Molecular structure and conformational composition of 1,1-dichlorobutane: a gas-phase electron diffraction and ab initio investigation

Abstract

Gas-phase electron diffraction data obtained at 23°C, together with results from ab initio molecular orbital calculations ( HF 6-31 G(d) ). were used to determine the structure and conformational composition of 1,1-dichlorobutane. Of the five distinguishable conformers (AA, G + A, AG +, G + G + and G + G −), the G + A conformer was found to be the low-energy form, and the investigation also indicated that certain amounts of the AA and G + G − conformers might be present. The symbols describing the conformers refer to torsion about the C 1C 2 and C 2C 3 bonds, anti (A) with H 5C 1C 2C 3 and C 1C 2C 3C 4 torsion angles of 180° and gauche (G + or G −) with torsion angles of + 60° or 300° (−60°) respectively. The results for the principal distances ( r g) and angles (∠ α) from the combined electron diffraction/ab initio study for the G + A conformer, with estimated 2σ uncertainties, were as follows: r( C 1 C 2) = 1.521(4) A ̊ , r( C 2 C 3) = 1.539(4) A ̊ , r( C 3 C 4) = 1.546(4) A ̊ , r( C Cl 6) = 1.782(3) A ̊ , r( CCl 7) = 1.782(3) A ̊ , 〈r( CH)〉 = 1.106(6) A ̊ , ∠C 1C 2C 3 = 114.4(13)°, ∠C 2C 3C 4 = 112.5(13)°, ∠CCCl 6 = 110.4(7)°, ∠CCCl 7 = 111.9(7)°, 〈∠CCH〉 = 108.9(47)°. Only average values for r(CC), r(CCl), r(CH), ∠CCC, ∠CCX and ∠CCH were determined in the least-square refinements; the differences between the values for these parameters in the same conformer and between the different conformers were kept constant at the values obtained from the ab initio molecular orbital calculations.