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Related Concept Videos

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,...
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...
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...
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...
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...
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...

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High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy
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Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study.

Gennaro Pescitelli1, Daniele Padula, Fabrizio Santoro

  • 1Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, I-56126 Pisa, Italy. ripes@dcci.unipi.it

Physical Chemistry Chemical Physics : PCCP
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Summary

Solid-state electronic circular dichroism (CD) spectra of sesquiterpenoid 1 exhibit unique vibrational fine structure and new bands compared to solution spectra. Theoretical calculations successfully reproduced these solid-state spectral features.

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Area of Science:

  • Organic Chemistry
  • Spectroscopy
  • Computational Chemistry

Background:

  • Electronic circular dichroism (CD) spectroscopy is a powerful tool for determining molecular structure.
  • Solid-state CD spectra can differ significantly from solution spectra due to intermolecular interactions.
  • Sesquiterpenoids are a diverse class of natural products with complex structures.

Purpose of the Study:

  • To investigate the differences between solid-state and solution electronic CD spectra of sesquiterpenoid 1β,10β-epoxydesacetoxymatricarin (1).
  • To elucidate the role of intermolecular interactions and vibrational effects in shaping the solid-state CD spectrum.
  • To validate theoretical methods for predicting solid-state CD spectra.

Main Methods:

  • Measurement of electronic CD spectra in both microcrystalline solid state and solution.
  • X-ray crystallography to obtain the solid-state geometry of compound 1.
  • Time-Dependent Density Functional Theory (TDDFT) CD calculations using X-ray derived structures.
  • Modeling of potential energy surfaces at the harmonic level, including Duschinsky and Herzberg-Teller effects, to reproduce vibrational structure.

Main Results:

  • The solid-state CD spectrum of compound 1 displayed a pronounced vibrational fine structure in the n-π* enone transition region and a new band in the π-π* region, absent in the solution spectrum.
  • TDDFT CD calculations incorporating exciton-type couplings between proximate molecules in the crystal lattice successfully reproduced the observed solid-state spectral features.
  • Vibrational structure in the CD spectrum was also accurately reproduced for the isolated molecule through advanced computational modeling.

Conclusions:

  • Intermolecular interactions in the microcrystalline solid state significantly influence the electronic CD spectrum of sesquiterpenoid 1.
  • Theoretical calculations, including exciton coupling and vibrational effects, are effective in predicting and explaining solid-state CD spectra.
  • This study provides insights into the solid-state behavior of sesquiterpenoids and advances the application of computational methods in spectroscopy.