| Abstract | The infrared spectrum of the fully substituted 13-C carbon monoxide dimer, (¹³C¹⁶O)₂, has been studied in the region of the C–O stretching vibration, around 2096 cm⁻¹. Over 120 transitions have been assigned to transitions involving 49 rotational levels in the excited state and 23 levels in the ground state. Relative energies for all these levels have been accurately ([approximate]0.0003 cm⁻¹) determined without model dependence, except for the interval between the lowest levels of A and B symmetry. The observed stacks of levels were similar to those determined previously for the normal isotope, (¹²C¹⁶O)₂, and could therefore be identified with the same labeling scheme, but the differences between them constitute a subtle probe of the intermolecular forces in this system. In both isotopes, the stacks fall into two groups, characterized by larger (4.4 Å) or smaller (4.0 Å) intermolecular separations. The energy splitting between the ground states of these two isomers is 0.877 cm⁻¹ in (¹²C¹⁶O)₂, and 1.285 cm⁻¹ in (¹³C¹⁶O)₂. The observed isotope shifts may be explained in terms of a simple anharmonic interaction model. |
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