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C-C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study
Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-5341-2637
2014 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 118, no 49, 11641-11651 p.Article in journal (Refereed) Published
Abstract [en]

The presence of specific hydrocarbon gas molecules in various types of water cavities in natural gas hydrates (NGHs) are governed by the relative stabilities of these encapsulated guest molecule-water cavity combinations. Using molecular quantum chemical dispersion-corrected hybrid density functional computations, the interaction (Delta E(host-)guest) and cohesive energies (Delta E-coh), enthalpies, and Gibbs free energies for the complexes of host water cages and hydrocarbon guest molecules are calculated at the pi B97X-D/6-311++G(2d,2p) level of theory. The zero-point energy effect of ?Ehost-guest and ?Ecoh is found to be quite substantial. The energetically optimal host-guest combinations for seven hydrocarbon gas molecules (CH4, C2H6, C3H6, C3H8, C4H8, i-C4H10, and n-C4H10) and various water cavities (D, ID, T, P, H, and I) in NGHs are found to be CH4@D, C2H6@T, C3H6@T, C3H8@T, C4H8@T/P/H, i-C4H10@H, and n-C4H10@H, as the largest cohesive energy magnitudes will be obtained with these host-guest combinations. The stabilities of various water cavities enclosing hydrocarbon molecules are evaluated from the computed cohesive Gibbs free energies: CH4 prefers to be trapped in a ID cage; C2H6 prefer T cages; C3H6 and C3H8 prefer T and H cages; C4H8 and i-C4H10 prefer H cages; and n-C4H10 prefer I cages. The vibrational frequencies and Raman intensities of the C-C stretching vibrational modes for these seven hydrocarbon molecules enclosed in each water cavity are computed. A blue shift results after the guest molecule is trapped from gas phase into various water cages due to the host-guest interactions between the water cage and hydrocarbon molecule. The frequency shifts to the red as the radius of water cages increases. The model calculations support the view that C-C stretching vibrations of hydrocarbon molecules in the water cavities can be used as a tool to identify the types of crystal phases and guest molecules in NGHs.

Place, publisher, year, edition, pages
American Chemical Society , 2014. Vol. 118, no 49, 11641-11651 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-113497DOI: 10.1021/jp510118pISI: 000346320800021PubMedID: 25406092OAI: oai:DiVA.org:liu-113497DiVA: diva2:781941
Note

Funding Agencies|Swedish Research Council (VR); Swedish supercomputer center (NSC); State Scholarship Fund of China Scholarship Council [201206060016]

Available from: 2015-01-19 Created: 2015-01-19 Last updated: 2016-11-10
In thesis
1. Theoretical Studies of Natural Gas Hydrates and H-bonded Clusters and Crystals
Open this publication in new window or tab >>Theoretical Studies of Natural Gas Hydrates and H-bonded Clusters and Crystals
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis H-bonded systems (natural gas hydrates, water clusters, and crystal ice) are studied by density functional theory (DFT) computations.

Natural gas hydrates (NGHs) play an important role in energy and environmental fields: NGHs are considered as a promising backup energy resource in the near-future due to their tremendous carbon content; improper exploration of NGHs could induce geological disasters and aggravate the greenhouse effect. In addition, many technologies based on gas hydrates are being applied and developed. The thermodynamic stabilities of various water cavities in different clathrate crystalline phases occupied by hydrocarbon gas molecules are studied by dispersion-corrected hybrid functionals. The Raman spectra of C-C and C-H stretching vibrations of hydrocarbon molecules in various water cavities in the solid state are derived. The trends of C-H stretching vibrational frequencies are found to follow the prediction by the “loose cage ─| tight cage” model. In addition, the trends and origins of 13C NMR chemical shifts of hydrocarbon molecules in various NGHs are presented. These theoretical results will enlarge the database of C-C and C-H stretching vibrational frequencies and 13C NMR parameters of hydrocarbon molecules in NGHs, and provide valuable information to help identify the types of clathrate phases and varieties of guest molecules included in NGHs samples taken from natural sites.

The behavior of water clusters may help to understand the properties of its liquid and solid states. The thermodynamic stabilities and IR spectra of a small-, medium-, and large-sized water cluster are studied in this work. After full optimization of (H2O)20,54,100 using the hybrid functional B3LYP, the electronic energies, zero-point energies, internal energies, enthalpies, entropies, and Gibbs free energies of the water clusters are computed. The OH stretching vibrational IR spectra of (H2O)20,54,100 are also presented and split into sub-spectra for different H-bond types based on the specific contributions from each group. It is found that the OH stretching vibrational frequencies of water are sensitive to the conformations of the H-bonds and the vibrations of the H-bonds belonging to different types are located in separated regions in the IR spectra. Thus, the spectroscopic fingerprints will reflect the H-bond topology of the water molecules in a water cluster.

Ice XI has been suggested to be involved in the process of planetary formation as a considerable electric field might be formed from the ferroelectric ice XI in space. IR and Raman spectroscopic technology can be directly used to identify the occurrence of ferroelectric ice XI in laboratory or extraterrestrial settings. Due to the difficulty for DFT to describe non-covalent systems, the performance of 16 different DFT methods applied on the ice Ih, VIII, IX, and XI crystal phases are assessed. Based on the computational accuracy and cost, the IR and Raman spectra of ice Ih and XI are derived and compared. The librational vibrations are found to be the identifier which can be used to distinguish ice Ih and ice XI in the universe. In addition, the existence only one kind of H-bond in ice Ih is demonstrated from the overlapping sub-spectra for different types of H-bonded pair configurations in 16 isomers of ice Ih.

The region of water under negative pressure is an exotic land in lack of exploitation. Guest free clathrate hydrate (clathrate ice) of sII type has been recently confirmed experimentally at negative pressure. Does any other clathrate ice phase exist at negative pressure region? Since clathrate hydrate are isostructural with silica clathrate minerals and semiconductor clathrates, and crystal structure prediction by analogy with known structures and first-principles computations is an effective way to find new crystalline phases of solid materials, we are motived to look for new clathrate ice phases from silica or semiconductor clathrate materials based on first-principles computations. Borrowing the idea new clathrate frameworks of ZnO and SiC can be constructed by connecting their bubble clusters in different ways, new clathrate ice phases (sL, sL_I, sL_II, and sL_III) are generated by connecting the water bubble clusters according to different rules. Using the non-local dispersion-corrected vdW-DF2 functional, clathrate ice sL with ultralow density (0.6 g/cm3) is predicted by first-principles phase diagram computations to be stable under larger negative pressures than the sII phase. The phase diagram of water is thus extended into the lower negative pressure region.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 45 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1808
National Category
Theoretical Chemistry Physical Chemistry Inorganic Chemistry Other Chemistry Topics Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:liu:diva-132428 (URN)10.3384/diss.diva-132428 (DOI)9789176856321 (Print) (ISBN)
Public defence
2016-12-01, Plank, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-11-10 Created: 2016-11-10 Last updated: 2016-11-11Bibliographically approved

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