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Related Concept Videos

Atomic Nuclei: Magnetic Resonance01:05

Atomic Nuclei: Magnetic Resonance

638
The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
638
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

632
In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
632
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

1.7K
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
1.7K
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

981
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
981
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Molecular influence on nuclear-quadrupole-coupling effects in laser induced alignment.

Linda V Thesing1,2,3, Andrey Yachmenev1,2, Rosario González-Férez4

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Nuclear-quadrupole interactions significantly influence molecular alignment after laser pulses. The interplay between hyperfine and rotational energy levels dictates spin-rotational dynamics in asymmetric-top molecules.

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Area of Science:

  • Physical Chemistry
  • Quantum Mechanics
  • Molecular Physics

Background:

  • Nuclear-quadrupole interactions affect molecular energy levels.
  • Field-free impulsive alignment is a technique to orient molecules using laser pulses.
  • Asymmetric-top molecules exhibit complex rotational dynamics.

Purpose of the Study:

  • To computationally investigate the impact of nuclear-quadrupole interactions on molecular alignment.
  • To analyze the influence of hyperfine- and rotational-energy-level structures on alignment dynamics.
  • To compare these effects in various asymmetric-top molecules with large nuclear spins.

Main Methods:

  • Computational study of molecular dynamics.
  • Analysis of nuclear spin and external field interactions.
  • Focus on hyperfine and rotational energy level structures.

Main Results:

  • The magnitude of hyperfine splittings relative to rotational energy splittings is critical for post-laser pulse spin-rotational dynamics.
  • The influence of quadrupole coupling on rotational dynamics diminishes when highly excited rotational states are dominant.
  • Specific molecules studied include iodobenzene and its diiodo derivatives.

Conclusions:

  • Nuclear-quadrupole interactions play a key role in field-free impulsive alignment.
  • Molecular spin-rotational dynamics are sensitive to the balance between hyperfine and rotational energy scales.
  • Understanding these interactions is crucial for controlling molecular alignment.