NMR Relaxation of Tetrahydrofuran in Clathrate Hydrate

Abstract

The reorientational motion of the tetrahydrofuran guest molecule in the clathrate hydrate C4H8O · 17D2O was studied by proton NMR spin‐lattice relaxation measurements in the temperature range 4 — 278 K. The results are interpreted by a Davidson‐Cole distribution of correlation times with a distribution parameter ß = 0.12. The maximum or cutoff time τ0 in this distribution obeys an activation law with prefactor 3.75 · 10−13 s and activation energy 4140 J mol−1. At temperatures below 20 K the motion of the guest molecules can be described by a coupled ensemble of quantummechanical hindered rotators where the reorientation rate is again described by a Davidson‐Cole distribution.

Modeling the THF clathrate hydrate dynamics by combining molecular dynamics and quasi-elastic neutron scattering

“…with a characteristic time longer than the picosecond. Such a timescale is in full agreement with dielectric spectroscopy [34], 1 H NMR [37,[47][48][49]] and 2 H NMR [39][40][41][42]48], according to these measurements the characteristic time for THF reorientation is shorter than 10 À10 s for temperature above ca. 100 K. Thus, the MD trajectories lead to spatial and time characteristics of the THF dynamics similar to those observed with the help of QENS experiments.…”

“…Such combined methodology is particularly appropriated for investigating guest dynamics in clathrate hydrates [53][54]. This investigation provides new insight onto the THF dynamics with complementary timescale with respect to previous investigations by means of NMR [37][38][39][40][41][42][47][48][49], such information is essential to understand guest-water interactions in view of the extended timescale met in the clathrate hydrates [54]. Moreover, MD-QENS combined analyzis consists in a ''direct showdown" of experimental and theoretical results through the comparison of QENS-measured and MD-derived scattering laws, leading to the validation of the potential used in the MD simulations.…”

“…1 Present address: UTINAM, France. dynamics has been explored by means of dielectric spectroscopy [34], of 1 H NMR [37,[47][48][49], of 2 H NMR [39][40][41][42]48] and of MD simulations [50,51]. From the 2 H NMR investigation, the THF dynamics occur in the rapid regime (i.e.…”

Guest motion in tetrahydrofuran clathrate hydrate studied by deuteron nuclear magnetic resonance

“…16 In the present work, we address the question regarding the interaction of the guests with the host lattice via a multidimensional deuteron NMR investigation thus complementing previous NMR studies which mostly exploited relaxometry, line-shape analysis, and double-resonance techniques. 10,14,[17][18][19][20][21][22][23][24][25][26][27] Relatively broad, Gaussian-type NMR spectra have been reported to occur at temperatures well above that at which the spin-lattice relaxation time T 1 is at its minimum. 25 A conclusion drawn from these experimental results is that "each THF molecule is surrounded by a slightly different water cage and the resultant distribution of local cage geometries confers differing degrees of anisotropic character on the reorientational motions of different THF molecules in the structure."…”

Reply to the ‘Comment on “A deuteron NMR study of the tetrahydrofuran clathrate hydrate. Part II: Coupling of rotational and translational dynamics of water”’ by M. Bach-Vergés, S. J. Kitchin, K. D. M. Harris, M. Zugic and C. A. Koh, Phys. Chem. Chem. Phys., 2004,6, 871

“…At the beginning of our clathrate work 4,5 we spent about a whole year to obtain satisfactory samples, which would give reproducible results for the NMR relaxation times T 1 of the enclathrated guest molecules. After this experience the preparation of all clathrate samples in later work 6 proved to be reliable and reproducible following this same procedure.…”