Knowledge Bank

The millimeter wave spectrum of the isotopically substituted CO dimer, $(13C16O)2$ 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 $(13C16O)2$, 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 $(13C16O)2$ to be $3.769cm-1$, slightly larger than the $(12C16O)2$ value [2] of $3.731cm-1$. 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.