Prototypical cyclohexane dimers: spectroscopic evidence for σ stacking at low temperatures
N Ramanathan1, Shubhra Sarkar, K Sundararajan
1Materials Chemistry and Metal Fuel Cycle Group, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. nram@igcar.gov.in.
Physical Chemistry Chemical Physics : PCCP
|December 18, 2019
Summary
This study provides the first spectroscopic evidence for sigma (σ) stacking in cyclohexane dimers. These interactions, crucial for structural motifs, are stabilized by dispersion forces and a unique
Area of Science:
- Physical Chemistry
- Spectroscopy
- Computational Chemistry
Background:
- Sigma (σ) stacking interactions are fundamental in determining the structures of aliphatic saturated cyclic hydrocarbons.
- Previous research has primarily focused on pi (π) stacking, with less emphasis on σ stacking in such systems.
Purpose of the Study:
- To provide unambiguous spectroscopic evidence for σ stacking interactions in cyclohexane dimers.
- To investigate the structural motifs and stabilization mechanisms of cyclohexane dimers.
Main Methods:
- Molecular beam experiments using effusive nozzle and supersonic jet sources.
- Infrared spectroscopy for dimer characterization.
- Ab initio computations, including Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analyses.
- Energy Decomposition Analysis (EDA).
Main Results:
- Spectroscopic evidence confirmed the existence of cyclohexane dimers stabilized by σ stacking at low temperatures.
- Computational studies identified eclipsed, parallel displaced, and T-shaped dimer structures.
- NBO analysis revealed significant σ → σ* interactions, while AIM analysis suggested 'dihydrogen bonding' as an inducing factor.
- EDA indicated substantial contributions from dispersion interactions to dimer stability.
Conclusions:
- Sigma (σ) stacking is a significant interaction in stabilizing cyclohexane dimers.
- The observed stacking is a result of a combination of σ → σ* interactions, 'dihydrogen bonding', and dispersion forces.
- This work highlights the importance of σ stacking in understanding the supramolecular chemistry of saturated hydrocarbons.
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