CH₄（H₂O）_n（n=1-20） Complex: A Theoretical Investigation Of Configurations And Properties

Natural gas hydrates are high energy density and reserved abundantly. They areknown as the strategic resources having business development prospects in the21stcentury and they are one of the first choices of new clean energy in the future. Howhydrates nucleate and what’s the mechanism they grow have been the focus all thoseyears.In this paper, based on the optimal structures of TIP4P/TIP5P water clusters（H₂O）_n（n≤21） and a regular dodecahedron water structure, a series of initialconfigurations of methane hydrate clusters CH₄（H₂O）_n（n=1-20） are produced, by themethod of replacing water molecules by methane molecules at random. In order tomake more initial configurations,624cage-like and256compact structures ofCH₄（H₂O）_n（n=1-20） are generated also using a computer program randomly. Based onthe density functional theory, about1800kinds of the initial configurations ofCH₄（H₂O）_n（n=1-20） are optimized in this paper. A series of stable structures ofCH₄（H₂O）_n（n=1-20） are picked out and the low-lying isomers are analyzed withaspects of compositions and characteristics. In addition, some proposed complex ofCH₄（H₂O）_n（n=1-20） in literature are count for more comparison. The calculationresults are as follows:1. For the low-lying structures of CH₄（H₂O）_n（n=1-8）, water molecules existencein the form of three-membered rings, four-membered rings or five-membered rings.For n≤5, water molecules make a plane structure. From n=6, the subunits of watermolecules go from two-dimensional structures to3-dimensions. Relative to thelowest-energy structures, the low-lying isomers have different O-H bonds that hang todifferent directions. The distribution of the charge in water molecules andinteractional characteristics make light influence on the stability of the wholestructures.2. For the low-lying structures of CH₄（H₂O）_n（n=9-16）, the subunits of water molecules are almost made of cube, pentagonal prism and their derivative structure, ortheir combination. The lowest-energy structure of CH₄（H2O）15has its water moleculesa cage-like structure. These subunits are not big enough to package a methanemolecule as an object.3. For the low-lying structures of CH₄（H₂O）_n（n=17-20）, the surfaces of watermolecules are almost made of four-membered rings or five-membered rings.Compared with the small size of the clusters, the number of four-membered rings hasa slight decrease while the number of five-membered rings increases rapidly,especially at n=20, reaching to nine. It shows that， for larger size hydrates,five-membered rings may be the better choice for water molecules groups. Whenn=20, water group is a compact structure with a water molecule in the middle bondedto the surrounding molecules with hydrogen bonds, but methane molecules is stillindependent of the water group.4. Comparing the the lowest-energy structures of CH₄（H₂O）_n（n=1-20） with theirinitial configurations, statistic shows that the structures of water can be well simulatedby TIP4P experience potential or TIP5P experience potential. The subunits of thelowest-energy structures of CH₄（H₂O）_n（n=8-11） are nearly single bodies, the numberof four-mumbered rings keeping four to five. When n=12, connected bodies ofwater molecules appear, so the number of little ring, like four-mumbered rings, soarsuntil n=19. At n=15, CH₄（H2O）15a cage-like structure emerges for water group andthe number of five-mumbered ring has a surge into six. When n=20, a compactstructure arises for water molecules and the number of five-mumbered ring increasesto nine. Dipole Moments （DM） analysis for lowest-energy structures proof that, whenthe water molecules joint to four-mumbered ring, the dipole-dipole effect is weak sothat the DM value is lower; when the water molecules joint to five-mumbered ring,the dipole-dipole effect is strong, causing a increasing DM value. It can be predictedthat, when the size of the cluster increases, the water molecules exist in the form ofpentagon.5. For the lowest-energy structures of CH₄（H₂O）_n（n=1-20）, the average formation energy per water molecule Eb/nis increased with n. It proves astrengthened interaction between water molecules and a more stable structure. Theaverage formation energy per hydrogen bond Eb/Nis between7.120-9.710kcal/mol,which is close to the bond energy of hydrogen bond in moderate intensity. After n=14,the value of Eb/Nbecomes more stable with little changes among8.280-8.540kcal/mol, indicating a more fixed stability of hydrogen bond. The formation energy ofmethane molecule E bCH₄isincreased with n suggesting an increase force between amethane molecule and a water network. The value of EbCH₄grows gradually from5.430kcal/mol to22.478kcal/mol, which proofs that the interaction between amethane molecule and a water group can be comparable to a hydrogen bond and thethere is a adsorption comparable to a hydrogen bond between and methane moleculeand water molecules.