Probing London Dispersion in Proton-Bound Onium Ions: Are Alkyl-Alkyl Steric Interactions Reliably Modeled?

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• London dispersion forces are crucial noncovalent interactions influencing molecular structure and properties.
• Accurate computational modeling of dispersion interactions, especially for bulky alkyl groups like tert-butyl, remains a challenge.

Purpose of the Study:

• To experimentally probe and quantify London dispersion interactions between tert-butyl substituents in protonated bis-pyridines.
• To evaluate the performance of dispersion-corrected density functional theory (DFT-D3BJ) in predicting structures and interactions involving tert-butyl groups.

Main Methods:

• Gas-phase infrared multiphoton dissociation spectroscopy to analyze N-H stretching frequencies and infer ground-state geometries.
• Trapped ion mobility spectrometry to determine collision cross sections, providing an independent structural readout.
• Dispersion-corrected density functional theory (DFT-D3BJ) calculations with various exchange-correlation functionals.

Main Results:

• Systematic modulation of London dispersion interaction strength was observed with increasing substituent size (H to Me to tert-Bu).
• Experimental data revealed discrepancies with DFT-D3BJ predictions, particularly for tert-butyl-tert-butyl interactions.
• DFT-D3BJ calculations were found to significantly overbind alkyl-alkyl interactions, failing to reliably predict minimum energy structures when dispersion competes with hydrogen bonding.

Conclusions:

• Experimental methods provide reliable insights into London dispersion interactions in complex molecular systems.
• Current DFT-D3BJ functionals may require refinement for accurately describing strong alkyl-alkyl dispersion interactions, especially in the presence of competing noncovalent forces.