Alternative Low‐Energy Mechanisms for Isotopic Exchange in Gas‐Phase D₂O–H⁺(H₂O)_n Reactions

Abstract

Molecular-dynamics (MD) trajectories and high-level ab initio methods have been used to study the low-energy mechanism for D(2)O-H(+)(H(2)O)(n) reactions. At low collisional energies, MD simulations show that the collisional complexes are long-lived and undergo fast monomolecular isomerization, converting between different isomers within 50-500 ps. Such processes, primarily involving water-molecule shifts along a water chain, require the surmounting of very-low-energy barriers and present sizable non- Rice-Ramsperger-Kassel-Marcus (RRKM) effects, which are interpreted as a lack of randomization of the internal kinetic energy. Interestingly, the rate of water shifts was found to increase upon increasing the size of the cluster. Based on these findings, we propose to incorporate the following steps into the mechanism for low-energy isotopic scrambling these D(2)O-H(+)(H(2)O)(n) reactions: a) formation of the collisional complex [H(+)(H(2)O)(n)D(2)O]* in a vibro-rotational excited state; b) incorporation of the heavy-water molecule in the cluster core as HD(2)O(+) by means of isomerization involving molecular shifts; c) displacement of solvation molecules from the first shell of HD(2)O(+) inducing de-deuteration (shift of a D(+) to a neighbor water molecule); d) reorganization of the clusters and/or expulsion of one of the isotopic variants of water (H(2)O, HDO or D(2)O) from the periphery of the complex.

Isotope exchange in reactions between D2O and size-selected ionic water clusters containing pyridine, H+(pyridine)m(H2O)n

“…This observation is in good accord with Car-Parrinello simulations of proton migration in bulk water [144]. Furthermore, the simulations of Mella and Ponti indicate that isomerisation due to water shifts occur on the time scale 50-500 ps at room temperature, thereby not always allowing for full statistical energy randomization [111]. The rate of water shifts was found to increase upon increasing the size of the cluster.…”

Insights into the dynamics of evaporation and proton migration in protonated water clusters from Large‐scale Born–Oppenheimer direct dynamics

“…We also observe a putative contradiction between the conclusions from experimental[58] and computational[36] studies of proton transfer in protonated water clusters and the fact that the proton solvation shell in clusters in general is stable. [59] This contradiction may be resolved by realizing that proton transfer resulting in the reformation of a similar solvation structure is a fast and a rare event which does not affect the results of time‐unresolved experiments.…”

“…Mella and Ponti modelled such proton‐migration events in small water clusters ( n < 6) by performing extensive dynamics simulations with an empirical potential, as well as by conducting MP2 calculations of relevant single geometries of the PES. [36] According to these simulations, structural rearrangements induced by water migrations occur within 50–500 ps, driving the proton migration over negligible energy barriers, eventually leading to H/D randomization.…”

On the origin and consequences of high DMAEMA reactivity ratio in ATRP copolymerization with MMA: An experimental and theoretical study#

“…Additional insights on the microstructure of the MMA-DMAEMA (M-D) copolymers emerge noticing that a local decrease in probability of finding the ionizable monomer along a chain, de facto, lowers the probability of finding two such monomers sufficiently close to form a charged hydrogen bond interaction, 57,58 if one of them is protonated. To show that this is just the case, we have collected the probability of forming all possible triads as a function of the position of the first monomer in each triad.…”