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Related Concept Videos

NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

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 in...
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
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.
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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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...
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...
UV–Vis Spectroscopy: Molecular Electronic Transitions01:16

UV–Vis Spectroscopy: Molecular Electronic Transitions

In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this process,...

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A switchable molecular rotator: neutron spectroscopy study on a polymeric spin-crossover compound.

J Alberto Rodríguez-Velamazán1, Miguel A González, José A Real

  • 1Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain. jarv@unizar.es

Journal of the American Chemical Society
|February 28, 2012
PubMed
Summary

Molecular rotation in spin-crossover compounds is linked to spin state changes. This study reveals pyrazine ligand rotation is suppressed in the low-spin state, impacting material properties.

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Area of Science:

  • Materials Science
  • Solid-State Chemistry
  • Spectroscopy

Background:

  • Spin-crossover (SCO) compounds exhibit distinct high-spin and low-spin states.
  • Molecular dynamics play a crucial role in SCO phenomena.
  • Understanding the interplay between molecular motion and spin states is key for material design.

Purpose of the Study:

  • To investigate the relationship between molecular rotation and spin-crossover transitions.
  • To characterize the dynamics of the pyrazine ligand in a polymeric SCO compound.
  • To explore the influence of external stimuli on molecular motion and spin states.

Main Methods:

  • Quasielastic neutron scattering (QENS) for probing molecular dynamics.
  • Solid-state Deuterium Nuclear Magnetic Resonance (2H NMR) spectroscopy.
  • Utilizing a wide time scale (10⁻¹³–10⁻³ s) for dynamic analysis.

Main Results:

  • Demonstrated a clear correlation between pyrazine ligand rotation and spin-state switching.
  • Observed 4-fold jump motion of pyrazine rings in the high-spin state.
  • Found that motion is suppressed in the low-spin state and further restricted by benzene guest molecules.

Conclusions:

  • Molecular rotation is intrinsically coupled to the spin-crossover transition.
  • The dynamics of the pyrazine ligand are directly modulated by the spin state.
  • External stimuli (temperature, chemical environment) can control both spin state and molecular motion.