Theoretical and Experimental Study of Weakly Bound CO2–(pH2)2 Trimers

The infrared spectrum of CO2–(pH2)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 pH2–CO2 (J. Chem. Phys. 2010, 132, 214309) and pH2–pH2 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 pH2 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 pH2 density in the body-fixed frame is used, and shows that in the ground state the two pH2 molecules are localized much more closely together than is the case for the two He atoms in the analogous CO2–(He)2 species. A clear tunneling splitting is evident for the torsional motion of the two pH2 molecules on a ring about the CO2 molecular axis, in contrast to the case of CO2–(He)2 where a more regular progression of vibrational levels reflects the much lower torsional barrier.