ChemInform Abstract: MOLECULAR STRUCTURES OF SILACYCLOHEXANE AND SILACYCLOPENTANE AS DETERMINED BY GAS PHASE ELECTRON DIFFRACTION

Abstract

Abstract The molecular structures of silacyclopentane and silacyclohexane are determined by gas phase electron diffraction. Silacyclohexane is found to exist in a modified chair conformation which is somewhat flattened (relative to cyclohexane) at the silicon atom and somewhat puckered at the C 4 position. The SiC ( r g = 1.885(3) A) and CC ( r g = 1.550(3) A) bond lengths were found to be comparable with those observed in more highly strained polycyclic molecules such as 1-methyl-1-silaadamantane and 1-methyl-1-silabicyclo(2.2.1)heptane. Valence angles for the ring were found to be: ∠C 5 SiC 1 = 104.2(1.4)°, ∠SiC 1 C 2 = 110.6(0.6)°, ∠C 1 C 2 C 3 = 113.7(1.1)° while the observed dihedral angles were: τ(SiC) = 44.0(4.2)°, τ(C 1 C 2 ) = 57.3(2.0)° and τ(C 2 C 3 ) = 67.5(2.0)°. Combined analysis of electron diffraction and microwave spectroscopic data for silacyclopentane shows that the molecule exists in the C 2 or twist conformation with SiC and average CC bond lengths of 1.892(2) A and 1.549(3) A respectively. The valence angles obtained for the ring are: ∠C 4 SiC 1 = 96.3(0.3)°, ∠SiC 1 C 2 = 103.6(0.3)° and ∠C 1 C 2 C 3 = 108.4(0.7)°, while the ring dihedral angles are: τ(SiC) = 13.3(0.4)°, τ(C 1 C 2 ) = 36.1(1.0)° and τ(C 2 C 3 ) = 49.7(1.4)°. Molecular mechanics calculations are found to be helpful in interpreting the structures and conformations of these two molecules in terms of simple Bayer and Pitzer strain energy concepts.