The effect of the condensed-phase environment on the vibrational frequency shift of a hydrogen molecule inside clathrate hydrates
We report a theoretical study of the frequency shift (redshift) of the stretching fundamental transition of an H molecule confined inside the small dodecahedral cage of the structure II clathrate hydrate and its dependence on the condensed-phase environment. In order to determine how much the hydrat...
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Published in | The Journal of chemical physics Vol. 148; no. 14; p. 144304 |
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Main Authors | , , , , , |
Format | Journal Article |
Language | English |
Published |
United States
14.04.2018
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Online Access | Get more information |
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Summary: | We report a theoretical study of the frequency shift (redshift) of the stretching fundamental transition of an H
molecule confined inside the small dodecahedral cage of the structure II clathrate hydrate and its dependence on the condensed-phase environment. In order to determine how much the hydrate water molecules beyond the confining small cage contribute to the vibrational frequency shift, quantum five-dimensional (5D) calculations of the coupled translation-rotation eigenstates are performed for H
in the v=0 and v=1 vibrational states inside spherical clathrate hydrate domains of increasing radius and a growing number of water molecules, ranging from 20 for the isolated small cage to over 1900. In these calculations, both H
and the water domains are treated as rigid. The 5D intermolecular potential energy surface (PES) of H
inside a hydrate domain is assumed to be pairwise additive. The H
-H
O pair interaction, represented by the 5D (rigid monomer) PES that depends on the vibrational state of H
, v=0 or v=1, is derived from the high-quality ab initio full-dimensional (9D) PES of the H
-H
O complex [P. Valiron et al., J. Chem. Phys. 129, 134306 (2008)]. The H
vibrational frequency shift calculated for the largest clathrate domain considered, which mimics the condensed-phase environment, is about 10% larger in magnitude than that obtained by taking into account only the small cage. The calculated splittings of the translational fundamental of H
change very little with the domain size, unlike the H
j = 1 rotational splittings that decrease significantly as the domain size increases. The changes in both the vibrational frequency shift and the j = 1 rotational splitting due to the condensed-phase effects arise predominantly from the H
O molecules in the first three complete hydration shells around H
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ISSN: | 1089-7690 |
DOI: | 10.1063/1.5024884 |