The formation of sulfate, a major component of fine particles posing significant threats to human health, is closely linked to the speciation of Fe(III) complexes with S(IV) at the air-water interface, a crucial yet underexplored aspect of atmospheric sulfur cycling. The speciation of Fe(III)-sulfito complexes at the interface remains poorly understood and thus hinders the understanding of the oxidizing properties of transition metals in atmospheric aerosol. Here, we reveal the coordination preference of the active reaction center (Fe(III)-sulfito complex) and the ligand substitution mechanism between Fe(III)-sulfito complexes and H2O at the air-water interface. Our results demonstrate that the Fe(III) complexes coordinated by HSO3- undergo a rapid and spontaneous transformation from S-coordination to O-coordination within picoseconds. Furthermore, the substitution reaction of HSO3- in Fe(H2O)4(OH)(HSO3)+/Fe(H2O)3(OH)2(HSO3) by the H2O nucleophile proceeds via both a kinetically and thermodynamically favorable pathway, characterized by free energy barriers of ∼6.39/3.81 kcal/mol. These findings further demonstrate that HSO3- preferentially occupies the second hydration shell of hydrolyzed Fe(III) complexes. The dynamic equilibrium between competing coordination forms provides molecular-scale insights into interfacial metal-sulfur chemistry, where distinct Fe(III) complexes govern the catalytic behavior of the system, thereby bridging coordination chemistry and atmospheric science.
Zhang et al. (Thu,) studied this question.