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The Cyclohexasilanes Si 6 H 11 X and Si 6 Me 11 X with X=F, Cl, Br and I: A Quantum Chemical and Raman Spectroscopic Investigation of a Multiple Conformer Problem
Author(s) -
Hölbling Margit,
Flock Michaela,
Hassler Karl
Publication year - 2007
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.200600731
Subject(s) - conformational isomerism , crystallography , chemistry , substituent , raman spectroscopy , basis set , cyclohexane conformation , ring (chemistry) , density functional theory , molecule , stereochemistry , computational chemistry , physics , hydrogen bond , organic chemistry , optics
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 ∼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 ∼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.