H2, HD, and D2 in the small cage of structure II clathrate hydrate: vibrational frequency shifts from fully coupled quantum six-dimensional calculations of the vibration-translation-rotation eigenstates

Lauvergnat, David; Felker, Peter; Scribano, Yohann; Benoit, David M.; Ba?i?, Zlatko

Abstract

We report the first fully coupled quantum six-dimensional (6D) bound-state calculations of the vibration-translation-rotation eigenstates of a flexible H2, HD, and D2 molecule confined inside the small cage of the structure II clathrate hydrate embedded in larger hydrate domains with up to 76 H2O molecules, treated as rigid. Our calculations use a pairwise-additive 6D intermolecular potential energy surface for H2 in the hydrate domain, based on an ab initio 6D H2–H2O pair potential for flexible H2 and rigid H2O. They extend to the first excited (v = 1) vibrational state of H2, along with two isotopologues, providing a direct computation of vibrational frequency shifts. We show that obtaining a converged v = 1 vibrational state of the caged molecule does not require converging the very large number of intermolecular translation-rotation states belonging to the v = 0 manifold up to the energy of the intramolecular stretch fundamental (≈4100 cm−1 for H2). Only a relatively modest-size basis for the intermolecular degrees of freedom is needed to accurately describe the vibrational averaging over the delocalized wave function of the quantum ground state of the system. For the caged H2, our computed fundamental translational excitations, rotational j = 0 → 1 transitions, and frequency shifts of the stretch fundamental are in excellent agreement with recent quantum 5D (rigid H2) results [A. Powers et al., J. Chem. Phys. 148, 144304 (2018)]. Our computed frequency shift of −43 cm−1 for H2 is only 14% away from the experimental value at 20 K.

Citation

Lauvergnat, D., Felker, P., Scribano, Y., Benoit, D. M., & Bačić, Z. (2019). H2, HD, and D2 in the small cage of structure II clathrate hydrate: vibrational frequency shifts from fully coupled quantum six-dimensional calculations of the vibration-translation-rotation eigenstates. The Journal of chemical physics, 150(15), Article 154303. https://doi.org/10.1063/1.5090573

• Journal Article Type: Article
• Acceptance Date: Mar 19, 2019
• Online Publication Date: Apr 15, 2019
• Publication Date: Apr 21, 2019
• Deposit Date: Apr 24, 2019
• Publicly Available Date: Apr 24, 2019
• Journal: The Journal of Chemical Physics
• Print ISSN: 0021-9606
• Publisher: American Institute of Physics
• Peer Reviewed: Peer Reviewed
• Volume: 150
• Issue: 15
• Article Number: 154303
• DOI: https://doi.org/10.1063/1.5090573
• Keywords: Physical and Theoretical Chemistry; General Physics and Astronomy
• Public URL: https://hull-repository.worktribe.com/output/1624344
• Additional Information: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Lauvergnat, D., Felker, P., Scribano, Y., Benoit, D. M., & Bačić, Z. (2019). H2, HD, and D2 in the small cage of structure II clathrate hydrate: vibrational frequency shifts from fully coupled quantum six-dimensional calculations of the vibration-translation-rotation eigenstates. The Journal of chemical physics, 150(15), https://doi.org/10.1063/1.5090573 and may be found at https://doi.org/10.1063/1.5090573
• Contract Date: Apr 24, 2019