Hydrogen clathrate hydrates are promising sources of clean energy and are known to exist in a sII hydrate lattice, which consists of H2 molecules in dodecahedron (512) and hexakaidecahedron (51264) water cages. The formation of these hydrates which occur in extreme thermodynamic conditions is known to be considerably reduced by an inclusion of tetrahydrofuran (THF) in cages of these hydrate lattice. In this present work, we employ the density functional theory with a dispersion corrected (B97-D) functional to characterize vibrational Raman modes in the cages of pure and THF doped hydrogen clathrate hydrates. Our calculations show that the symmetric stretch of the H2 molecule in the 51264H2·THF cage is blueshifted compared to the 51264H2 cage. However, all vibrational modes of water molecules are redshifted which suggest reduced interaction between the H2 molecule and water molecules in the 51264H2·THF cage. The symmetric and asymmetric O–H stretch of water molecules in 512H2, 51264H2, and 51264H2·THF cages are redshifted compared with the corresponding guest free cages due to interactions between encapsulated H2 molecules and water molecules of the cages. The low frequency modes contain contributions from contraction and expansion of water cages and vibration of water molecules due to hydrogen bonding and these modes could possibly play an important role in the formation of the hydrate lattice.
Received 28 November 2012Accepted 05 March 2013Published online 25 March 2013
This work used the computing resources provided by the Indian Institute of Science Education and Research, Pune (IISER Pune). K. R. Ramya acknowledges IISER Pune for graduate fellowship support. The authors thank DST Nanomission (Grant No. SR/NM/NS-42/2009) for generous financial support towards this work.
Article outline: I. INTRODUCTION II. COMPUTATIONAL DETAILS III. RESULTS AND DISCUSSION A. High frequency modes B. Mid and low frequency modes IV. CONCLUDING REMARKS
21.See supplementary material at http://dx.doi.org/10.1063/1.4795610 for (a) a document listing all vibrational modes in the H2, 512, 512H2, 51264, 51264H2, 51264THF, and 51264H2·THF cages, (b) Video 1 showing vibrational Raman frequencies of water and hydrogen molecules in the 51264H2 cage, (c) Video 2 showing vibrational Raman frequencies of water and hydrogen molecules in the 51264H2·THF cages, and (d) Video 3 showing vibrational Raman frequencies of THF in the 51264H2·THF cage. [Supplementary Material]
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