Isotopic probing of weak intermolecular forces: Infrared spectrum and energy levels of the (CO)-C-13-O-16 dimer

The infrared spectrum of the fully substituted 13-C carbon monoxide dimer, ((CO)-C-13-O-16)(2), has been studied in the region of the C-O stretching v ibration, around 2096 cm(-1). Over 120 transitions have been assigned to tr ansitions involving 49 rotational levels in the excited state and 23 levels in the ground state. Relative energies for all these levels have been accu rately (approximate to0.0003 cm(-1)) determined without model dependence, e xcept for the interval between the lowest levels of A and B symmetry. The o bserved stacks of levels were similar to those determined previously for th e normal isotope, ((CO)-C-12-O-16)(2), and could therefore be identified wi th 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 Angstrom) or smaller (4.0 Angstrom) intermolecular separations. The energy splitting be tween the ground states of these two isomers is 0.877 cm(-1) in ((CO)-C-12- O-16)(2), and 1.285 cm(-1) in ((CO)-C-13-O-16)(2). The observed isotope shi fts may be explained in terms of a simple anharmonic interaction model. (C) 2001 American Institute of Physics.