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NMR Spectroscopy of Benzene Derivatives

NMR Spectroscopy of Benzene Derivatives

Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...

Structure of Benzene: Molecular Orbital Model

Structure of Benzene: Molecular Orbital Model

According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...

Mass Spectrometry: Aromatic Compound Fragmentation

Mass Spectrometry: Aromatic Compound Fragmentation

Upon ionization, aromatic compounds generate a molecular ion that is observed as a prominent peak in their mass spectra. For example, the molecular ion peak for benzene appears at a mass-to-charge ratio of 78, while toluene is observed at a mass-to-charge ratio of 92. The molecular ion benzene is highly stable and does not readily undergo further fragmentation due to the significant amount of energy required to disrupt the aromatic stability of the benzene ring. In contrast, the molecular ion...

UV–Vis Spectroscopy: Molecular Electronic Transitions

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...

UV–Vis Spectroscopy of Conjugated Systems

UV–Vis Spectroscopy of Conjugated Systems

Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π → π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λmax.
One of the factors influencing λmax is the extent...

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Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−

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Threshold Photoelectron Spectrum of m-Benzyne.

M Gerlach¹, E Karaev¹, D Schaffner¹

¹Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.

The Journal of Physical Chemistry Letters

|November 30, 2022

Summary

Researchers studied the electronic structure of meta-benzyne (C6H4) biradicals. They determined its ionization energy and vibrational modes, confirming its monocyclic structure in the cation state.

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Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

Published on: October 9, 2012

Area of Science:

Physical Chemistry
Quantum Chemistry
Spectroscopy

Background:

Meta-benzyne (C6H4) biradicals possess unique electronic structures, making them subjects of significant chemical interest.
Understanding the electronic properties and structural dynamics of such reactive intermediates is crucial in organic chemistry.

Purpose of the Study:

To experimentally determine the adiabatic ionization energy of the meta-benzyne biradical.
To investigate the vibrational modes of the meta-benzyne cation.
To elucidate the structural characteristics of the meta-benzyne cation using spectroscopic methods.

Main Methods:

Photoion mass-selected threshold photoelectron spectroscopy was employed to study the meta-benzyne biradical.
Franck-Condon spectral modeling was utilized to analyze the electronic transition.
High-level computational chemistry methods were used for comparison.

Main Results:

An adiabatic ionization energy of 8.65 ± 0.015 eV was experimentally derived for meta-benzyne.
A vibrational progression of 0.10 eV was assigned to the ν9+ ring breathing mode of the cation.
The experimental spectrum strongly supports a monocyclic C6 framework for the observed 2A1 cation state.

Conclusions:

The study successfully characterized the electronic and vibrational properties of the meta-benzyne cation.
Computational and experimental results agree well, confirming the monocyclic structure of the dominant cation state.
The energetically close bicyclic 2A2 cation state was found to have low Franck-Condon factors and was not observed experimentally.