ChemInform Abstract: 2‐Bromo‐3‐chloro‐1‐propene: Gas Phase Molecular Structure and Conformational Composition as Determined by Electron Diffraction

Abstract

Abstract The molecular structure and conformational composition of 2-bromo-3-chloro-1-propene have been determined by gas-phase electron diffraction at nozzle temperatures of 20, 90 and 187°C. There is a mixture of two conformers in the gas phase with the halogen atoms anti or gauche to each other. The anti conformer (torsion angle τ = 0) is the most stable with 48(11), 45(9) and 38(9)% at 20, 90 and 187°C, respectively. This corresponds to an energy difference (with standard deviation) of Δ E = E g − E a = 3 ± 1 kJ mol −1 , and an entropy difference Δ S = 4 ± 3 J mol −1 K −1 . The geometry of the molecule at 20°C is described by the following r a and ∠ α parameters (error limits are 2σ including corrections for correlation among observations and other experimental uncertainties): r (CC) = 1.360(14) A, r (CC) = 1.491(16) A, r (CBr) = 1.911(11) A, r (CCl) = 1.803(9) A, r (CH)> = 1.139(30) A (average CH distance), ∠CCC ( anti ) = 128.4(1.0)°, ∠CCBr ( anti ) = 121.3(3.0)°, ∠CCCl ( anti ) = 113.0(2.6)°, = 112.8(4.2)°, τ ( gauche torsional angle) = 109.5(3.5)° and 2 > 1 2 (r.m.s. torsional amplitude for the anti form) = 10.6(3.6)°. The difference between bond angles in the anti and gauche forms were estimated by molecular mechanics calculations and kept constant at the values Δ∠CCC = ∠CCC ( anti ) — ∠CCC ( gauche ) = 2.4°, Δ∠CCBr = −0.8° and Δ∠CCCl = 0.7°.