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¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
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In the late 1800s, the revelation that light extended beyond visible wavelengths led to the discovery of X-rays by Wilhelm Roentgen. Recognized as high-energy electromagnetic radiation with short wavelengths, X-rays prompted exploration into their interaction with crystals. Max von Laue proposed in 1912 that the periodic arrangement of atoms, ions, or molecules in crystals would cause them to diffract X-rays, a hypothesis confirmed through experiments with copper sulfate and zinc sulfide...
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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...

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Variable temperature neutron diffraction and X-ray charge density studies of tetraacetylethane.

Paula M B Piccoli1, Thomas F Koetzle, Arthur J Schultz

  • 1Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439, USA. pmbpiccoli@hotmail.com

The Journal of Physical Chemistry. A
|July 3, 2008
PubMed
Summary

The study of enolized tetraacetylethane (TAE) reveals its intramolecular hydrogen bond behavior changes with temperature. Disorder at 298 K vanishes at low temperatures due to intermolecular contacts, impacting hydrogen bond characteristics.

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Area of Science:

  • Solid-state chemistry
  • Crystallography
  • Hydrogen bonding

Background:

  • Intramolecular hydrogen bonds are crucial in molecular structure and reactivity.
  • Understanding temperature-dependent hydrogen bond dynamics is key to materials science.
  • Enolized 3,4-diacetyl-2,5-hexanedione (tetraacetylethane, TAE) features a short, strong intramolecular hydrogen bond.

Purpose of the Study:

  • To investigate the temperature-dependent behavior of the intramolecular hydrogen bond in TAE.
  • To characterize structural changes and hydrogen bond dynamics from 20 K to 298 K.
  • To elucidate the role of intermolecular interactions and computational methods in understanding hydrogen bonds.

Main Methods:

  • Single crystal neutron diffraction at variable temperatures (20–298 K).
  • X-ray diffraction at 20 K for charge density analysis.
  • Quantum mechanical calculations with periodic boundary conditions.

Main Results:

  • Hydrogen bond distances (O2-H1 and O1...H1) show significant temperature dependence.
  • Resonance-assisted hydrogen bonding is suggested by C-O bond lengths at 298 K.
  • Structural disorder at 298 K diminishes at low temperatures, potentially due to C-H...O contacts.
  • Quantum calculations with periodicity better reproduced experimental structures.

Conclusions:

  • The intramolecular hydrogen bond in TAE exhibits distinct temperature-dependent characteristics.
  • Intermolecular C-H...O contacts play a role in ordering the structure at low temperatures.
  • Computational methods, particularly with periodic boundary conditions, are valuable for studying such systems.