The Cyclohexasilanes Si₆H₁₁X and Si₆Me₁₁X with X=F, Cl, Br and I: A Quantum Chemical and Raman Spectroscopic Investigation of a Multiple Conformer Problem

Abstract

The conformers of the monohalocyclohexasilanes, Si(6)H(11)X (X=F, Cl, Br or I) and the haloundecamethylcyclohexasilanes, Si(6)Me(11)X (X=F, Cl, Br or I) are investigated by DFT calculations employing the B3LYP density functional and 6-31+G* basis sets for elements up to the third row, and SDD basis sets for heavier elements. Five minima are found for Si(6)H(11)X-the axial and equatorial chair conformers, with the substituent X either in an axial or equatorial position-and another three twisted structures. The equatorial chair conformer is the global minimum for the X=Cl, Br and I, the axial chair for X=F. The barrier for the ring inversion is approximately 13 kJ mol(-1) for all four compounds. Five minima closely related to those of Si(6)H(11)X are found for Si(6)Me(11)X. Again, the equatorial chair is the global minimum for X=Cl, Br and I, and the axial chair for X=F. Additionally, two symmetrical boat conformers are found as local minima on the potential energy surfaces for X=F, Cl and Br, but not for X=I. The barrier for the ring inversion is approximately 14-16 kJ mol(-1) for all compounds. The conformational equilibria for Si(6)Me(11)X in toluene solution are investigated using temperature dependent Raman spectroscopy. The wavenumber range of the stretching vibrations of the heavy atoms X and Si from 270-370 cm(-1) is analyzed. Using the van't Hoff relationship, the enthalpy differences between axial and equatorial chair conformers (H(ax-)H(eq.)) are 1.1 kJ mol(-1) for X=F, and 1.8 to 2.8 kJ mol(-1) for X=Cl, Br and I. Due to rapid interconversion, only a single Raman band originating from the "averaged" twist and boat conformers could be observed. Generally, reasonable agreement between the calculated relative energies and the experimentally determined values is found.

Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

“…Only the introduction of a second trimethylsilyl group at position 4 giving heptasilane Si 7 Me 16 (1), which further raises the number of 1,3-interactions, leads to a decrease of the number of predicted conformers to five, with just two of them having relative energies smaller than 6 kJmol -1 . We are confident that these results will not change substantially by using other methods, as the suitability of DFT for the calculation of conformer compositions of methylated oligosilanes has been demonstrated with the systems Si 6 Me 12 and Si 6 Me 11 X, X = H, F, Cl, Br and I [15,16]. Our calculations show that conformers which have a t + t + or t + t -pentasilane subunit with the two SiSiSiSi dihedral angles in the range ±180-150°are lowest in energy.…”

Ge6Me12 and Ge6H12 Cyclohexagermanes: Conformer Equilibria Studied by Raman Spectroscopy and Quantum Chemical Calculations

“…15 kJ mol –1 ) and Si 6 Me 12 (ca. 16 kJ mol –1 ) 13,14,26. Therefore, a calculation at the aug‐cc‐pVTZ level was not performed for Ge 6 Me 12 in the interest of conserving time (the barrier is 10.0 kJ mol –1 for Ge 6 H 12 at the B3LYP/aug‐cc‐pVTZ level).…”

“…Dynamic NMR spectroscopic studies for C 6 Me 12 and Si 6 Me 12 gave barriers for the chair‐to‐chair interconversion of 70 kJ mol –1 and 20 kJ mol –1 , respectively 7,12. In two Raman spectroscopic studies of Si 6 Me 12 and Si 6 Me 11 X (X = H, F, Cl, Br and I), we were able to identify chair and twist conformers and to determine their enthalpy differences 13,14. It turned out that boat conformations also represent minima for these cyclohexasilanes, and that the boat‐to‐twist interconversion is fast even on the timescale of Raman spectroscopy.…”