Cage fusion from bi-cages to tri-cages during nucleation of methane hydrate: a DFT-D simulation. Issue 18 (24th January 2019)

• Record Type: Journal Article
• Authors: Li, Keyao; Shi, Ruili; Tang, Lingli; Huang, Yingying; Cao, Xiaoxiao; Su, Yan
• Is Part Of: Physical chemistry chemical physics. Volume 21:Issue 18(2019)
• Page Start: 9150
• Page End: 9158
• Publication Date: 2019-01-24
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c8cp07207h
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Water-cages tend to form dense accumulation configurations; and 5 12 and 4 1 5 10 6 2 cages are abundant in the early nucleation stage of CH4 hydrates. Abstract : Water-cage clusters encapsulating guest molecules are the basic components of hydrate crystal structures. Herein, we investigated the fusion process from bi-cages to tri-cages to probe the nucleation mechanism at the initial stage of CH4 hydrate formation by employing dispersion-corrected density functional theory. We found that tri-cages possess high stability by sharing three, rather than two, polygonal faces. In addition, any mono-cage combined with a nonstandard 4 1 5 10 6 2 cage could achieve considerable stability regardless of which face is shared; this finding illustrates that 4 1 5 10 6 2 cages are more likely to appear at the early stages of CH4 hydrate nucleation than other nonstandard cages. We then simulated the Raman spectra of CH4 molecules in water-cage to characterize the spectral characteristics of the CH4 hydrate. The C–H symmetric stretching frequency of encapsulated CH4 molecules red-shifted with increasing mono-cage size, which follows the prediction of the "loose cage–tight cage" model. The symmetric stretching vibrational frequencies of trapped CH4 molecules in the tri-cage revealed a clear red-shift compared with those in the component mono- and bi-cages. The cage fusion process and spectroscopic properties described in this work are expected to provide new atomistic insights into CH4 hydrates at the initial nucleation stage.