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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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 π orbitals.
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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

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...
Trends in Lattice Energy: Ion Size and Charge02:54

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure of the strength of this attraction. The lattice energy (ΔHlattice) of an ionic compound is defined as the energy required to separate one mole of the solid into its component gaseous ions. For the ionic solid sodium chloride, the lattice energy is the enthalpy change of the process:
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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,...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...

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Confocal Imaging of Confined Quiescent and Flowing Colloid-polymer Mixtures
10:56

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Lennard-Jones binary mixture in disordered matrices: exploring the mode coupling scenario at increasing confinement.

P Gallo1, M Rovere

  • 1Dipartimento di Fisica, Università Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy. gallop@fis.uniroma3.it

Journal of Physics. Condensed Matter : an Institute of Physics Journal
|May 27, 2011
PubMed
Summary

Mode coupling theory accurately predicts glass-forming liquid dynamics under confinement. Simulations show agreement, with deviations linked to hopping effects at higher confinement levels.

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Area of Science:

  • Computational physics
  • Soft matter physics
  • Statistical mechanics

Background:

  • Glass-forming liquids exhibit complex dynamics upon cooling.
  • Confinement in disordered matrices can alter liquid behavior.
  • Mode coupling theory (MCT) is a theoretical framework for dense liquids.

Purpose of the Study:

  • To investigate the dynamics of a binary Lennard-Jones liquid confined in a soft sphere matrix.
  • To assess the predictive power of MCT for confined glass-forming liquids.
  • To analyze the impact of packing fraction and confinement on liquid dynamics.

Main Methods:

  • Molecular dynamics (MD) simulations were employed.
  • A binary Lennard-Jones liquid was confined within soft sphere matrices.
  • Two distinct methods were used to construct the confining environment.
  • Single particle density correlators were analyzed.

Main Results:

  • Simulations showed close agreement between observed dynamics and MCT predictions across various packing fractions.
  • Discrepancies were observed and attributed to hopping effects.
  • Hopping effects become more significant with increasing confinement.

Conclusions:

  • MCT provides a good description of confined glass-forming liquid dynamics.
  • Hopping phenomena are crucial for understanding deviations from MCT under confinement.
  • The study validates MCT's applicability in certain confined systems.