Fundamentals of Silicon Chemistry: Molecular States of Silicon‐Containing Compounds

Abstract

Silicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the "renaissance" of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic "molecular fingerprints" as well as the resulting benefit for the preparative chemist. The comparison of "equivalent" states of "chemically related" molecules is emphasized, based on perturbation arguments and supporting quantum-chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon "building blocks", the energy-dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of "silicon substituents" on molecular properties. Future-directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion-pair formation of silicon-substituted K systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.

Infrared spectrum of the disilane cation (Si2H6+) from Ar-tagging spectroscopy

“…The low-resolution photoelectron spectrum exhibits a progression in the symmetric SiH 3 umbrella mode measured to be n 2 (a 1g ) B 0.11 eV, which corresponds to B900 cm À1 . 45,46 The current calculations predict for this mode a significantly lower frequency of only B800 cm À1 (817 and 787 cm À1 at the MP2 and B3LYP level, respectively). As the vibrational assignment is probably correct and the calculations are accurate to better than AE20 cm À1 , the discrepancy is probably due to the large uncertainty in the photoelectron measurement arising from the low spectral resolution.…”

“…The fundamentals with a 2u (n 5/6 ) and e u (n 7-9 ) symmetry are IR active, whereas those with a 1g (n [1][2][3] ) and e g (n [10][11][12] As a result of the large geometry change upon ionization and the induced long Franck-Condon progressions in the totally symmetric SiH 3 umbrella and low-frequency Si-Si stretch modes, the He(I) photoelectron spectrum of Si 2 H 6 is broad (>1 eV) and essentially unresolved at the achieved spectral resolution (B200 cm À1 ), largely preventing vibrational characterization of the cation ground state. 44,45 The only vibrationally resolved feature that has been assigned to the symmetric SiH 3 umbrella mode is n 2 (a 1g ) B 0.11 eV (B900 cm À1 ). 45,46 Photoionization mass spectrometry of Si 2 H 6 yields an adiabatic ionization potential of AIE = 9.74(2) eV, and low appearance energies of r10.04(2) and r10.…”

“…44,45 The only vibrationally resolved feature that has been assigned to the symmetric SiH 3 umbrella mode is n 2 (a 1g ) B 0.11 eV (B900 cm À1 ). 45,46 Photoionization mass spectrometry of Si 2 H 6 yields an adiabatic ionization potential of AIE = 9.74(2) eV, and low appearance energies of r10.04(2) and r10. Corresponding geometrical, vibrational, and energetic parameters are listed in Tables 1 and 2.…”

Let’s Play with Valence Isomers: The Influence of Different Main Group Elements on Their Relative Stability

“…1 Even, excluding the diastereomers and enantiomers, a computer-aided procedure generated 217 possible C 6 H 6 isomers. 2 In 1966, in order to better classify all these isomers, Romanian chemist Balaban defined an interesting new type of isomeric relationship that he named ''valence isomerism. '' 3 Valence isomers of benzene were described by the formula (CH) 6 , each carbon bearing a single hydrogen.…”

Maximum hardness in P6 isomers

“…P6 is valence isoelectronic with C6&. Qing conventional valence rules, it is possible to draw 217 isomers of C6H6 [5], but the five valence isomers 1-5 are structures in which six CH units are connected by single and double bonds [6]. It is these CH units that P atoms should most resemble and, therefore, the five valence isomers of benzene should be the most likely structures for P6: Indeed, all five structures turned out to be real minima on the energy surfaces that we studied and the energies of the HOMO and LUMO and the HOMO-LUMO gap for the five structures all turned out to be the reasonable values that one might anticipate for stable molecules.…”