Theoretical and experimental study of weakly bound CO 2-(pH 2) 2 trimers

The infrared spectrum of CO 2-(pH 2) 2 trimers is predicted by performing exact basis-set calculations on a global potential energy surface defined as the sum of accurately known two-body pH 2-CO 2 (J. Chem. Phys.2010, 132, 214309) and pH 2-pH 2 potentials (J. Chem. Phys.2008, 129, 094304). These results are compared with new spectroscopic measurements for this species, for which 13 transitions are now assigned. A reduced-dimension treatment of the pH 2 rotation has been employed by applying the hindered-rotor averaging technique of Li, Roy, and Le Roy (J. Chem. Phys.2010, 133, 104305). Three-body effects and the quality of the potential are discussed. A new technique for displaying the three-dimensional pH 2 density in the body-fixed frame is used, and shows that in the ground state the two pH 2 molecules are localized much more closely together than is the case for the two He atoms in the analogous CO 2-(He) 2 species. A clear tunneling splitting is evident for the torsional motion of the two pH 2 molecules on a ring about the CO 2 molecular axis, in contrast to the case of CO 2-(He) 2 where a more regular progression of vibrational levels reflects the much lower torsional barrier. \ua9 2011 American Chemical Society.Peer reviewed: YesNRC publication: Ye