2,3-dibromo-1-propene: gas phase molecular structure and conformational composition as determined by electron diffraction and molecular mechanics calculations

Abstract

From electron diffraction data recorded at 20, 90 and 180°C, 2,3-dibromo-1-propene was found to exist as a mixture of two conformers with the halogen atoms anti or gauche to each other. The percentages of anti conformer (torsion angle τ a = 0) at 20, 90 and 180°C were 46(7), 39(5) and 33(12), respectively, from which were calculated, with estimated standard deviation, an energy difference Δ E = E g — E a =4 ± 1 kJ mol −1 and an entropy difference Δ S = 8 ± 4 J mol −1 K −1. The following molecular structure ( r a bond distances and ∠ ga bond angles) was obtained from 90°C experiment with uncertainties estimated at 2σ: r (CH) = 1.114(37) Å, r(CC) = 1.333(13) Å, r(CC) = 1.480(14) Å, r(CBr) = 1.904(17) Å, r(=CBr) = 1.940(19) Å, ∠CCC( gauche) = 124.2(1.7)°, ∠CCBr( gauche) = 120.1(4.4)°, ∠CCBr( gauche) = 113.4(3.5)°, the values for the angles ∠CCC, ∠CCBr and ∠CCBr in the anti conformer were different from the gauche angles by an amount of 2.4, −0.6 and 1.9°, respectively. These differences were determined from molecular mechanics calculations. τ g (BrCCBr torsional angle for the gauche form) = 112.0(4.3)° and 〈(Δτ) 2〉 1 2 (r.m.s. torsional amplitude for the anti form) = 18.8(7.0)°.