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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.
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...
Mass Spectrum: Interpretation01:24

Mass Spectrum: Interpretation

An unknown compound can be established by identifying the molecular ion peak in the mass spectrum. The molecular ion peak is often weak or absent due to the predominance of fragmentation in high-energy electron beams. In such cases, a soft-energy electron beam can be used to scan the spectrum to enhance the intensity of the molecular ion peak. Additionally, chemical ionization, field ionization, and desorption ionization spectra are used to obtain a relatively intense molecular ion peak.To...
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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.
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Microwave spectrum and structural parameters for the formamide-formic acid dimer.

Adam M Daly1, Bryan A Sargus, Stephen G Kukolich

  • 1Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA.

The Journal of Chemical Physics
|November 9, 2010
PubMed
Summary

We studied the formamide-formic acid complex using microwave spectroscopy. The structure reveals hydrogen bond distances and a bent formic acid angle, with calculations supporting experimental findings.

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Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Computational Chemistry

Background:

  • Formamide and formic acid are fundamental molecules with significant roles in chemistry.
  • Understanding intermolecular interactions, such as hydrogen bonding, is crucial for chemical and biological processes.

Purpose of the Study:

  • To investigate the structure and hydrogen bonding of the formamide-formic acid complex.
  • To determine key structural parameters and compare experimental data with theoretical calculations.

Main Methods:

  • Pulsed-beam Fourier transform microwave spectroscopy was employed to measure rotational spectra.
  • Analysis of rotational constants and quadrupole coupling parameters.
  • Least-squares structure fitting using data from six isotopologues.
  • Density functional theory (DFT) and ab initio calculations (MP2, CCSD) were performed for comparison.

Main Results:

  • Accurate rotational constants and quadrupole coupling parameters were obtained for six isotopologues.
  • Key structural parameters determined include hydrogen bond distances (1.78 Å and 1.79 Å).
  • The angle(C-O-H) of formic acid in the complex (121.7°) is significantly larger than the monomer value.
  • The complex is nearly planar with a small inertial defect (-0.158 amu Ų).
  • Formamide proton deviates from the molecular plane by 15(3)°.

Conclusions:

  • The study provides a detailed structural characterization of the formamide-formic acid complex.
  • Experimental results are in good agreement with computational predictions.
  • The findings enhance our understanding of hydrogen bonding in molecular complexes.