Increasing Complexity in a Conformer Space Step-by-Step: Weighing London Dispersion against Cation-π Interactions

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Organic Chemistry

Background:

• London dispersion is a weak, short-range intermolecular force.
• Its contribution to the stability and structure of organic molecules is debated.
• Understanding these forces is crucial for predicting molecular behavior.

Purpose of the Study:

• To evaluate the importance of London dispersion in moderately large organic molecules.
• To investigate the interplay between London dispersion and other noncovalent interactions.
• To assess the predictive accuracy of computational methods for molecular structure.

Main Methods:

• Experimental evaluation using molecular torsion balance.
• Gas-phase cryogenic ion vibrational predissociation (CIVP) spectroscopy.
• Solid-state Fourier transform infrared (FT-IR) spectroscopy and single-crystal X-ray crystallography.
• Density functional theory (DFT) calculations with conformational analysis.

Main Results:

• London dispersion provides a small attractive contribution, as expected.
• The interplay of London dispersion with other noncovalent interactions significantly impacts molecular structure.
• Pairwise interactions, like between tert-butyl groups, are modest.
• In silico predictions of structure for flexible organic molecules are less reliable than anticipated.

Conclusions:

• London dispersion's role is subtle and context-dependent.
• Accurate prediction of complex molecular structures requires improved computational models.
• Experimental and computational studies are essential for understanding noncovalent interactions.