MILLIMETER WAVE SPECTRUM OF THE $^{13}C^{16}O$ DIMER

References [1] A.R.W. McKellar, J. Chem. Phys. 115, 3571 (2001). [2] J. Tang, A.R.W. McKellar, L.A. Surin, D.N. Fourzikov, B.S. Dumesh, and G. Winnewisser, J. Mol. Spectrosc. 214, 87 (2002).Author Institution: I. Physikalisches Institut, Universit\""{a}t zu K\""{o}ln, Z\""{u}lpicher Str.; Steacie Institute for Molecular Sciences, National Research Council of CanadaThe millimeter wave spectrum of the isotopically substituted CO dimer, $(^{13}C^{16}O)_{2}$ has been studied for the first time, confirming and extending a recent infrared study [1]. A total of 87 transitions in the 77 - 180 GHz region have been assigned and analyzed with a model-independent term value scheme involving 57 rotational levels with $J = 0$ to 8. The levels can be classified into 7 ``stacks'' which have symmetry classifications of either $A^{+}/B^{-}$ or $A^{-}/B^{+}$, and K -values of either 0 or 1. For the normal isotope, symmetry and nuclear spin statistics cause alternate rotational levels to be missing, but for $(^{13}C^{16}O)_{2}$, all levels are present with an intensity alternation of 1:3 between the A and B symmetries. The four $A^{+}/B^{-}$ stacks have not previously been observed, and the lowest of them establishes the tunneling splitting of $(^{13}C^{16}O)_{2}$ to be $3.769 cm^{-1}$, slightly larger than the $(^{12}C^{16}O)_{2}$ value [2] of $3.731 cm^{-1}$. Even though a considerable amount of precise experimental data is now available for the CO dimer, we still have little theoretical insight into its structure and tunneling dynamics