13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: Experiment and computation

Jong Won Lee, Hailong Lu, Igor L. Moudrakovski, Christopher I. Ratcliffe, Ryo Ohmura, Saman Alavi, John A. Ripmeester

Research output: Contribution to journalArticle

25 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)1650-1657
Number of pages8
JournalJournal of Physical Chemistry A
Volume115
Issue number9
DOIs
Publication statusPublished - 2011 Mar 10

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Fingerprint Dive into the research topics of '<sup>13</sup>C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: Experiment and computation'. Together they form a unique fingerprint.

  • Cite this

    Lee, J. W., Lu, H., Moudrakovski, I. L., Ratcliffe, C. I., Ohmura, R., Alavi, S., & Ripmeester, J. A. (2011). 13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: Experiment and computation. Journal of Physical Chemistry A, 115(9), 1650-1657. https://doi.org/10.1021/jp1118184