The Dense Phase Reaction Mechanism – A New Pathway for Multi-Molecular Reactions

Classical kinetics in the gas and solution phases require chemical reactions to go either through unimolecular or bimolecular elementary steps. Elementary reactions involving three molecules or above are considered impractical due to the statistical improbability of a simultaneous collision involving more than two species. An inherent assumption in this theory is that of an `ideal` medium, meaning that there are no intermolecular interactions between reactant molecules. This assumption is increasingly shown to be naïve for many solutions¹, where transient or at times permanent supramolecular species are shown to form.

We now show a new mechanistic model for chemical reactivity that is adapted from recent advances in the understanding of homogeneous crystal nucleation². Here, a dynamic supramolecular aggregate of reactant molecules is first formed which induces a simultaneous reaction between multiple molecules. Experimental evidence for this mechanism will be shown in the self-assembly reaction of a polyoxometalate molecule where a seemingly elementary 6^th order reaction was observed.

In this reaction, a molecule containing one tungsten atom, [WO₂F₄]^-2 reacts with silicon dioxide in aqueous solution to form a single type of atomically precise 18 tungsten molecule [H₂F₆NaW₁₈O₅₆]^-7. An overview of the whole formation mechanism will be shown, with an emphasis on the dense phase reaction step.

References

(1) Sedlák, J. Phys. Chem. B 110, 4329-4338 (2006).

(2) J. De Yoreo et al., Science 349, 6760-1 -6760-6 (2015).