Isotope effects in the CO dimer: Millimeter wave spectrum and rovibrational calculations of ((CO)-C-12-O-18)(2)

Abstract

The millimeter wave spectrum of the isotopically substituted CO dimer, ((CO)-C-12-O-18)(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 (CO)-C-12-O-18 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 84-127 GHz 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 (CO)-C-12-O-18 dimer at 3.607 cm(-1). Accompanying rovibrational calculations using a recently developed hybrid potential from ab initio coupled cluster [CCSD(T)] and symmetry-adapted perturbation theory calculations [G. W. M. Vissers , J. Chem. Phys. 122, 054306 (2005)] gave very good agreement with experiment. The isotopic dependence of the A(+)/A(-) energy splitting, the intermolecular separation R, and the energy difference of two ground state isomers, which change significantly when O-18 or C-13 are substituted into the normal ((CO)-C-12-O-16)(2) isotopolog [L. A. Surin , J. Mol. Spectrosc. 223, 132 (2004)], was explained by these calculations. It turns out that the change in anisotropy of the intermolecular potential with respect to the shifted monomer centers of mass is particularly significant. (c) 2006 American Institute of Physics