Theoretical Study of Infrared Spectra of OCS-( pH2)2, OCS-( oD2)2, OCS-(HD)2, and Mixed OCS- pH2-He Trimers.

The calculated rovibrational energy levels and infrared spectra for OCS-( pH2)2, OCS-( oD2)2, OCS-(HD)2 and mixed OCS- pH2-He trimers are obtained by performing the exact basis-set calculations for the first time based on the newly developed potential energy surfaces ( J. Chem. Phys. 2017, 147, 044313). The "adiabatic-hindered-rotor" (AHR) method is used for reduced-dimension treatment of the hydrogen rotation. The predicted band origin shifts and the infrared spectra are in good agreement with the available experimental values: for the band origin shifts and infrared transitions, the root-mean-square(rms) deviations are smaller than 0.044 and 0.002 cm-1, respectively. We extend the assignments to the unrecorded infrared transitions for OCS-( pH2)2 and OCS-(HD)2 complexes and identify the infrared spectra for OCS-( oD2)2 and OCS- pH2-He for the first time. Three-dimensional density distributions for the ground states of OCS-( pH2)2, OCS- pH2-He, and OCS-(He)2 show that the pH2 molecules are localized in their corresponding global minimum regions, while the pronounced locations of the He atoms are missing in OCS- pH2-He and OCS-(He)2 with forming incomplete circles around the OCS axis. A clear tunneling splitting is observed for the torsional motion of the two hydrogen molecules ( pH2, HD, or oD2) on a ring about the OCS molecular axis, whereas no tunneling splitting is found in OCS- pH2-He or OCS-(He)2 due to a much lower torsional barrier.