Isotope effects in the CO dimer: millimeter wave spectrum and rovibrational calculations of (12C18O)2.

The millimeter wave spectrum of the isotopically substituted CO dimer, (C12O18)2, was studied with the Orotron jet spectrometer, confirming and extending a previous infrared study [A. R. W. McKellar, J. Mol. Spectrosc. 226, 190 (2004)]. A very dilute gas mixture of CO in Ne was used, which resulted in small consumption of C12O18 sample gas and produced cold and simple spectra. Using the technique of combination differences together with the data from the infrared work, six transitions in the 84–127GHz region have been assigned. They belong to two branches, which connect four low levels of A+ symmetry to three previously unknown levels of A− symmetry. The discovery of the lowest state of A− symmetry, which corresponds to the projection K=0 of the total angular momentum J onto the intermolecular axis, identifies the geared bending mode of the C12O18 dimer at 3.607cm−1. Accompanying rovibrational calculations using a recently developed hybrid potential from ab initio coupled cluster [CCSD(T)] and symmetry-ad...