Interstellar Germanium Dust Molecules: a Computational Study on Their Structure, Spectroscopy, and Detectability; or, One Small Step Down the Periodic Table, One Giant Leap in Interstellar Chemistry
Creators: Flowers, Adam M
Publisher: name: University of Alberta Library
Year: 2024

<p>Alongside observational and experimental work, theoretical and computational research is just as vital to understanding the chemical composition of space. Although more molecules continue to be discovered in space, the molecular structures in which many of the elements in the periodic table exist in the interstellar medium (ISM) remain a mystery. With growing capabilities to discern more of the chemical composition of the 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, this thesis presents a 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. Silicon-carbide dust grains are known to be prevalent in interstellar dust, and, therefore, by taking these as reference, germanium carbide and germanium silicide structures are proposed for detection in the ISM. An extensive, high-level theoretical study on tetra-atomic germanium carbide/silicide clusters, including calculations of accurate harmonic and anharmonic vibrational frequencies, rotational constants, and dipole moments done at the CCSD(T)-F12a(b)/cc-pVT(Q)Z-F12 levels of theory is presented in this work. Structures studied herein include rhomboidal (r-), diamond (d-), and trapezoidal (t-) tetra-atomic molecules of the form Ge(x)C(4-x) and Ge(x)Si(4-x), where x=0-4. The most promising structure for detection is r-Ge(2)C(2) via the v(4) mode with a frequency of 802.7 cm-1 (12.5 micrometres) and an intensity of 307.2 km mol-1. Other molecules potentially detectable, i.e., through vibrational modes or rotational transitions, include r-Ge(3)C, r-GeSi(3), d-GeC(3),  r-GeC(3), and t-Ge(2)C(2).</p>

<pre><code>Alongside proposing germanium-containing molecules for detection, a thorough determination of the transition states between the different isomers of the cyclic tetra-atomic silicon-carbide, germanium carbide, and germanium silicide clusters is presented. Through use of density functional theory (B3LYP-D3BJ, M06-2X, wB97X-D4, and B2GP-PLYP) in conjunction with the aug-cc-pVTZ basis set, transition state structures and their barrier heights are determined for the interconversions between the various isomers for the family of tetra-atomic SiC, GeC, and GeSi compounds.  Determining which structures might be detectable not only depends on their intrinsic spectroscopic features, but whether or not they are likely to exist as isomers in interstellar environments. By examining the energy barrier heights for transitions between isomers, we determined that many of these structures are unlikely to exhibit interconversion in the ISM, outside of hotter circumstellar environments. Although Boltzmann population ratios at approximate circumstellar temperatures suggest the presence of higher energy minima, it is likely that once interconversion happens, as molecules travel away from a star and cool, they will get kinetically trapped in the potential energy well they inhabit, making how the ratios freeze out dependent on the time and pathways the molecules take to cool down. As such, many of these higher energy minima may still be good candidates for detection including r-SiC$_3$, r-GeC(3), r-GeSi(3), t-Si(2)C(2), r-Ge(2)C(2), and d-Si(3)C.
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