title: Competing quantum effects in the free energy profiles and diffusion rates of hydrogen and deuterium molecules through clathrate hydrates.
abstract: Clathrate hydrates hold considerable promise as safe and economical materials for hydrogen storage. Here we present a quantum mechanical study of H2and D2diffusion through a hexagonal face shared by two large cages of clathrate hydrates over a wide range of temperatures. Path integral molecular dynamics simulations are used to compute the free-energy profiles for the diffusion of H2and D2as a function of temperature. Ring polymer molecular dynamics rate theory, incorporating both exact quantum statistics and approximate quantum dynamical effects, is utilized in the calculations of the H2and D2diffusion rates in a broad temperature interval. We find that the shape of the quantum free-energy profiles and their height relative to the classical free energy barriers at a given temperature, as well as the rate of diffusion, are strongly affected by competing quantum effects: above 25 K, zero-point energy (ZPE) perpendicular to the reaction path for diffusion between cavities decreases the quantum rate compared to the classical rate, whereas at lower temperatures tunneling outcompetes the ZPE and as a result the quantum rate is greater than the classical rate.
authors.affiliation: Department of Chemistry, New York University, New York, NY 10003, USA. zlatko.bacic@nyu.edu.
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Cendagorta
authors.affiliation: Department of Chemistry, New York University, New York, NY 10003, USA. zlatko.bacic@nyu.edu.
authors.forename: Anna
authors.initials: A
authors.lastname: Powers
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14850, USA.
authors.forename: Timothy J H
authors.initials: TJ
authors.lastname: Hele
authors.affiliation: Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, CA 94305, USA.
authors.forename: Ondrej
authors.initials: O
authors.lastname: Marsalek
authors.affiliation: Department of Chemistry, New York University, New York, NY 10003, USA. zlatko.bacic@nyu.edu and NYU-ECNU Center for Computational Chemistry at NYU Shanghai, 3663 Zhongshan Road North, Shanghai, 200062, China.
authors.forename: Zlatko
authors.initials: Z
authors.lastname: Bačić
authors.affiliation: NYU-ECNU Center for Computational Chemistry at NYU Shanghai, 3663 Zhongshan Road North, Shanghai, 200062, China and Department of Chemistry and Courant Institute of Mathematical Sciences, New York University, New York, NY 10003, USA. mark.tuckerman@nyu.edu.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Tuckerman