Rotational spectrum and structure of the HCN-(CO₂)₂ trimer

Rotational spectra have been observed and rotational constants determined for HCN-(CO₂)₂, H¹³CN-(CO₂)₂, HC¹⁵N-(CO₂)₂, and HCN-¹³CO₂CO₂ by using the Fourier transform, Flygare/Balle Mark II spectrometer with a pulsed nozzle. Less extensive observations were made of DCN-(CO₂)₂, HCN-(¹³CO₂)₂, and two ¹⁸O-substituted isotopic species. The rotational constants found for the parent asymmetric top are 1852.844, 1446.159, and 981.48 MHz for A, B, and C and -0.1085, -0.0297, -0.0241, -0.0190, and -0.0066 MHz for τ₁, τ₂, τ_aaaa, τ_bbbb, and τ_cccc, respectively. The isotopic substitution reveals a ground-state geometry with the C₂ symmetry of the slipped parallel (CO₂)₂ subunit and having the HCN along the C₂ axis, the N end closest to the (CO₂)₂. The C₂ symmetry is confirmed by the absence of eo and oe states as predicted for 2-fold symmetry with only equivalent bosons off-axis. The two carbons of the (CO₂)₂ lie in a plane R = 3.098 Å below the center of mass of the HCN. The C-C distance in this subunit is 3.522 Å, which is 0.077 Å shorter than reported for the free (CO₂)₂ dimer. Also, an inertial analysis shows the individual CO₂'s to be counterrotated by γ = 20.3° out of the ac plane containing the carbons, the inner oxygens rotated away from the HCN. The OCC "slip" angle β is 60.8° in the (CO₂)₂. The torsional oscillations of the HCN are anisotropic, with an average displacement of 12.4°, as determined from isotopic substitution and the ¹⁴N hyperfine structure (hfs). Virtually all of the hf components are doublets separated by 10-200 kHz. We attribute the doubling to an inversion of the clusters by a 140° counterrotation of the CO₂'s. The inversion does not affect the dipole moment of the cluster, so the observed doubling is the difference in tunneling splittings of the rotational states for each transition.