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

Abstract

Large-scale on-the-fly Born-Oppenheimer molecular dynamics simulations using recent advances in linear scaling electronic structure theory and trajectory integration techniques have been performed for protonated water clusters around the magic number (H(2)O)(n)H(+) , for n = 20 and 21. Besides demonstrating the feasibility and efficiency of the computational approach, the calculations reveal interesting dynamical details. Elimination of water molecules is found to be fast for both cluster sizes but rather insensitive to the initial geometry. The water molecules released acquire velocities compatible with thermal energies. The proton solvation shell changes between the well-known Eigen and Zundel motifs and is characterized by specific low-frequency vibrational modes, which have been quantified. The proton transfer mechanism largely resembles that of bulk water but one interesting variation was observed.

Velocity of a Molecule Evaporated from a Water Nanodroplet: Maxwell–Boltzmann Statistics versus Non‐Ergodic Events

“…[19] As seen in Figures 2a nd 3, the theoretical calculations (shifted by aconstant value of À740 ms À1 to account for the shorter detection window used in the simulation) well reproduce the behavior of the distribution function at low velocities.F or am ore quantitative comparison, Figure 4i llustrates the agreement and evolution of the distribution as afunction of the droplet size n,where the calculated and measured values of the mean hVi and the full-width-at-half-maximum DV are both observed to decrease and saturate with an increasing number of molecules in the droplet. Thep resent results for hVi are also in good agreement with the values obtained by Rybkin et al [12] using largescale Born-Oppenheimer molecular dynamics simulations (BOMD) for protonated water droplets near the magic number n = 21. They are also consistent with the conclusions reached by Va rilly and Chandler in the bulk limit.…”

“…[11] After excitation, the outof-equilibrium nanodroplet relaxes through the evaporation of one or several molecules.The protonated fragment is mass analyzed at least 80 ns after the collision. Thet ypical time required for evaporation is af ew picoseconds, [12] and further evaporation after the mass analysis of the residue is negligible. Thet otal kinetic energy released (KER) during the dissociation is partitioned because of momentum conservation, that is,t he evaporated molecule acquires an additional velocity randomly oriented in the center-of-mass reference frame (CMF) of the parent droplet.…”

“…In the cited study using infrared laser excitation, the deposited energies were much smaller than those involved in the high-energy collisions of the present study.Because of the COINTOF technique used here,s ingle-molecule evaporation can be specifically selected from among all the collision-induced evaporation events.S ince these events span the entire regime where none to all water molecules are evaporated, the measured velocity distributions of the present study (Figure 3) represent the most extensive data [12] based on ab initio moleculard ynamics trajectories (BOMD). The full black symbols correspond to the transition path sampling study (TPS) for ab ulk sample of 900 water molecules by Varilly and Chandler.…”

Velocity of a Molecule Evaporated from a Water Nanodroplet: Maxwell–Boltzmann Statistics versus Non‐Ergodic Events

“…For a more quantitative comparison, Figure 4 illustrates the agreement and evolution of the distribution as a function of the droplet size n , where the calculated and measured values of the mean 〈 V 〉 and the full-width-at-half-maximum Δ V are both observed to decrease and saturate with an increasing number of molecules in the droplet. The present results for 〈 V 〉 are also in good agreement with the values obtained by Rybkin et al12 using large-scale Born–Oppenheimer molecular dynamics simulations (BOMD) for protonated water droplets near the magic number n =21. They are also consistent with the conclusions reached by Varilly and Chandler in the bulk limit 22…”

The Dalton quantum chemistry program system

“…Potential-energy surfaces can be mapped by the calculation of intrinsic reaction coordinates. Born–Oppenheimer direct-dynamics studies can be performed by calculating classical molecular trajectories on the potential-energy surfaces, iteratively solving Newton's equation of motion for the atoms in the system, allowing dynamical studies of systems containing more than one hundred atoms 47,48…”