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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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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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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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Conformations of Ethane and Propane

19.7K
In an organic molecule, free rotation about the carbon-carbon single bond results in energetically different conformers of the molecule. Due to this rotation, called the internal rotation, ethane has two major conformations — staggered and eclipsed.
Staggered conformation is a low energy and more stable conformation with the C-H bonds on the front carbon placed at 60°dihedral angles relative to the C-H bonds on the back carbon, leading to a reduced torsional strain. In staggered...
19.7K
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

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The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this...
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Stability of Conjugated Dienes

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Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
4.7K
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

17.4K
Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal...
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Theoretical in-Solution Conformational/Tautomeric Analyses for Chain Systems with Conjugated Double Bonds Involving

Peter I Nagy1

  • 1Center for Drug Design and Development, the University of Toledo, Toledo, OH 43606, USA. pnagy@utnet.utoledo.edu.

International Journal of Molecular Sciences
|May 19, 2015
PubMed
Summary

The s-trans conformation is preferred for X=CH-CH=Y structures in gas and solution phases. Computational methods predict conformational preferences and transition barriers, exploring solvent effects on molecular behavior.

Keywords:
FEP/MCIEF-PCM/B97D/aug-cc-pvqzIEF-PCM/CCSD(T)/ CBSs-cis/s-trans equilibriumtautomerization mechanism

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Area of Science:

  • Computational Chemistry
  • Molecular Modeling
  • Physical Chemistry

Background:

  • Investigates conformational and tautomeric transformations in X=CH-CH=Y systems.
  • Extends previous studies on s-cis/s-trans conformational equilibria of related molecules.

Purpose of the Study:

  • To study conformational/tautomeric transformations for X=CH-CH=Y structures in various phases.
  • To predict conformational preferences and transition barriers in gas and solution phases.
  • To explore the influence of solvent models on molecular behavior.

Main Methods:

  • Utilized B97D/aug-cc-pvqz and CCSD(T)CBS computational levels for gas-phase analysis.
  • Employed IEF-PCM (Integral Equation Formalism of the Polarizable Continuum Model) for solution-phase calculations.
  • Incorporated a tetrahydrate model and Monte Carlo simulations with free energy perturbation for explicit solvent effects.

Main Results:

  • The s-trans conformation is energetically favored in the gas phase and in solution.
  • Transition state barriers for central C-C bond rotations range from 29-36 kJ/mol in the gas phase.
  • The tetrahydrate model reduced the transition barrier to 27 kJ/mol; explicit solvent models showed discrepancies with PCM but predicted the same prevalent conformation.

Conclusions:

  • The s-trans conformation is consistently preferred across different environments.
  • Solvent models significantly impact calculated solvation energies, but conformational predictions remain robust.
  • Water-assisted double proton-relay is not viable in non-protic solvents.