Electronic Structure of Silicon-Containing Clusters and Nanodiamondoids
Final Report Abstract
In this project, a variety of complementary mass spectrometry and laser spectroscopy methods were combined with quantum chemical tools to characterize the geometric, electronic, optical, chemical bonding and reactivity properties of a large variety of silicon-containing clusters and diamondoids. To this end, a variety of cluster sources and a new laser-vaporization time-of-flight mass spectrometer were constructed and commissioned. Silicon clusters and related nanostructures are of great current interest because of their potential applications in semiconductor industry, materials science, astronomy, and organic and theoretical chemistry. As pure silicon clusters are quite reactive, recent activities explore the possibilities to stabilize them by various means of modification (e.g. hydrogen passivation, doping) and to develop new nanostructures with tailored chemical, optical, and magnetic properties. In this project, a number of fundamental SixHy+ cations and their adducts, ranging from Si2H4+ to Si8H31+, were studied for the first time. The computational analysis of measured vibrational spectra provides unprecedented insight into their structure and often unusual chemical bonding properties, which differ substantially from those of the well-known hydrocarbon species. In particular, symmetric and asymmetric two-center three-electron (2c-3e) Si-H-Si bonds were identified for the first time in isolated SixHy molecules, and found to be very stable and slightly bent Si-H-Si bridges. These can also be described as rarely occurring charge-inverted hydrogen bonds. In this context, also the first spectrum of a member of the family of the elusive silanols, namely protonated silanol, was measured and its analysis reveals that its SiH3+-OH2 structure can best be described by charge-dipole bonding. Along another route, the large effects of doping neutral silicon clusters by first-row elements (X=Be-N) on their geometric and electronic structure and their optical properties (HOMO-LUMO gaps) were elucidated by IR spectroscopy and sophisticated quantum chemical calculations coupled to global optimization techniques. The properties of the silicon clusters can largely be varied by the type (X) and number (m) of doping atoms in SinXm, including the mass, valency, and electronegativity of X, and the force constant of the Si-X bond. To unravel the effects of the charge, a large number of SinOm+ cations were studied. Although the structures depend strongly on the cluster size (n) and oxidation state (m), several typical structural motifs are identified (Si2O2 rhombus, Si2O3 pentagon, Si3O3 hexagon, SiO4 silicate). In an effort to record optical spectra of mass-selected clusters, the new mass spectrometer has been optimized by recording optical spectra of Au4+ clusters and their adducts, and the sensitivity of the setup was shown to be two orders of magnitude better than current state-of-the-art. Diamondoids, a recently established new class of hydrocarbons, and their derivatives have unique and largely variable properties, which make them promising building blocks for new nanomaterials. Their transient radical cations are postulated as intermediates in astrochemistry and organic chemistry. Here, we characterized the geometric and electronic structure of the prototypical adamantane radical cation, and elicudated in great detail the Jahn-Teller distortion in its ground electronic state. Further studies on its microhydrated clusters probe the effects of aqueous solvation and reveal that two H2O molecules are sufficient for proton transfer to the solvent, which generates the reactive adamantyl radical involved in its functionalization. These studies on the spectroscopy and mechanistic reactivity are valuable for astronomers and organic chemists working on synthetic routes for modification of diamondoids in solution chemistry.
Publications
- IR Spectrum and Structure of the Adamantane Cation: Direct Evidence for Jahn-Teller Distortion, Angew. Chem. Int. Ed. 51, 4925-4929, 2012
A. Patzer, M. Schütz, T. Möller, O. Dopfer
(See online at https://doi.org/10.1002/anie.201108937) - Infrared spectrum of the Si3H8+ cation: Evidence for a bridged isomer with an asymmetric three-center two-electron Si-H-Si bond, Chem. Eur. J. 19, 15315-15328, 2013
M.A.R. George, M. Savoca, O. Dopfer
(See online at https://doi.org/10.1002/chem.201302189) - IR Spectrum and Structure of Protonated Disilane: Probing the Si-H-Si Proton Bridge, Angew. Chem. Int. Ed. 52, 1579, 2013
M. Savoca, J. Langer, O. Dopfer
(See online at https://doi.org/10.1002/anie.201208958) - Vibrational Spectra and Structures of Neutral SimCn Clusters (m+n=6): Sequential Doping of Silicon Clusters with Carbon Atoms, J. Phys. Chem. A 117, 1158-1163, 2013
M. Savoca, J. Langer, A. Lagutschenkov, D.J. Harding, A. Fielicke, O. Dopfer
(See online at https://doi.org/10.1021/jp305107f) - Size and Shape dependent Photoluminescence and Excited State Decay Rates of Diamondoids, Phys. Chem. Chem. Phys. 16, 3070-3076, 2014
R. Richter, D. Wolter, T. Zimmermann, L. Landt, A. Knecht, C. Heidrich, A. Merli, O. Dopfer, P. Reiß, A. Ehresmann, J. Petersen, J.E. Dahl, R.M.K. Carlson, C. Bostedt, T. Möller, R. Mitric, T. Rander
(See online at https://doi.org/10.1039/c3cp54570a) - Vibrational Spectra and Structures of Bare and Xe-tagged Cationic SinOm+ Clusters, J. Chem. Phys. 141, 104313, 2014
M. Savoca, J. Langer, D.J. Harding, D. Palagin, K. Reuter, O. Dopfer, A. Fielicke
(See online at https://doi.org/10.1063/1.4894406) - Vibrational spectra and structures of neutral Si6X clusters (X = Be, B, C, N, O), Phys. Chem. Chem. Phys. 16, 22364-22372, 2014
N.X. Truong, M. Savoca, D.J. Harding, A. Fielicke, O. Dopfer
(See online at https://doi.org/10.1039/c4cp03414g) - Vibrational spectra and structures of neutral SinC clusters (n=3-8), Phys. Chem. Chem. Phys. 17, 18961-18970, 2015
N.X. Truong, M. Savoca, D.J. Harding, A. Fielicke, O. Dopfer
(See online at https://doi.org/10.1039/c5cp02588e) - Characterization of neutral boron-silicon clusters using infrared spectroscopy: The case of Si6B, Int. J. Mass Spectrom. 395, 1-6, 2016
N.X. Truong, M. Haertelt, B.K.A. Jaeger, S. Gewinner, W. Schöllkopf, A. Fielicke, O. Dopfer
(See online at https://doi.org/10.1016/j.ijms.2015.11.006) - Infrared Spectroscopy and Structures of Boron-Doped Silicon Clusters (SinBm, n=3-8, m=1-2), J. Phys. Chem. C. 121, 10767-10776, 2017
N.X. Truong, B.K.A. Jaeger, S. Gewinner, W. Schöllkopf, A. Fielicke, O. Dopfer
(See online at https://doi.org/10.1021/acs.jpcc.7b01290)
