The cyclohexasilanes Si6H11X and Si6Me11X with X=F, Cl, Br and I: a quantum chemical and Raman spectroscopic investigation of a multiple conformer problem.
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.
Hölbling
Margit
M
Institute of Inorganic Chemistry, University of Technology, Stremayrgasse 16, 8010 Graz, Austria.
Flock
Michaela
M
Hassler
Karl
K