Aromatic molecules in anion recognition: electrostatics versus H-bonding.
Holger Schneider1, Kristen M Vogelhuber, Florian Schinle
1JILA, University of Colorado, Boulder, Colorado 80309, USA.
Journal of the American Chemical Society
|October 9, 2007
Summary
Negative ions prefer hydrogen bonding to hydrogen atoms in fluorinated aromatic molecules, even when carbons carry positive charges. Bifurcated hydrogen bonds are more stable than linear ones.
Area of Science:
- Physical Chemistry
- Chemical Physics
- Spectroscopy
Background:
- Investigating interactions between anions and aromatic molecules is crucial for understanding chemical bonding.
- Fluorination offers a method to tune the electronic properties and charge distribution within aromatic systems.
- Understanding anion-ligand interactions is key in fields ranging from atmospheric chemistry to materials science.
Purpose of the Study:
- To explore the binding preferences of various anions (Cl-, I-, SF6-) to fluorinated benzene ligands (C6FnH(6-n)).
- To investigate how varying degrees of fluorination influence the interaction sites between anions and aromatic systems.
- To elucidate the nature of hydrogen bonding in anion-aromatic complexes.
Main Methods:
- Infrared photodissociation spectroscopy was employed to study mass-selected anion complexes.
- Computational chemistry methods were utilized to analyze binding energies and charge distributions.
- Systematic variation of fluorination levels (n=0-5) on the benzene ring.
Main Results:
- Anions preferentially form hydrogen bonds with the hydrogen atoms of the aromatic ligand, not the carbon atoms.
- This preference persists even at high fluorination levels where carbon atoms exhibit positive partial charges.
- Bifurcated hydrogen bonds, involving two adjacent C-H groups, were found to be energetically more favorable than linear hydrogen bonds to a single C-H group.
Conclusions:
- The study reveals a surprising preference for hydrogen bonding over interactions with positively charged carbon centers in fluorinated aromatics.
- Hydrogen bonding to C-H groups is a dominant interaction pathway for anions in these systems.
- Bifurcation of hydrogen bonds offers enhanced stability, influencing the structural preferences of anion-aromatic complexes.
More Related Videos
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...
01:18Aromatic Hydrocarbon Cations: Structural Overview
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...
Removing one hydrogen from the intervening CH2 group with both...
01:25Basicity of Heterocyclic Aromatic Amines
Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).
01:18Basicity of Aromatic Amines
The basicity of aromatic amines is much weaker than that of aliphatic amines due to the involvement of the lone pair of electrons over the N atom in resonance with the aryl rings. Generally, the electron-donating ability of any substituents on the aryl ring of aromatic amines increases the basicity of the amine by increasing electron density, and hence the availability of lone pair on the nitrogen. On the other hand, electron-withdrawing functional groups on the aryl ring of amines decrease the...
01:21Basicity of Aliphatic Amines
Amines can behave as Brønsted–Lowry bases by accepting a proton from the acid to form corresponding conjugate acids. Due to a lone pair of nonbonding electrons, aliphatic amines can also act as Lewis bases by forming a covalent bond with an electrophile.
To measure the basicity of amines, two conventions are generally used. The first defines Kb as the basicity constant for the deprotonation reaction of water by the amine, as presented in Figure 1. Conventionally, lower Kb indicates higher...
To measure the basicity of amines, two conventions are generally used. The first defines Kb as the basicity constant for the deprotonation reaction of water by the amine, as presented in Figure 1. Conventionally, lower Kb indicates higher...
01:18π Molecular Orbitals of the Allyl Cation and Anion
An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl anions can be obtained by treating propene with a strong base that can deprotonate methyl groups. Allyl cations are formed as intermediates during substitution reactions involving allylic halides. In both cases, the hybridization of the allylic carbon changes from sp3 to sp2, giving rise to a carbon chain with three sp2-hybridized carbons, each with an...