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Mass Spectrometry: Amine Fragmentation00:55

Mass Spectrometry: Amine Fragmentation

Amines can be identified using mass spectroscopy based on their characteristic fragmentation patterns. The molecular ions of amines undergo fragmentation via ⍺-cleavage. The ⍺-cleavage of the carbon-carbon bonds in amines generates an alkyl radical and resonance-stabilized nitrogen-containing cation.
In amines, the number of nitrogen atoms affects the mass of the molecular ion, which is described by the nitrogen rule of mass spectrometry. This rule states that a compound containing a single or...
Mass Spectrometry of Amines01:15

Mass Spectrometry of Amines

In mass spectroscopy, amines undergo fragmentation to give parent ions with odd molecule weights. This observed mass spectrum follows the nitrogen rule; a molecule with an odd number of nitrogen atoms produces a molecular ion with an odd molecular weight. Amines undergo fragmentation through α cleavage, producing nitrogen-containing cations—iminium ions—and alkyl radicals. Mass spectra of aromatic and cyclic aliphatic amines exhibit strong molecular ion peaks, but acyclic aliphatic amines show...
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...
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.
NMR Spectroscopy Of Amines01:19

NMR Spectroscopy Of Amines

In proton NMR spectroscopy, primary amines and secondary amines showcase their N–H protons as a broad signal in the chemical shift range between δ 0.5 and 5 ppm. The exact position in this range depends on several factors, including sample concentration, hydrogen bonding, and the type of solvent used. Since amine protons undergo fast proton exchange in solution, the protons are labile and therefore do not participate in any splitting with adjacent protons. Thus, the observed peak is broad and...
2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)01:19

2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does...

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Scattering resonances in slow NH3-He collisions.

Koos B Gubbels1, Sebastiaan Y T van de Meerakker, Gerrit C Groenenboom

  • 1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany. K.Gubbels@science.ru.nl

The Journal of Chemical Physics
|February 25, 2012
PubMed
Summary

We theoretically investigated collisions between ammonia (NH3) molecules and helium (He) atoms, identifying key scattering resonances. These findings are crucial for future molecular beam experiments involving Stark-decelerated NH3.

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Area of Science:

  • Physical Chemistry
  • Quantum Mechanics
  • Atomic and Molecular Collisions

Background:

  • Understanding molecular collisions is fundamental in physical chemistry.
  • Ammonia (NH3) molecules are important in various chemical processes and can be manipulated with electric fields (Stark deceleration).
  • Helium (He) atoms are often used as collision partners due to their inert nature.

Purpose of the Study:

  • To theoretically investigate slow collisions between ammonia (NH3) and helium (He) atoms.
  • To identify and analyze scattering resonances in NH3-He collisions.
  • To provide insights for future experimental observations of these resonances.

Main Methods:

  • Development of a four-dimensional potential energy surface for NH3-He interactions based on ab initio calculations.
  • Application of fully converged quantum close-coupling calculations.
  • Calculation of state-to-state integral and differential cross sections over a wide energy range (10^-4 to 130 cm^-1).

Main Results:

  • Identification of pronounced shape and Feshbach resonances in NH3-He collisions.
  • Resonances were particularly prominent for inelastic collisions leading to the symmetric umbrella state of NH3 (j=k=1).
  • Detailed analysis of resonant structures using scattering wavefunctions, phase shifts, and lifetimes.

Conclusions:

  • The study predicts observable scattering resonances in NH3-He collisions.
  • These findings have implications for experiments using Stark-decelerated NH3 beams.
  • The theoretical framework provides a basis for understanding and predicting resonance phenomena in similar molecular systems.