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 ‚CO 2 van der Waals trimers. Three barrel-like structures [C 1 (two) and C 2 symmetry] and three planar (C s ) structures were located on the potential energy surface. Their CBS-MP2/cc-pVXZ (X ) D, T, Q) stabilization energies are 1760 (C 1 ), 1514 (C 2 ), 1660 (C 1 ), 1325 (C s ), 1556 (C s ), and 1398 (C s ) cm -1 , respectively. The most stable structure (one of the C 1 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 (CO 2 ) 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 ‚CO 2 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.

Theoretical approach to the mechanism of reactions between halogen atoms and unsaturated hydrocarbons: The Cl + propene reaction

“…It is interesting to observe that the CPR correction changes the activation energy in the correct direction but it is too big, probably as a consequence of the overcorrection associated with the CP algorithm to correct for BSSE at the correlated level. 33 As concluded in recent studies 34,35 where the CP values are compared with those arising from the application of BSSE-free methods, the real values must lie somewhere between the NCP and CPR estimates.…”

“…A number of authors 33 noticed overcorrections when the CP method is applied at the correlated levels. Further, recent works 34,35 where BSSE-free techniques, like the symmetryadapted perturbation theory (SAPT) 36 or complete basis set (CBS) 37 extrapolations, are employed seem to provide arguments confirming that apparent limitation of the CP method. However, in any case, experience suggests that the CP correction is useful to provide upper limits for the interaction energies.…”

Mechanism and Kinetics of the Reaction of OH Radicals with Glyoxal and Methylglyoxal: A Quantum Chemistry+CVT/SCT Approach

“…However, the use of this method is polemic since several authors have reported that it overestimates the BSSE (see refs. [36][37][38][39][40][41][42][43][44] and references therein).…”

Infrared spectra of the OCS-CO2 complex: Observation of two distinct slipped near-parallel isomers

“…So there can be little doubt that isomer a is the lowest energy ground state form of OCS-CO 2 , in agreement with ab initio results. 3 Interestingly, however, we observed that the 2055.7 cm −1 band ͑isomer b͒ seemed to be stronger than the 2066.1 cm −1 band ͑isomer a͒ by a factor of approximately 1.5-2.0 when helium was the carrier. Because of the variation of laser power and other experimental parameters, this intensity ratio is subject to large uncertainties.…”

“…These two isomers agree fairly well with the predicted structures from an ab initio calculation by Valdés and Sordo. 3 Following them, we label the two forms as isomers a ͑the original one͒ 1 and b ͑the new one observed here͒. In our experiment, the band of isomer b actually appears to be stronger than that of the isomer a, but the calculations and our experimental test both suggest that isomer a is the lowest energy form.…”