Structure and Dynamics of the Methane-Propane van der Waals Complex

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Computational Chemistry

Background:

• Van der Waals complexes provide insights into intermolecular forces.
• Internal rotation in weakly bound molecules is a complex phenomenon.
• Methane-propane is a relevant system for studying non-covalent interactions.

Purpose of the Study:

• To investigate the structure and dynamics of the methane-propane van der Waals complex.
• To characterize the internal rotation of methane within the complex.
• To compare experimental results with theoretical calculations.

Main Methods:

• Microwave spectroscopy was used to measure transitions between 7-26 GHz.
• Rotational constants and distortion constants were determined by fitting spectral lines to a semirigid rotor model.
• Ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level were performed to determine equilibrium structures and energies.

Main Results:

• Eighteen microwave transitions were assigned to the lowest energy state of the methane-propane complex.
• The experimental structure shows a van der Waals bond distance of 3.98 Å, larger than the calculated equilibrium value of 3.71 Å.
• Calculations revealed that large zero-point energy contributions flatten the potential, increasing the radial distance and facilitating internal rotation.

Conclusions:

• The internal rotation of methane within the methane-propane complex is nearly free and does not relax even at low temperatures.
• Zero-point energy plays a crucial role in determining the effective structure and dynamics of the complex.
• The interplay between angular motion and radial distance influences the van der Waals interaction in this system.