J . Phys. Chem. 1993,97, 3685-3689
3685
Microwave Spectroscopic Investigation of the Weakly Bound Dimer Carbon Monoxide-Chlorine, OCCl* Wolfgang Jager, Yunjie Xu, and M. C. L. Gerry' Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T l Z 1 Received: October 21, 1992; In Final Form: December 18, 1992 The rotational spectrum of the van der Waals complex OCCl2 has been observed in the frequency range 5-12 GHz using a cavity-pulsed microwave Fourier transform spectrometer. The isotopmers investigated were O C W 1 2 , OC37C135C1,OC3sC137C1,0'3C35C12,013C37C13sC1,013C3sC137C1, and 1eO'3C35C12.It is established that the complex is linear in the ground vibrational state, and a complete substitution (rs)structure has been obtained. The nuclear quadrupole hyperfine structures due to the 35Cl and 37Cl nuclei have been resolved, yielding the corresponding nuclear quadrupole coupling constants. The differences in these constants between the inner and outer C1 nuclei, along with the derived geometry and the relatively low centrifugal distortion constants, are consistent with a picture of a rather rigid complex with a significant charge redistribution in the C12 monomer on complexation with CO.
Introduction Several van der Waals dimers consisting of nonpolar or only weakly polar monomers, like ArCO' and ArN2,2 have been investigated recently in our laboratory, using a cavity microwave (MW) Fourier transform (FT) spectrometer. One purpose of these studies was to provide information about the structures and about the van der Waals dynamics of such fundamental systems. These investigations have now been extended to the carbon monoxide-chlorine, OCC12, system which has recently been studied in the infrared region.3 In the infrared study, rotationally resolved absorption spectra were detected by exciting the CO stretch and clearly indicated a linear structure for this complex. Rotational and centrifugal distortion constants of both ground and excited vibrational states, as well as the band center, were obtained and yielded values for the van der Waals distance and the van der Waals stretching force constant and frequency. As the authors pointed out, however, it could not be shown whether the dimer is carbon or oxygen bonded, because of the lack of isotopic data. In the present work, the rotational spectra of seven isotopic species of OCClz have been measured using a cavity-pulsed MWFT spectrometer. A major goal was toestablish the geometry of the complex. Information of this kind is especially important in light of current discussions concerning the possibility of observing different conformers of halogen-containing dimers in molecular expansion experiment^.^ The high resolution of the microwave experiments should also permit observation of the hyperfine structure due to the two quadrupolar chlorine nuclei and hence an estimate of the charge redistribution in Cl2 on complex formation.
Experimental Details The experiments were done with a cavity MWFT spectrometer of the Balle-Flygare type,5 which has been described in detail earlierS6 Recently, the frequency range of the instrument has been extended to lower frequencies down to 4 GHz. In the range from 8 to 12 GHz, the available excitation pulse power from the MW synthesizer was sufficient to achieve maximum polarization conditions with moderate pulse lengths (
