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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.
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

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.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
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...
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...

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High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy
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Phonon dynamics and electron-phonon coupling in pristine picene.

Alberto Girlando1, Matteo Masino, Ivano Bilotti

  • 1Dip. di Chimica G.I.A.F. and INSTM-UdR Parma, Universitá di Parma, Parco Area delle Scienze 17/A, I-43124 Parma, Italy. girlando@unipr.it

Physical Chemistry Chemical Physics : PCCP
|December 24, 2011
PubMed
Summary

This study analyzes the phonon structure of crystalline picene, an organic semiconductor. Researchers detailed lattice and intramolecular vibrations, providing a complete understanding of its vibrational properties.

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Area of Science:

  • Solid-state physics
  • Materials science
  • Organic electronics

Background:

  • Picene is a recently developed organic semiconductor with potential applications in electronics.
  • Understanding its vibrational properties is crucial for predicting and optimizing its electronic behavior.

Purpose of the Study:

  • To conduct a comprehensive analysis of the phonon structure in crystalline picene.
  • To investigate both lattice and intramolecular vibrations and their couplings.

Main Methods:

  • Polarized Raman spectroscopy and lattice dynamics calculations for lattice phonons.
  • Raman, infrared spectroscopy, and density functional theory (DFT) for intramolecular modes.
  • Semiempirical INDO/S method for calculating transfer integrals and coupling constants.

Main Results:

  • An exhaustive assignment of lattice phonons was achieved.
  • Intramolecular modes were characterized, and coupling with low-frequency molecular vibrations was analyzed.
  • Molecule-to-molecule transfer integrals and Peierls/Holstein coupling constants were evaluated.

Conclusions:

  • The study provides a complete phonon structure analysis of crystalline picene.
  • The findings contribute to a deeper understanding of picene's vibrational dynamics and electronic coupling.
  • This research supports the development of picene-based organic electronic devices.