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

H. S. Gutowsky, Jane Chen, P. J. Hajduk, R. S. Ruoff

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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, τaaaa, τbbbb, and τcccc, 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.

Original languageEnglish (US)
Pages (from-to)7774-7780
Number of pages7
JournalJournal of physical chemistry
Issue number20
StatePublished - Dec 1 1990


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