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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.