π-Stacking Isomerism in Polycyclic Aromatic Hydrocarbons: The 2-Naphthalenethiol Dimer

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• π-Stacking interactions between aromatic hydrocarbons are common but their physical origin is debated.
• Understanding these forces is crucial for fields like materials science and drug design.
• Previous studies often focused on unsubstituted aromatic systems.

Purpose of the Study:

• To investigate the nature of π-stacking in a model system using thiol-substituted naphthalene.
• To determine the dominant forces governing the interaction in a specific dimer orientation.
• To provide experimental and computational insights into the π-stacking mechanism.

Main Methods:

• Rotational spectroscopy was employed to identify and characterize isomers of the 2-naphthalenethiol dimer.
• Molecular orbital calculations were used to rationalize experimental data and explore the potential energy surface.
• Symmetry-Adapted Perturbation Theory (SAPT) was applied for energy decomposition analysis.

Main Results:

• Two isomers of the 2-naphthalenethiol dimer were identified with a parallel-displaced crossed orientation.
• The potential energy surface for the dimer was found to be shallow.
• SAPT analysis revealed that dispersion forces are the dominant noncovalent interaction, with Pauli repulsion also playing a role.

Conclusions:

• The study provides experimental evidence that dispersion forces are key contributors to π-stacking in this model system.
• The findings suggest π-stacking can be viewed as a balance between attractive dispersion forces and repulsive Pauli forces.
• This work offers a refined understanding of noncovalent interactions in aromatic systems.