Molecular structures and conformational compositions of 2-chlorobutane and 2-bromobutane; an investigation using gas-phase electron-diffraction data and ab initio molecular orbital calculations

Abstract

The structure and conformational composition of 2-chlorobutane and 2-bromobutane have been studied by gas-phase electron diffraction (GED) at 25°C, together with ab initio molecular orbital calculations (HF/6-311+G(d,p)). These molecules may exist as three distinguishable conformers (G+, A, and G−). The symbols refer to anti (A) with a torsion angle Φ2(X8–C2–C3–C4) of about 180° and gauche (G+ and G−) with torsion angles Φ2(X8–C2–C3–C4) of about +60° and 300°(−60°), respectively. It was not possible; from our GED-data alone, to accurately determine the conformational composition because the distance distributions for two of the conformers (G+ and G−) are very similar. The conformational composition for 2-chlorobutane obtained from the ab initio calculations (G+ 62%, A 25% G− 13%) was found to fit the experimental data quite well. For 2-bromobutane the ab initio calculated conformational composition (G+ 58%, A 28% G− 14%) did not, however, fit the experimental data. Here a much better fit was obtained by using only 10% of the A conformer and using the relative energy for the two gauche conformers, as obtained in the ab initio molecular orbital calculations, to calculate the relative amounts of the two gauche forms (G+ 73%, A 10% G− 17%). The results for the principal distances (rg) and angles ∠α for the G+ conformer of 2-chlorobutane, with estimated 2σ uncertainties, obtained from the combined GED/ab initio study are: r(C1–C2)=1.524(3)Å,r(C2–C3)=1.528(3)Å,r(C3–C4)=1.539(3)Å,r(C–Cl)=1.812(3)Å,r(C–H)ave=1.098(4)Å, ∠C1C2C3=111.5(16)°, ∠C2C3C4=113.3(5)°, ∠C1C2C1=110.4(9)°. The results for the G+ conformer of 2-bromobutane are: r(C1–C2)=1.526(4)Å,r(C2–C3)=1.530(4)Å,r(C3–C4)=1.540(4)Å,r(C–Br)=1.982(5)Å,r(C–H)ave=1.111(8)Å, ∠C1C2C3=112.5(16)°, ∠C2C3C4=114.6(15)°, ∠C1C2Br=110.1(16)°. Only average values for r(C–C), r(C–H), ∠CCC, and ∠CCH could be determined in the least-squares refinements, the differences between these parameters in the same conformer, and between the different conformers, were kept constant at the values obtained in the ab initio molecular orbital calculations.