The molecular symmetry group of the CO dimer and the assignments of the intermolecular vibrations

CO Dimer: The Infrared Spectrum Revisited

“…There are four intermolecular modes of vibration, which can be described as van der Waals stretch (A + symmetry), antigeared bend (A + ), torsion (A − ), and geared bend (B + ). 25 Excitation of the latter two modes in v CO = 0 gives rise to A − rotational levels, and it is transitions between these levels and the A + levels of the true vibrational ground state that are observed in the millimeter wave spectrum. The lowest such stack with A − levels, a geared bend intermolecular mode, lies at only 3.7 cm −1 .…”

“…Background. As discussed in detail by Yamada, 25 the CO dimer can be classified using the G 4 molecular symmetry group, 32 with possible symmetry labels A + , A − , B + , and B − . The overall symmetry of a given rotational level is given by the product of its vibrational and rotational labels.…”

“…In order of energy, the next four ground state stacks beyond d are e, f, j, and k. These levels, with overall symmetry A – , have been assigned − to the first excited level of the geared bend intermolecular mode, which we label v b = 1, though Yamada has proposed an alternate assignment for k . − These are K = 0 (e, j) and K = 1 (f, k) stacks for the two isomers, with (e, f) and (j, k) belonging together on the basis of their energies. In addition, there are also associated K = 2 stacks, labeled p and o.…”

“…With the help of calculations based on this hybrid potential, a comprehensive survey spectrum of ( 12 C 16 O) 2 in the millimeter wave range was analyzed to obtain 53 new ground state rotational level energies in addition to the 49 already known . There has also been discussion in the literature regarding the appropriate labeling of CO dimer rotational levels. − This labeling is not self-evident because of the floppy nature of the dimer and the presence of two isomers. Finally, an infrared spectrum of the CO trimer has recently been assigned …”

“…The ground vibrational state of the dimer (A + vibrational symmetry) thus has only even- J levels for K = 0 (i.e., J K a K c = 0 00 , 2 02 , 4 04 , etc., in asymmetric rotor notation) and similarly only half the expected levels for K > 0. There are four intermolecular modes of vibration, which can be described as van der Waals stretch (A + symmetry), antigeared bend (A + ), torsion (A – ), and geared bend (B + ) . Excitation of the latter two modes in v CO = 0 gives rise to A – rotational levels, and it is transitions between these levels and the A + levels of the true vibrational ground state that are observed in the millimeter wave spectrum.…”