Reports: UR655264-UR6: Quantum simulations of lithium ion solvation dynamics in mixed Stockmayer clusters
printer friendlyBeyond the hexamer, the global minimum structures are quite complex and in some cases rather unusual compared to the familiar growth pattern seen in other clusters. The molecules in the second coordination layer aggregate at first by coordinating directly the vertices of the octahedral hexamer. However, with five or more molecules coordinated to the octahedron, the balance between the energy gained by adsorbing directly to a vertex and the energy gained by orienting the molecules in the second layer along the same direction, begins to shift in favor of the latter. In the n = 11,12,13 range, rather than binding at vertices that are as far apart as possible, the molecules in the second layer coordinate adjacent vertices of the octahedron, and these “lines of molecules" fold into a braid to gain the additional stability of aligning their mutual dipoles. Beginning with n = 14, the global minima structures resemble those observed in the work of Lu and Singer, where rings of solvent molecules crown the octahedral sheath and grow asymmetrically, always crowning the same vertex. By comparing the global minima found in our investigation with those of the corresponding pure Stockmayer clusters we conclude that the charge - dipole interaction profoundly impacts the Stockmayer clusters structures, as anticipated.
The Metropolis algorithm enhanced by the Parallel Tempering strategy is used to measure internal energies and heat capacities. The octahedral feature remains the dominant theme in the structure of clusters with n > 6 at all the temperatures investigated. The first “magic number" size is identified using successive differences in the adiabatic solvent dissociation energies. This size is taken to represent the completion of the second solvation layer for the lithium ion - nitromethane clusters. It corresponds to the n = 18 system, a solvated ion with the first sheath having octahedral symmetry, weakly bound to an eight - membered and a four - membered ring crowning a vertex of the octahedron. For n = 9, 10, 11, 12, 14, and 18, one can clearly distinguish three peaks in the heat capacity, providing evidence of a distinct pattern where solid-to-solid changes evolve with increasing size to take on more of a melting like character. At the same time, those peaks that one would call “melting” as they appear to be in the middle for n = 9, 10, 11, 12, and 14 seem to morph gradually into the “boiling” peaks. This behavior is similar to what has been observed in mid-sized Lennard–Jones clusters. The pattern has been confirmed by a inherent structure analysis at several temperatures.
We have used Diffusion Monte Carlo, enhanced with importance sampling and a bias-free Smart Darting strategy to determine the ground state energies and wave functions for Li+(nitromethane)n (n = 8 – 20). The zero point energy is approximately 5% of the total binding energy and seems to be size independent in the range considered. The Smart Darting Diffusion Monte Carlo simulations find the same ground state energy and mixed-distribution as the traditional approach in the n = 8 – 13 range. For n > 13 we find that the ground state energies agree quantitatively with or without smart darting moves, but the population distributions can be significantly different. We note that the relatively large mass of nitromethane confines the distribution for clusters with less than 16 dipoles to a single minimum. Implying that the ground state wavefunction only occupies that minimum and none others, because the trial wavefunction used to provide importance sampling was chosen to spread over several minima. Nevertheless two isomers are identified in separate Diffusion Monte Carlo simulation carried out without darting moves. These are therefore likely to be long-lived states detectable in experiments.
Some evidence is offered to conclude that introducing Smart Darting - like moves in traditional DMC simulations may produce a more reliable ground state distribution. The lightness of the lithium ion produces substantial nuclear quantum effects and the combination of these together with the profound impact the ion – dipole interaction has on the structure and thermodynamics of the Stockmayer clusters should make the dynamics that follow a charge transfer into a solvated lithium ion both interesting and insightful. Both Variational and Diffusion Monte Carlo estimates of the adiabatic solvent dissociation energy reveal that quantum effects further enhance the stability of the n = 18 system relative to its neighbors. Therefore, this size seems particularly suited for future equilibrium and dynamic path integral simulations. The next step in the investigation proposed is to simulate the microsolvation of the lithium ion and the neutral lithium atom by mixed Stockmayer clusters. Simulations of these systems for various compositions using parameters that approximate nitromethane and THF molecules are in progress.
