Germanium Dicarbide: Evidence for a T-Shaped Ground State Structure

Abstract

The equilibrium structure of germanium dicarbide GeC 2 has been an open question since the late 1950s. Although most high-level quantum calculations predict an Lshaped geometry, a T-shaped or even a linear geometry cannot be ruled out because of the very flat potential energy surface. By recording the rotational spectrum of this dicarbide using sensitive microwave and millimeter techniques, we unambiguously establish that GeC 2 adopts a vibrationally averaged T-shaped structure in its ground state. From analysis of 14 isotopologues, a precise r 0 structure has been derived, yielding a Ge−C bond length of 1.952(1) Å and an apex angle of 38.7(2)°.

Ab initio potential energy surface and vibration–rotation energy levels of germanium dicarbide, GeC2

“…The effective rotational and quartic centrifugal distortion constants for the ground vibrational state of the isotopologues 74 Ge 12 C 2 , 74 Ge 13 C 2 , 72 Ge 13 C 2 , 70 Ge 13 C 2 , and 74 Ge 12 C 13 C are given in Table . The predicted constants are compared with the values derived recently by Zingsheim et al in the analysis of the rotational spectra of GeC 2 . Experimental determination of the spectroscopic constants of GeC 2 is somewhat challenged because for the symmetric 12 C‐containing species, the energy levels labeled with the odd rotational quantum number K a are not populated due to nuclear spin statistic.…”

“…These constants were kept fixed at the theoretical estimates obtained at the CCSD(T)/cc‐pwCVQZ level of theory. The rotational and centrifugal distortion constants of the GeC 2 isotopologues were determined using the second‐order vibrational perturbation theory (VPT2) based on the predicted equilibrium structural parameters and harmonic vibrational frequencies . For all of the GeC 2 isotopologues considered here, the experimental rotational constants B and C are reproduced by the present ab initio results to about 3 MHz, whereas the experimental rotational constant A is underestimated by about 240 MHz.…”

“…Using the CCSD(T) method, it was predicted by Sari et al to be 144 cm −1 for the cc‐pwCVTZ basis set, whereas by Zingsheim et al to be −7 cm −1 for the cc‐pwCVQZ basis set. As found in the previous studies, the linear CCGe configuration is a transition state, with the energy predicted at the CCSD(T)/aug‐cc‐pV6Z level of theory to be 1351 cm −1 above the minimum. Changes in the predicted equilibrium CC and CGe bond lengths beyond the aug‐cc‐pV6Z basis set are estimated to be about 0.0003 and 0.0006 Å, respectively.…”

“…This is not the case for the symmetric 13 C‐containing species. Moreover, Zingsheim et al observed only the rotational transitions between the energy levels labeled with 0 and 1. As a result, several spectroscopic constants of GeC 2 could not be determined in the least‐squares fit to the experimental data.…”

“…The potential energy surface of GeC 2 was found by Sari et al to be very flat near the cyclic L‐ and T‐shaped configurations. In the recent study by Zingsheim et al, the rotational spectra of fourteen isotopologues of GeC 2 were observed and the effective structure of the GeC 2 molecule in its ground vibrational state was concluded to be cyclic T‐shaped of symmetry. The vibrationally averaged r 0 structure of GeC 2 was determined, with the GeC bond length of 1.952(1) Å and the CGeC valence angle of 38.7(2)°.…”