Anharmonic Vibrational Spectroscopy of Germanium-Containing Clusters, Ge_xC_4–x and Ge_xSi_4–x (x = 0–4), for Interstellar Detection

An extensive, high-level theoretical study on tetra-atomic germanium carbide/silicide clusters is presented. Accurate harmonic and anharmonic vibrational frequencies and rotational constants are calculated at the CCSD­(T)-F12a­(b)/cc-pVT­(Q)­Z-F12 levels of theory. With growing capabilities to discern more of the chemical composition of the interstellar medium (ISM), an accurate database of reference material is required. The presence of carbon is ubiquitous in the ISM, and silicon is known to be present in interstellar dust grains; however, germanium-containing molecules remain elusive. To begin understanding the presence and role of germanium in the ISM, we present this study of the vibrational and rotational spectroscopic properties of various germanium-containing molecules to aid in their potential identification in the ISM with modern observational tools such as the James Webb Space Telescope. Structures studied herein include rhomboidal (r-), diamond (d-), and trapezoidal (t-) tetra-atomic molecules of the form Ge_xC_4–x and Ge_xSi_4–x, where x = 0–4. The most promising structure for detection is r-Ge₂C₂ via the ν₄ mode with a frequency of 802.7 cm^–1 (12.5 μm) and an intensity of 307.2 km mol^–1. Other molecules that are potentially detectable, i.e., through vibrational modes or rotational transitions, include r-Ge₃C, r-GeSi₃, d-GeC₃, r-GeC₃, and t-Ge₂C₂.