C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Abstract

Abstract: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 energies (ΔEhost-guest) and cohesive energies (ΔEcoh), enthalpies and Gibbs free energies for the complexes of host water cages and hydrocarbon guest molecules are calculated at the ωB97X-D/6-311++G(2d,2p) level of theory. The zero point energy effect of ΔEhost-guest and ΔEcoh 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 prefer 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.2

CH-Stretching Vibrational Trends in Natural Gas Hydrates Studied by Quantum-Chemical Computations

“…[19][20][21] Theoretically, the nucleation and growth process of methane hydrate have been simulated and explored on the atomic and molecular level through molecular dynamics simulations. [23][24][25][26][27][28][29][30] The interaction between host water cages and guest molecules, 32,41 stability, diffusion and vibrations of guest molecules in water cavities of clathrate hydrates, 31,34,[38][39][40][42][43] and phase transition between ice and methane clathrates have been studied by firstprinciple calculations. 37,44 Experimentally, Raman spectra of hydrocarbon hydrates have demonstrated that the CH stretching frequency of the guest molecule is commonly (but not always) lower when in a large cage than when in a small cage [13][14][15] .…”

Raman and IR Spectra of Ice Ih and Ice XI with an Assessment of DFT Methods

“…No obvious difference between ice Ih and XI is found for the OH stretching part of the spectrum, which might suggest that the intramolecular vibrations are little affected by the proton-ordering. Figure S6) as in previous studies, [75][76][77][78] and the contribution from each group to the total spectra was projected out (in accordance with the number of H atoms of the group and their vibrational amplitudes squared [75][76][77][78] ). In Figure 6, the IR spectra of stretching vibrations in 16 different isomers of ice Ih are shown in the range of 3090 -3460 cm -1 .…”

“…In addition, the OH stretching vibrations are projected into different categories based on the contributions from different H-bond configurations as in previous studies. [75][76][77][78]  RESULTS AND DISCUSSION Assessment of DFT Methods. H-bonding between water molecules is the dominating interaction in ice.…”

Theoretical Studies of Natural Gas Hydrates and H-bonded Clusters and Crystals

“…sI, sII, sH, sK, etc. [1][2][3]6 Clathrate ice is the guest free clathrate hydrate. Clathrate ice sII is recently obtained through pumping off the guest molecules of sII Ne clathrate hydrate by Falenty et al, 51 which has negative thermal expansion below 55 K and is stable at negative pressure in the range of 0.4 and 1 GPa.…”

“…In this thesis H-bonded systems (natural gas hydrates, [1][2][3] water clusters, 4 and crystal ice 5 ) are studied by density functional theory (DFT) computations.…”