¹³C NMR Studies of Hydrocarbon Guests in Synthetic Structure H Gas Hydrates: Experiment and Computation

Abstract

13C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The 13C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 Å) but a much smaller change in the c-axis (in the range of 0.01 Å) is observed. The 13C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest−water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.

Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations

“…The spectrum between 20 and 40 ppm shows the chemical shifts of MCH in the large cage, there is some overlap with those of the liquid phase. 37 The 13 C NMR chemical shifts of CH 3 F in the different clathrate hydrate structures are reported in Table I.…”

Molecular dynamics screening for new kinetic inhibitors of methane hydrate

“…Clathrate hydrates are crystalline solids in which a water lattice (the "host") creates cages entrap another type of molecule (the "guest"). Many different compounds that can form clathrate hydrates have been documented, [1][2][3] but for practical applications the most important the components of natural gas, particularly methane; hence these solids are often termed "gas hydrates", or simply "hydrates". At moderate to high pressures, gas hydrates will form at temperatures that can be significantly above the 273 K. Such conditions are common geologically, in permafrost and at and below the sea floor, and have led to the formation of vast natural deposits of gas hydrates.…”

Spectroscopic and thermodynamic investigations of clathrate hydrates of methacrolein

“…[8][9][10][11][12][13][14][15][16][17][18][19][20] Recent studies have reported an interesting phenomenon for the inclusion of large guest molecules (LGMs) in the large cages of sII or sH hydrates. [8][9][10][11][12][13][14][15][16][17][18][19][20] It was reported that the trans conformation of some large guest molecules (LGMs), such as n-butane and isopentane, is thermodynamically preferred in the neat phase, but the gauche conformation of LGMs is observed in the large cages of sII or sH hydrates. [8][9][10][11]15 In addition, it was recently reported that some alcohols with molecular sizes above 7.5Å form sII hydrates in the presence of methane gas.…”

“…[8][9][10][11][12][13][14][15][16][17][18][19][20] It was reported that the trans conformation of some large guest molecules (LGMs), such as n-butane and isopentane, is thermodynamically preferred in the neat phase, but the gauche conformation of LGMs is observed in the large cages of sII or sH hydrates. [8][9][10][11]15 In addition, it was recently reported that some alcohols with molecular sizes above 7.5Å form sII hydrates in the presence of methane gas. 13,19 1-Butanol with a molecular size of 9.05Å and a trans-trans conformation may not be considered as an sII hydrate former from the viewpoint of guest inclusion, but 1-butanol with a molecular size of 7.72Å and a gauche-gauche conformation is reported to be an sII hydrate former in the presence of methane gas.…”