Generating Excess Protons in Microsolvated Acid Clusters under Ambient Conditions: An Issue of Configurational Entropy versus Internal Energy

Abstract

Acid dissociation, and thus liberation of excess protons in small water droplets, impacts on diverse fields such as interstellar, atmospheric or environmental chemistry. At cryogenic temperatures below 1 K, it is now well established that as few as four water molecules suffice to dissociate the generic strong acid HCl, yet temperature‐driven recombination sets in simply upon heating that cluster. Here, the fundamental question is posed of how many more water molecules are required to stabilize a hydrated excess proton at room temperature. Ab initio path integral simulations disclose that not five, but six water molecules are needed at 300 K to allow for HCl dissociation independently from nuclear quantum effects. In order to provide the molecular underpinnings of these observations, the classical and quantum free energy profiles were decomposed along the dissociation coordinate in terms of the corresponding internal energy and entropy profiles. What decides in the end about acid dissociation, and thus ion pair formation, in a specific microsolvated water cluster at room temperature is found to be a fierce competition between classical configurational entropy and internal energy, where the former stabilizes the undissociated state whereas the latter favors dissociation. It is expected that these are generic findings with broad implications on acid–base chemistry depending on temperature in small water assemblies.

Unveiling Zwitterionization of Glycine in the Microhydration Limit

“… 16 − 18 More recently, it has been shown that a minimum of four water molecules is required to dissociate a simple strong acid in the microhydration limit, HCl(H 2 O) n . 14 , 15 , 19 − 21 …”

“…Both formation and stabilization of charges are advantageous if plenty of solvating water molecules are available, notably in bulk aqueous solution. The situation is radically different under the limit of having only a very few water molecules available: A piece of metallic sodium is known to most vigorously dissolve in a bucket of water by dissociating into Na + (aq) and e – (aq), while Na­(H 2 O) n are quite stable molecular complexes for sufficiently small cluster sizes n as shown by computation and experiment. − More recently, it has been shown that a minimum of four water molecules is required to dissociate a simple strong acid in the microhydration limit, HCl­(H 2 O) n . ,,− …”

Zwitter Ionization of Glycine at Outer Space Conditions due to Microhydration by Six Water Molecules

“…The corresponding mechanism leading to proton transfer, e.g. for HCl, greatly differs from that of the very same reaction at high temperatures due to thermal activation at constant cluster size [24,25]. Thus, not only the reaction pathway but also the required number of water molecules may be different at ultra-low compared to ambient temperatures [22][23][24][25].…”

“…for HCl, greatly differs from that of the very same reaction at high temperatures due to thermal activation at constant cluster size [24,25]. Thus, not only the reaction pathway but also the required number of water molecules may be different at ultra-low compared to ambient temperatures [22][23][24][25]. Moreover, in case of glycine-water clusters, the energy differences between non-ionic and zwitterionic species are very small in the relevant cluster size regime [26], which hampers computational predictions.…”