Ab Initio Molecular Dynamics Reveal Spectroscopic Siblings and Ion Pairing as New Challenges for Elucidating...

The characterization of prenucleation species is essential to understand crystallization mechanisms across many chemical systems and often involves the use of vibrational spectroscopy. Nowhere is this more evident than in the development of "green" aluminum processing technologies, where detailed understanding of the speciation of aluminum and its polynuclear analogues in highly alkaline, low water solutions is elusive. The aluminate anion Al(OH)₄^- predominates in alkaline conditions, yet equilibrium with dimeric species, either μ-oxo Al₂O(OH)₆^2- or di-μ-hydroxo Al₂(OH)₈^2-, can be assumed. Using ab initio molecular dynamics with full solvation and the presence of counterions, this work reconciles previous contradictory studies that had concluded only a single species under relevant solution conditions. We reveal that the two dimers are energetically separated by 2 kcal/mol in pure water but that the stability of each can be reversed by ion pairing expected in saturated salt solutions. Simulated Raman and IR spectra for each species (accounting for anharmonicity and the fluctuating solvating environment) provide the first proof that the considered species are "spectroscopic siblings", whose multiple overlapping bands prevent definitive assertions in terms of speciation when compared to the experimental spectra. These observations are likely to hold in higher order aluminate oligomers and as such present a massive challenge toward understanding the crystallization mechanisms relevant to aluminum processing.