Ions interacting with planar aromatic molecules: modeling electron transfer reactions.
B O Forsberg1, J D Alexander, T Chen
1Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden.
The Journal of Chemical Physics
|February 15, 2013
Summary
We developed a model to calculate charge exchange cross sections for multiply charged ions interacting with Polycyclic Aromatic Hydrocarbons (PAHs). This model accurately predicts potential energy barriers and electron transfer reactions, aiding interstellar chemistry studies.
Area of Science:
- Theoretical Chemistry
- Astrochemistry
- Atomic and Molecular Physics
Background:
- Polycyclic Aromatic Hydrocarbons (PAHs) are abundant in interstellar environments and play crucial roles in astrochemistry.
- Understanding ion-molecule interactions with PAHs is essential for modeling chemical processes in space.
- Multiply charged cations interacting with PAHs are relevant to various astrophysical phenomena.
Purpose of the Study:
- To develop a theoretical model for calculating absolute charge exchange cross sections for multiply charged cations interacting with PAHs.
- To provide analytical solutions for the potential energy surface experienced by electrons in the field of charged PAH discs.
- To compare model predictions with Density Functional Theory (DFT) calculations and experimental data.
Main Methods:
- Modeling PAHs as conducting, infinitely thin, randomly oriented circular discs.
- Deriving an analytical solution for the potential energy surface of electron-ion-disc interactions.
- Utilizing DFT-calculated PAH ionization energies as model inputs.
- Including orientation-dependent polarizabilities for larger PAHs.
Main Results:
- The model accurately predicts the location and height of potential energy barriers, showing good agreement with DFT calculations.
- Model results favorably compare with experimental data for single- and multiple electron transfer reactions.
- The importance of orientation-dependent polarizabilities for larger PAHs was demonstrated.
Conclusions:
- The presented theoretical model provides accurate charge exchange cross sections for ion-PAH interactions.
- The model's agreement with DFT and experimental data validates its utility.
- These findings contribute to understanding the role of PAHs and their ions in interstellar chemistry and related spectroscopies.
Related Concept Videos
01:14π 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...
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:16Electrophilic Aromatic Substitution: Overview
In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound.
01:11ortho–para-Directing Activators: –CH3, –OH, –NH2, –OCH3
All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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...
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.