Higher Energy States in the CO Dimer:  Millimeter-Wave Spectra and Rovibrational Calculations

Abstract

New extensive millimeter-wave measurements of the 12 C 16 O dimer have been made, and more than 300 new spectral transitions have been observed in the frequency range 81-135 GHz. A joint analysis of these and previous millimeter-wave data yielded the precise location of 33 new energy levels of A + symmetry and 20 levels of A -symmetry. These energy levels are located at 8-18 cm -1 above the zero-point level. Some of them belong to already known stacks, and others make up 9 new stacks of the dimer. Newly determined stacks have K ) 0, 1, and, for the first time, 2, where K is the projection of the total angular momentum on the intermolecular axis. The energy levels from accompanying rovibrational calculations with the use of a recently developed hybrid CCSD(T)/DFT-SAPT potential are in very good agreement with experiment. Analysis of the calculated wave functions revealed that two new stacks of A + symmetry with K ) 2 correspond to overall rotation of the dimer while the other newly observed stacks belong to the geared bend overtone modes. The ground vibrational states of the two "isomers" found are more or less localized at the two minima in the potential surface, whereas all the geared bend excited states show a considerable amount of delocalization.

The CO-H2van der Waals complex and complex organic molecules in cold molecular clouds: A TMC-1C survey

“…Laboratory data have clarified later a spectroscopic problem of these unsuccessful searches. The extensive millimeter-wave (MMW) studies of the CO dimer (Surin et al 2007) have shown that the radio astronomical search of this complex was based on frequencies which cannot be unambiguously attributed to (CO) 2 . In the case of the CO-paraH 2 complex, the interstellar search was outside the correct frequency position of the most promising R(0) line, as later identified by the first MMW study of CO-paraH 2 (Pak et al 1999), and only the weaker Q(1) line was correctly tuned.…”

THz and Submillimeter‐wave Spectroscopy of Molecular Complexes

“…Similar experiments have been carried out also for the isotopic species of the CO dimer, (C 18 O) 2 and ( 13 CO) 2 , with the OROTRON in the MMW region, and two very closely located "isomers" of A + symmetry were reported (Surin et al 2007). The energy gaps between the "Cbonded" and "O-bonded isomer" are found to be 0.638 and 1.287 cm −1 for (C 18 O) 2 and ( 13 CO) 2 (Surin et al 2007), respectively.…”

“…The very low bending vibration is an indication of the extreme floppiness of the CO dimer. The value of 3.731 cm −1 is almost the same as, or even slightly lower than, the rotational J = 1-0 transition frequency, Surin et al (2007).  American Chemical Society, 2007.…”

“…The energy gaps between the "Cbonded" and "O-bonded isomer" are found to be 0.638 and 1.287 cm −1 for (C 18 O) 2 and ( 13 CO) 2 (Surin et al 2007), respectively. The frequency of the geared bending mode of (C 18 O) 2 , 3.607 cm −1 , is slightly lower than that of the Table 5 Rovibrational assignment of the observed stacks of the CO dimer (Table 5 of Surin et al (2007) …”

“…Very recently, the observations have been extended to highly excited bending mode, and more than 100 lines were assigned to the rovibrational transitions related to additional eight stacks labeled as l, n, m, r with A + symmetry and o, p, s, u with A − symmetry (Surin et al 2007). From the calculated wavefuctions, the newly observed levels are assigned to highly excited states of the geared bending mode up to v b = 3 K = 2, as listed in Table 5.…”