Related Experiment Video
Updated: Aug 8, 2026
11:27
Synthesis and Characterization of Functionalized Metal-organic Frameworks
Published on: September 5, 2014
Density functional study of methyl chemisorption on polycyclic aromatic hydrocarbons
Barbara V Unterreiner1, Yannick Carissan, Wim Klopper
1Lehrstuhl für Theoretische Chemie, Institut für Physikalische Chemie, Universität Karlsruhe TH, 76128 Karlsruhe, Germany. klopper@chem-bio.uni-karlsruhe.de
Summary
Methyl radicals react with large polycyclic aromatic hydrocarbons (PAHs), forming stronger bonds at PAH edges. This explains carbon deposition on heated graphite surfaces during experiments.
Area of Science:
- Chemical Physics
- Materials Science
- Computational Chemistry
Background:
- Methyl radical reactions are crucial for carbon deposition on materials like highly oriented pyrolytic graphite (HOPG).
- Understanding these reactions informs processes involving carbon formation and surface modification.
Purpose of the Study:
- To investigate the reaction mechanisms between methyl radicals and large polycyclic aromatic hydrocarbons (PAHs).
- To elucidate the factors influencing carbon deposition on HOPG surfaces.
Main Methods:
- Density functional calculations were employed to study equilibrium and transition structures.
- Analysis included computing pi orbital axis vector (POAV) and carbon altitude above the PAH plane.
- Various density functionals (B3LYP, TPSSh, BP86, TPSS) were utilized.
Main Results:
- Strongest C-CH(3) bonds form at PAH edges with sp(3) hybrid orbitals (approx. 25% s character).
- Internal PAH carbon atoms form weaker bonds (approx. 16% s character) and move out of the molecular plane.
- Computed parameters align with Hammond's postulate and Hückel molecular orbital theory.
Conclusions:
- Reaction site (edge vs. interior) significantly impacts bond strength and product geometry.
- The findings provide a molecular-level understanding of experimental carbon deposition observations.
Related Concept Videos
01:18Aromatic Hydrocarbon Anions: Structural Overview
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous overlap of p...
Due to the absence of continuous overlap of p...
01:24π Molecular Orbitals of 1,3-Butadiene
Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...
02:30Stability of Substituted Cyclohexanes
This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
01:18Frost Circles for Different Conjugated Systems
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
01:29UV–Vis Spectroscopy: Woodward–Fieser Rules
UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the contributions...
01:05Mass Spectrometry: Cycloalkane Fragmentation
In mass spectrometry, cycloalkanes exhibit distinct fragmentation patterns due to the inherent stability of their molecular ions compared to linear or branched alkanes. The ring structure of cycloalkanes provides additional stability to the molecular ions, often resulting in prominent ion peaks in the mass spectrum.
For example, cyclohexane molecular ions have a mass-to-charge ratio (m/z) of 84, which tends to produce a stronger signal than linear alkanes like hexane. This stability comes from...
For example, cyclohexane molecular ions have a mass-to-charge ratio (m/z) of 84, which tends to produce a stronger signal than linear alkanes like hexane. This stability comes from...