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¹H NMR of Labile Protons: Temporal Resolution01:10

¹H NMR of Labile Protons: Temporal Resolution

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Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
The –OH proton in alcohols typically appears in the range of δ 2 to 5 ppm but can vary depending on the specific...
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Spin–Spin Coupling Constant: Overview01:08

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

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A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...
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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

906
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

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A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
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¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

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

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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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Coupled proton vibrations between two weak acids: the hinge complex between formic acid and trifluoroethanol.

Sophie M Schweer1, Arman Nejad1, Martin A Suhm1

  • 1Institute of Physical Chemistry, University of Goettingen, Tammannstr. 6, 37077 Goettingen, Germany. sschwee@gwdg.de.

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A novel hydrogen-bonded complex of formic acid and 2,2,2-trifluoroethanol was identified. Its vibrational properties were analyzed, providing a benchmark for studying proton transfer dynamics in similar systems.

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Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Quantum Chemistry

Background:

  • Hydrogen bonding plays a crucial role in chemical reactions and molecular interactions.
  • Understanding proton transfer dynamics in molecular complexes is essential for various chemical processes.

Purpose of the Study:

  • To investigate the structure and vibrational properties of a formic acid and 2,2,2-trifluoroethanol complex.
  • To establish a benchmark system for studying proton transfer in hydrogen-bonded complexes.

Main Methods:

  • Co-expansion of formic acid and 2,2,2-trifluoroethanol through a slit nozzle into vacuum.
  • Infrared absorption and Raman scattering spectroscopy for vibrational analysis.
  • Quantum chemical calculations (DFT) for theoretical validation.

Main Results:

  • Formation of a dominant, hinge-like 1:1 complex.
  • Assignment of two OH stretching fundamentals separated by 100 cm⁻¹.
  • Quantum chemical calculations successfully reproduced experimental findings.
  • In-phase OH stretch excitation remains below the proton transfer barrier.

Conclusions:

  • The formic acid-2,2,2-trifluoroethanol complex serves as a well-behaved model system.
  • This study provides a benchmark for exploring vibrational resonances in acid-alcohol systems.
  • Insights into proton transfer dynamics and vibrational coupling were gained.