration dynamics gives the distinctive brown color 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 the measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. From ab initio molecular dynamics simulations of the brown-ring complex in aqueous solution, we have identified that the classically established pseudo-octahedral [Fe(H2O)5(NO)]2+ complex is in equilibrium with a square-pyramidal [Fe(H2O)4(NO)]2+ complex through the exchange of one of the coordinated H2O molecules. We also find, using ab-initio multi-reference methods, that the mixture of these two complexes is what gives the distinctive brown coloration to the brown-ring test. We show that its UV-vis spectrum can be theoretically reproduced only by accounting these two species and not the [Fe(H2O)5(NO)]2+ complex alone. The energetics of the two complexes are also investigated with multi-reference methods.