TY - JOUR

T1 - Improved methods for feynman path integral calculations of vibrational-rotational free energies and application to isotopic fractionation of hydrated chloride ions

AU - Mielke, Steven L

AU - Truhlar, Donald G

PY - 2009/4/23

Y1 - 2009/4/23

N2 - We present two enhancements to our methods for calculating vibrational-rotational free energies by Feynman path integrals, namely, a sequential sectioning scheme for efficiently generating random free-particle paths and a stratified sampling scheme that uses the energy of the path centroids. These improved methods are used with three interaction potentials to calculate equilibrium constants for the fractionation behavior of Cl- hydration in the presence of a gas-phase mixture of H2O, D2O, and HDO. Ion cyclotron resonance experiments indicate that the equilibrium constant, Keq, for the reaction Cl(H2O)- + D2O h Cl(D2O)- + H2O is 0.76, whereas the three theoretical predictions are 0.946, 0.979, and 1.20. Similarly, the experimental Keq for the Cl(H2O)- + HDO h Cl(HDO)- + H2O reaction is 0.64 as compared to theoretical values of 0.972, 0.998, and 1.10. Although Cl(H2O)- has a large degree of anharmonicity, Keq values calculated with the harmonic oscillator rigid rotator (HORR) approximation agree with the accurate treatment to within better than 2% in all cases. Results of a variety of electronic structure calculations, including coupled cluster and multireference configuration interaction calculations, with either the HORR approximation or with anharmonicity estimated via second-order vibrational perturbation theory, all agree well with the equilibrium constants obtained from the analytical surfaces.

AB - We present two enhancements to our methods for calculating vibrational-rotational free energies by Feynman path integrals, namely, a sequential sectioning scheme for efficiently generating random free-particle paths and a stratified sampling scheme that uses the energy of the path centroids. These improved methods are used with three interaction potentials to calculate equilibrium constants for the fractionation behavior of Cl- hydration in the presence of a gas-phase mixture of H2O, D2O, and HDO. Ion cyclotron resonance experiments indicate that the equilibrium constant, Keq, for the reaction Cl(H2O)- + D2O h Cl(D2O)- + H2O is 0.76, whereas the three theoretical predictions are 0.946, 0.979, and 1.20. Similarly, the experimental Keq for the Cl(H2O)- + HDO h Cl(HDO)- + H2O reaction is 0.64 as compared to theoretical values of 0.972, 0.998, and 1.10. Although Cl(H2O)- has a large degree of anharmonicity, Keq values calculated with the harmonic oscillator rigid rotator (HORR) approximation agree with the accurate treatment to within better than 2% in all cases. Results of a variety of electronic structure calculations, including coupled cluster and multireference configuration interaction calculations, with either the HORR approximation or with anharmonicity estimated via second-order vibrational perturbation theory, all agree well with the equilibrium constants obtained from the analytical surfaces.

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U2 - 10.1021/jp900834u

DO - 10.1021/jp900834u

M3 - Article

C2 - 19290606

AN - SCOPUS:65649098645

SN - 1089-5639

VL - 113

SP - 4817

EP - 4827

JO - Journal of Physical Chemistry A

JF - Journal of Physical Chemistry A

IS - 16

ER -