TY - JOUR

T1 - Anharmonic Densities of States for Vibrationally Excited I-(H2O), (H2O)2, and I-(H2O)2

AU - Ma, Xinyou

AU - Yang, Nan

AU - Johnson, Mark A.

AU - Hase, William L.

N1 - Funding Information:
The research reported here was supported by the National Science Foundation under grant no. CHE-1416428, the Robert A. Welch Foundation under grant no. D-0005, and the Air Force Office of Scientific Research under AFOSR award no. FA9550-16-1-0133. The DFT calculations and Monte Carlo sampling of I−(H2O) and (H2O)2 were performed on the Chemdynm computer cluster of the Hase research group. The authors acknowledge the High Performance Computing Center (HPCC) at Texas Tech University (TTU) and the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources of CPU and GPU computation, respectively, for the I−(H2O)2 Monte Carlo sampling results reported within this article. M.A.J. also thanks the Air Force Office of Scientific Research (grant number FA9550-13-1-0007) for support of the temperature-controlled ion trap critical for the experimental measurements leading to the theoretical investigations reported here, and the application of this methodology to understanding the properties of the I−(H2O)2 system is supported by the National Science Foundation under grant CHE-1465100. X.M. and W.L.H. thank Jing Xie and Jiaxu Zhang for very important discussions.
Publisher Copyright:
© 2018 American Chemical Society.

PY - 2018/8/14

Y1 - 2018/8/14

N2 - Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, Nanh(E), for I-(H2O), (H2O)2, and I-(H2O)2 versus internal energy up to their dissociation energies. The anharmonic density of states, ρanh(E), is found from the energy derivative of Nanh(E). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H2O⋯H2O interactions and an accurate two-body potential for the I-⋯H2O fit to quantum chemical calculations. The extensive Monte Carlo samplings are computationally demanding, and the use of computationally efficient potentials was essential for the calculations. Particular emphasis is directed toward I-(H2O)2, and distributions of its structures versus internal energy are consistent with experimental studies of the temperature-dependent vibrational spectra. At their dissociation thresholds, the anharmonic to harmonic density of states ratio, ρanh(E)/ρh(E), is ∼2, ∼ 3, and ∼260 for I-(H2O), (H2O)2, and I-(H2O)2, respectively. The large ratio for I-(H2O)2 results from the I-(H2O)2 → I-(H2O) + H2O dissociation energy being more than 2 times larger than the (H2O)2 → 2H2O dissociation energy, giving rise to highly mobile H2O molecules near the I-(H2O)2 dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations.

AB - Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, Nanh(E), for I-(H2O), (H2O)2, and I-(H2O)2 versus internal energy up to their dissociation energies. The anharmonic density of states, ρanh(E), is found from the energy derivative of Nanh(E). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H2O⋯H2O interactions and an accurate two-body potential for the I-⋯H2O fit to quantum chemical calculations. The extensive Monte Carlo samplings are computationally demanding, and the use of computationally efficient potentials was essential for the calculations. Particular emphasis is directed toward I-(H2O)2, and distributions of its structures versus internal energy are consistent with experimental studies of the temperature-dependent vibrational spectra. At their dissociation thresholds, the anharmonic to harmonic density of states ratio, ρanh(E)/ρh(E), is ∼2, ∼ 3, and ∼260 for I-(H2O), (H2O)2, and I-(H2O)2, respectively. The large ratio for I-(H2O)2 results from the I-(H2O)2 → I-(H2O) + H2O dissociation energy being more than 2 times larger than the (H2O)2 → 2H2O dissociation energy, giving rise to highly mobile H2O molecules near the I-(H2O)2 dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations.

UR - http://www.scopus.com/inward/record.url?scp=85049175991&partnerID=8YFLogxK

U2 - 10.1021/acs.jctc.8b00300

DO - 10.1021/acs.jctc.8b00300

M3 - Article

C2 - 29944367

AN - SCOPUS:85049175991

VL - 14

SP - 3986

EP - 3997

JO - Journal of Chemical Theory and Computation

JF - Journal of Chemical Theory and Computation

SN - 1549-9618

IS - 8

ER -