Ab initio and DFT studies on van der Waals trimers: the OCS.(CO2)2 complexes

Area of Science:

• Computational chemistry
• Quantum chemistry
• Molecular interactions

Background:

• Van der Waals trimers are crucial for understanding intermolecular forces.
• Previous studies on similar systems like (CO2)3 indicate the existence of multiple isomers.

Purpose of the Study:

• To investigate the structure and stability of the OCS.(CO2)2 van der Waals trimer.
• To compare theoretical calculations with experimental microwave spectroscopy data.
• To analyze the role of dispersion forces and many-body interactions in trimer formation.

Main Methods:

• Ab initio calculations including MP2, MP4SDTQ, and QCISD(T) with various basis sets.
• Density Functional Theory (DFT) using B3LYP/6-31G(d,p).
• Many-body symmetry-adapted perturbation theory (SAPT) to analyze interaction contributions.

Main Results:

• DFT was found unsuitable for systems dominated by dispersion forces.
• Three distinct isomers (two noncyclic, one cyclic) were located and characterized.
• The most stable isomer, a noncyclic barrel-like structure, matches experimental observations.
• Energetic analysis suggests the existence of two unobserved isomers.

Conclusions:

• The barrel-like structure is the most stable isomer of the OCS.(CO2)2 trimer.
• Theoretical calculations support the potential existence of other isomers.
• Dispersion forces are critical for accurately modeling these van der Waals systems.