Gas phase structures and conformational properties of 1-silabutane and 2-silabutane

The molecular structures of 1-silabutane 1 and 2-silabutane 2 have been determined by gas electron diffraction (GED). The conformational equilibrium of 1 and 2 was studied experimentally in the gas phase (GED) and by quantum chemical (QC) calculations (HF with 6-31G* basis sets, MP2 with 6-31G* and cc-pVTZ basis sets, and B3LYP with 6-311G* basis sets). The potential function for internal rotation around the central C-C bond in 1 resembles that for n-butane, whereas the rotation around the Si-C bond in 2 has much lower energy barriers. The experimental relative enthalpies ΔH° (gauche-anti) are 0.76(10) and 0.14(18) kcal mol^-1 for 1 and 2, respectively. The enthalpy difference for 2 is reproduced within the experimental uncertainties by all four QC methods. Only the MP2 method with a large basis set reproduces the enthalpy difference for 1 correctly. Taking the different multiplicities of anti and gauche conformers into account, the conformational composition of 1 was found to be 65(5)% anti and 35(5)% gauche. The conformational composition of 2 was found to be 43(9)% anti and 57(9)% gauche.