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

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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...
NMR Spectroscopy of Benzene Derivatives01:37

NMR Spectroscopy of Benzene Derivatives

Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling constants depend...
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

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 first.
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.
UV–Vis Spectroscopy: Woodward–Fieser Rules01:29

UV–Vis Spectroscopy: Woodward–Fieser Rules

UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the contributions...

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Methylsalicylate: a rotational spectroscopy study.

Sonia Melandri1, Barbara Michela Giuliano, Assimo Maris

  • 1Dipartimento di Chimica G. Ciamician dell'Università, Via Selmi 2, I-40126 Bologna, Italy. sonia.melandri@unibo.it

The Journal of Physical Chemistry. A
|August 29, 2007
PubMed
Summary

Methyl salicylate exists in a stable form due to intramolecular hydrogen bonding. The internal rotation of its methyl group has a barrier of 5.38 kJ mol(-1), confirmed by rotational spectroscopy.

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Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Computational Chemistry

Background:

  • Methyl salicylate is a molecule known to exhibit tautomeric and conformational possibilities.
  • Understanding its stable structure and dynamics is crucial for various chemical applications.

Purpose of the Study:

  • To investigate the ground electronic state structure and conformational preferences of methyl salicylate.
  • To determine the energy barrier associated with the internal rotation of the methyl group.

Main Methods:

  • Free-jet rotational spectroscopy was employed to obtain high-resolution spectra.
  • Quantum chemical calculations were utilized to identify the lowest-energy conformer.

Main Results:

  • The study identified a stable conformer of methyl salicylate stabilized by an intramolecular hydrogen bond.
  • Rotational transitions exhibited splitting due to methyl group internal rotation.
  • The barrier to this internal rotation was quantified at V(3) = 5.38 kJ mol(-1).

Conclusions:

  • Methyl salicylate predominantly exists in a specific conformation stabilized by intramolecular hydrogen bonding.
  • The internal rotation of the methyl group is a significant dynamic feature with a well-defined energy barrier.