Ab Initio Study on the (OCS)₂·CO₂van der Waals Trimers

Abstract

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] were carried out to study the (OCS)2·CO2 van der Waals trimers. Three barrel-like structures [C1 (two) and C2 symmetry] and three planar (Cs) structures were located on the potential energy surface. Their CBS-MP2/cc-pVXZ (X = D, T, Q) stabilization energies are 1760 (C1), 1514 (C2), 1660 (C1), 1325 (Cs), 1556 (Cs), and 1398 (Cs) cm-1, respectively. The most stable structure (one of the C1 barrel-like isomers) has bond lengths, angles, rotational constants, and dipole moment that agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the rest of the isomers strongly suggests that the experimentally unobserved structures might also be present in the supersonic adiabatic expansion of the gas in the microwave spectroscopic studies as in the case of the (CO2)3 trimer where both barrel-like and planar isomers have been reported to exist. The many-body symmetry-adapted perturbation theory helps to shed some light on the nature of the interactions leading to the formation of the different isomers. While the dispersion forces make the most important attractive contributions to the interaction energies of the (OCS)2·CO2 isomers, the induction forces make contributions similar in magnitude to the electrostatic forces. The three-body contributions are small and stabilizing for the barrel-like structures and are less important for the cyclic isomers.