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Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study
Author(s) -
Tekautz Günter,
Binter Alexandra,
Hassler Karl,
Flock Michaela
Publication year - 2006
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500417
Subject(s) - chemistry , conformational isomerism , ab initio , raman spectroscopy , basis set , pseudorotation , ab initio quantum chemistry methods , crystallography , twist , enthalpy , density functional theory , ring (chemistry) , computational chemistry , molecular physics , molecule , physics , geometry , thermodynamics , mathematics , organic chemistry , optics
10.1002/cphc.200500417.abs The conformations of dodecamethylcyclohexasilane Si 6 Me 12 and undecamethylcyclohexasilane Si 6 Me 11 H 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 Si 6 Me 12 , 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 Si 6 (CH 3 ) 12 and Si 6 (CD 3 ) 12 fully corroborate these results. Below −40 °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 Si 6 (CH 3 ) 12 and 6.0±1.5 kJ mol − 1 for Si 6 (CD 3 ) 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 Si 6 Me 11 H, 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 °C only the axial and equatorial chairs are present, with an experimental Δ H ‐value of 0.46 kJ mol − 1 . With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental Δ H ‐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.