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Spin–Spin Coupling Constant: Overview01:08

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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.
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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.
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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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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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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
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Acoustic emissions from spin crossover complexes.

Sarah M Kamel1,2, Lajos Daróczi1, László Z Tóth1

  • 1Department of Solid State Physics, Doctoral School of Physics, University of Debrecen P.O. Box 2 H-4010 Debrecen Hungary dbeke@science.unideb.hu.

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Acoustic emission signals were detected during spin transitions in iron compounds, linked to microstructural changes. Despite reversible spin switching, acoustic activity decreased over cycles, indicating irreversible sample evolution.

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Area of Science:

  • Materials Science
  • Solid-State Physics
  • Chemistry

Background:

  • Spin crossover (SCO) materials exhibit a transition between high-spin and low-spin states.
  • Thermally induced SCO can involve significant volume and microstructural changes.
  • Acoustic emission (AE) is a technique sensitive to dynamic processes and material changes.

Purpose of the Study:

  • To investigate acoustic emission during thermally induced spin transitions in iron(II) SCO complexes.
  • To correlate AE signals with calorimetric data to understand the physical origin of the emission.
  • To explore the microstructural evolution of SCO materials during thermal cycling.

Main Methods:

  • Detection of acoustic emission signals during heating and cooling cycles.
  • Simultaneous measurement of calorimetric data (DSC) to monitor spin transitions.
  • Analysis of AE signal amplitude and energy probability distribution functions.
  • Investigation of temporal shapes of AE events (avalanches).

Main Results:

  • Acoustic emission was detected and correlated with calorimetric signals during spin transitions in [Fe(HB(tz)3)2] and [Fe(Htrz)(trz)2]BF4.
  • AE signals are attributed to elastic waves generated by volume and microstructural changes during the spin transition.
  • AE activity decreased with successive thermal cycles, despite reversible spin switching, indicating irreversible microstructural evolution.
  • AE amplitude and energy distributions followed power-law behavior with similar exponents for both samples, suggesting universality.

Conclusions:

  • Acoustic emission is a sensitive probe of microstructural changes accompanying spin transitions in SCO materials.
  • Irreversible microstructural evolution occurs in these SCO materials over thermal cycles, even when the spin transition itself is reversible.
  • The observed power-law behavior and scaled avalanche shapes suggest universal critical dynamics in the spin transition process.