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Guest molecule dynamics in clathrate hydrates change significantly above 200 K. Tetrahydrofuran (THF) shows increased motion barriers due to host-guest hydrogen bonding, unlike cyclopentane (CP).

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Area of Science:

  • Physical Chemistry
  • Materials Science
  • Solid-State NMR Spectroscopy

Background:

  • Clathrate hydrate guest-water interactions are typically van der Waals at low temperatures.
  • Above 200 K, water molecules in clathrates become more mobile, potentially enabling hydrogen bonding with guests.

Purpose of the Study:

  • To investigate the influence of host-guest hydrogen bonding on guest molecule dynamics in clathrate hydrates above 200 K.
  • To compare the dynamics of tetrahydrofuran (THF) and cyclopentane (CP) guests, differing in their hydrogen bonding capabilities.

Main Methods:

  • Magic Angle Spinning (MAS) solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
  • Measurement of proton spin-lattice relaxation times (T1) for guest molecules (THF and CP) as a function of temperature.
  • Analysis of activation barriers for guest molecular motion.

Main Results:

  • The activation barrier for THF motion significantly increased to 4.7 kcal/mol above 200 K, approximately five times higher than at lower temperatures.
  • The activation barrier for CP motion remained low at 0.67 kcal/mol, consistent with low-temperature data.
  • The increased barrier for THF indicates significant host-guest hydrogen bonding interactions.

Conclusions:

  • Host lattice mobility above 200 K facilitates significant hydrogen bonding between THF guests and the clathrate cage.
  • Hydrogen bonding plays a crucial role in modulating guest dynamics in clathrate hydrates at elevated temperatures.
  • The study highlights the temperature-dependent nature of host-guest interactions in clathrate systems.