| Abstract | 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. Eighty-seven transitions in the 77-180?GHz region have been assigned and analyzed in terms of a model-independent term value scheme involving 57 rotational levels with J=0-8. 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 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 3.769?cm-1, slightly larger than the (12C16O)2 value of 3.731?cm-1. A large amount of precise experimental data is now available for the CO dimer, which should lead to greater theoretical insight into its structure and tunneling dynamics. |
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