The Reactivity-Enhancing Role of Water Clusters in Ammonia Aqueous Solutions

Abstract

Among the many prototypical acid–base systems, ammonia aqueous solutions hold a privileged place, owing to their omnipresence in various planets and their universal solvent character. Although the theoretical optimal water–ammonia molar ratio to form NH4 + and OH– ion pairs is 50:50, our ab initio molecular dynamics simulations show that the tendency of forming these ionic species is inversely (directly) proportional to the amount of ammonia (water) in ammonia aqueous solutions, up to a water–ammonia molar ratio of ∼75:25. Here we prove that the reactivity of these liquid mixtures is rooted in peculiar microscopic patterns emerging at the H-bonding scale, where the highly orchestrated motion of 5 solvating molecules modulates proton transfer events through local electric fields. This study demonstrates that the reaction of water with NH3 is catalyzed by a small cluster of water molecules, in which an H atom possesses a high local electric field, much like the effect observed in catalysis by water droplets [e2301206120PNAS2023120].

Enticing a Proton using Single Ammonia Molecule as Bait

“…These results can be attributed to the symbiotic properties of ammonia, a better proton acceptor, and water, which forms better hydrogen bond networks. The present results are in accord with the ability of ammonia to enhance the protolysis in the NH 3 –H 2 O mixtures, which are attributed to the enhanced local electric fields, a consequence of ammonia being a better hydrogen bond acceptor than water . However, this effect is dominant only at a lower relative mole fraction of ammonia, which is once again attributed to the synergistic properties of ammonia as a better proton acceptor and water for its hydrogen bonding network.…”

“…The present results are in accord with the ability of ammonia to enhance the protolysis in the NH 3 −H 2 O mixtures, which are attributed to the enhanced local electric fields, a consequence of ammonia being a better hydrogen bond acceptor than water. 47 However, this effect is dominant only at a lower relative mole fraction of ammonia, 47 which is once again attributed to the synergistic properties of ammonia as a better proton acceptor and water for its hydrogen bonding network. and electric field (blue curve) as a function of simulation time for TFAA-W 5 (top panel), AA-Am 1 -W 3 (middle panel), and PhOH-Am 1 -W 7 (bottom panel) clusters in the time window of proton transfer.…”

Designed Local Electric Fields─Promising Tools for Enzyme Engineering

“…For example, a theoretical study by Cassone showed that the presence of an electric field causes delocalization of the electron density of a water molecule, which lowers the ionization threshold resulting in altered kinetics of proton transfer . In fact, the presence of electric fields under the form of different ions significantly increased the hydrogen-bonding pattern of water clusters and induces electro-freezing, which has been validated both theoretically and experimentally. − A very recent experimental study by Coote and co-workers demonstrates, in accord with observations of Zare et al, that surface O–H groups of water microdroplets induce electric fields that catalyze cleavage of a dihalogen by pyrimidine.…”

My Vision of Electric-Field-Aided Chemistry in 2050

“…Further impact of the O−H electric fields was discussed for the mechanism of ammonium hydroxide formation in water. 49…”

“…More recently, Zhang, Xie et al reported that water droplets accelerate, by 7 orders of magnitude, the Menshutkin S N 2 reaction between pyridine and CH 3 I. Further impact of the O–H electric fields was discussed for the mechanism of ammonium hydroxide formation in water …”