High-resolution infrared spectrum and energy levels of the weakly bound complex, CO–paraH2

The infrared spectrum of the weakly bound complex CO–$\mathit{para}{\mathrm{H}}_{\mathrm{2}},$ which accompanies the fundamental band of CO in the 4.7 μm region, has been recorded at high resolution $\text{(0.005\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}})$ using a long-path (200 m), low-temperature (48 K), absorption cell. A total of 135 transitions, constituting almost all of the observed lines, has been rotationally assigned in terms of 44 discrete quantum states of the complex, and the positions of these energy levels have been determined to high accuracy $\text{(<0.001\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}})$ for both the $v_{\mathrm{CO}}\text{=0}$ and 1 vibrational states. The binding energy of the complex, relative to the zero-point level, was determined to be about $\text{22\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ Predicted microwave and millimeter wave frequencies are given for the pure rotational spectrum of CO–$\mathit{para}{\mathrm{H}}_{\mathrm{2}}.$ The energy level pattern derived here provides the information needed to make precise determinations of the ${\mathrm{CO–H}}_{\mathrm{2}}$ intermolecular potential energy surface in the attractive region. However, determination of the dependence of this surface on the orientation of ${\mathrm{H}}_{\mathrm{2}}$ within the complex will have to await the analysis of the more complicated spectrum of CO–$\mathit{ortho}{\mathrm{H}}_{\mathrm{2}}.$