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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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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...
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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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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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Metal-Semiconductor Junctions

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The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
Schottky Barriers
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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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Transition from Strong to Weak Electronic Coupling in a Single-Molecule Junction.

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Charge transport in single-molecule junctions is temperature-insensitive for coherent transport. Stretching junctions can induce a transition from strong to weak electronic coupling, altering current-voltage characteristics.

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Area of Science:

  • Condensed matter physics
  • Molecular electronics
  • Nanotechnology

Background:

  • Investigating charge transport mechanisms in single-molecule junctions is crucial for advancing molecular electronics.
  • Understanding the influence of temperature and electrode coupling on charge transport is key to designing molecular devices.

Purpose of the Study:

  • To investigate charge transport in single-molecule junctions using gold nanoelectrodes.
  • To analyze the temperature dependence of conductance and the effect of electrode separation on junction behavior.
  • To elucidate the transition from strong to weak electronic coupling regimes induced by stretching.

Main Methods:

  • Fabrication and characterization of single-molecule junctions using gold nanoelectrodes.
  • Low-bias conductance measurements during mechanical stretching of molecular junctions.
  • Low-temperature current-voltage (I-V) measurements to analyze junction conformations and electronic coupling.
  • Statistical analysis of conductance and I-V characteristics as a function of electrode distance.

Main Results:

  • The most probable single-molecule conductance is insensitive to temperature, consistent with off-resonant coherent transport.
  • Low-temperature I-V measurements reveal smooth, tunneling-like shapes for some junction conformations.
  • A stretching-induced transition to a state with a zero-bias gap and finite-bias resonances was observed in approximately 25% of junctions.
  • This transition is attributed to a shift from strong to weak electronic coupling, involving significant renormalization of the injection barrier and electronic coupling.

Conclusions:

  • Single-molecule junction conductance is robust against temperature variations under coherent transport conditions.
  • Mechanical stretching can induce a significant electronic transition within molecular junctions, altering their transport properties.
  • The observed transition highlights the dynamic nature of molecular-electrode interfaces and their impact on charge transport.