Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

Abstract

The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.

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

“…[10] All geometry optimizations and minimum energy calculations, as well as the calculations of vibrational frequencies and IR and Raman intensities, were performed using either the Gaussian98 or the Gaussian03 package at the DFT level of theory. [14,15] The B3LYP density functional with the 6-31 + G* basis sets for H, C, Si, F, and Cl, [16] and the SDD basis sets incorporating quasi relativistic Stuttgart Dresden electron core potentials (ECPs) for Br and I were employed.…”

“…[10] For the twist minima the SiÀSi bonds are about 0.3 pm longer than for the chairs. The averaged SiSiSi angle of the chairs for 1 a-4 a is 111.7 AE 0.68, equal to Si 6 Me 12 (111.98) and Si 6 Me 11 H (111.78).…”

“…These differences were also detected for Si 6 Me 12 , where the equatorial SiÀC bond is longer, although only by 1 pm, and for Si 6 Me 11 H, where the equatorial SiÀH bond is longer by 2 pm. [10] In case of Si 6 Me 11 H, and further on generally, axial and equatorial SiÀX, SiÀH or SiÀC bonds are exclusively referred to as the bonds of the distinguished SiMeX or SiHX segment, respectively. For Si 6 Me 11 X we found, that the axial SiÀ X bond is longer for X = F and Cl by 0.5 and 2 pm, but shorter by 2 pm and 4 pm for X = Br and I, respectively.…”

“…[9] Results of a quantum chemical investigation of Si 6 Me 12 and Si 6 Me 11 H were published recently. [10] For Si 6 Me 12 a barrier for the ring inversion of 16 kJ mol À1 was found, and the corresponding twist and boat minima have relative energies of 8 and~11 kJ mol…”

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

“…X-ray structure analysis. Crystal data: C 16 A toluene solution of 1 (0.44 g; 1.0 mmol) was added to a toluene solution of potassium tert-butoxide (0.12 g; 1.05 mmol) and 18-crown-6 (0.28 g; 1.05 mmol) at room temperature in a glove-box. The reaction mixture was stirred overnight.…”