Raman peak frequencies of fluoromethane molecules measured in clathrate hydrate crystals: Experimental investigations and density functional theory calculations

Tsutomu Uchida, Ryo Ohmura, Akira Hori

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10 Citations (Scopus)

Abstract

Systematic observations of fluoromethane clathrate hydrates were carried out by Raman spectroscopy. The series of fluoromethanes, i.e., methane (CH 4), fluoromethane (CH3F), difluoromethane (CH 2F2), trifluoromethane (CHF3), and tetrafluoromethane (CF4), were used as standard guest molecules to investigate the vibration modes of the guest molecules in the hydrate phase, since all of these fluoromethanes are included in the same crystal structure and share similar functional groups. In this study, both the C-H and C-F vibration modes of the guest molecules were systematically collected and assigned each peak based on the density functional theory (DFT) calculations. The Raman peak table obtained by the DFT calculations was useful for assigning the Raman peaks measured by the experiments. The assignment of the Raman peaks of the C-H stretching mode of each fluoromethane hydrate coincided well with those estimated both experimentally and theoretically in previous studies. The empirical "loose cage-tight cage" model of the Raman peak shifts allowed us to estimate the unperturbed frequencies of the C-H symmetric stretching mode on CH3F molecules in the clathrate structure. Clathrate hydrates formed with deuterated water molecules indicated that the deuterium had little effect on the Raman spectra of the intramolecular vibration modes of the guest molecules within the experimental uncertainties.

Original languageEnglish
Pages (from-to)317-323
Number of pages7
JournalJournal of Physical Chemistry A
Volume114
Issue number1
DOIs
Publication statusPublished - 2010 Jan 14

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clathrates
Hydrates
hydrates
Density functional theory
density functional theory
Crystals
Molecules
crystals
molecules
vibration mode
Stretching
methylidyne
Deuterium
Methane
Functional groups
Raman spectroscopy
fluoromethane
Raman scattering
deuterium
methane

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

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abstract = "Systematic observations of fluoromethane clathrate hydrates were carried out by Raman spectroscopy. The series of fluoromethanes, i.e., methane (CH 4), fluoromethane (CH3F), difluoromethane (CH 2F2), trifluoromethane (CHF3), and tetrafluoromethane (CF4), were used as standard guest molecules to investigate the vibration modes of the guest molecules in the hydrate phase, since all of these fluoromethanes are included in the same crystal structure and share similar functional groups. In this study, both the C-H and C-F vibration modes of the guest molecules were systematically collected and assigned each peak based on the density functional theory (DFT) calculations. The Raman peak table obtained by the DFT calculations was useful for assigning the Raman peaks measured by the experiments. The assignment of the Raman peaks of the C-H stretching mode of each fluoromethane hydrate coincided well with those estimated both experimentally and theoretically in previous studies. The empirical {"}loose cage-tight cage{"} model of the Raman peak shifts allowed us to estimate the unperturbed frequencies of the C-H symmetric stretching mode on CH3F molecules in the clathrate structure. Clathrate hydrates formed with deuterated water molecules indicated that the deuterium had little effect on the Raman spectra of the intramolecular vibration modes of the guest molecules within the experimental uncertainties.",
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AB - Systematic observations of fluoromethane clathrate hydrates were carried out by Raman spectroscopy. The series of fluoromethanes, i.e., methane (CH 4), fluoromethane (CH3F), difluoromethane (CH 2F2), trifluoromethane (CHF3), and tetrafluoromethane (CF4), were used as standard guest molecules to investigate the vibration modes of the guest molecules in the hydrate phase, since all of these fluoromethanes are included in the same crystal structure and share similar functional groups. In this study, both the C-H and C-F vibration modes of the guest molecules were systematically collected and assigned each peak based on the density functional theory (DFT) calculations. The Raman peak table obtained by the DFT calculations was useful for assigning the Raman peaks measured by the experiments. The assignment of the Raman peaks of the C-H stretching mode of each fluoromethane hydrate coincided well with those estimated both experimentally and theoretically in previous studies. The empirical "loose cage-tight cage" model of the Raman peak shifts allowed us to estimate the unperturbed frequencies of the C-H symmetric stretching mode on CH3F molecules in the clathrate structure. Clathrate hydrates formed with deuterated water molecules indicated that the deuterium had little effect on the Raman spectra of the intramolecular vibration modes of the guest molecules within the experimental uncertainties.

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