Add time:07/17/2019         Source:sciencedirect.com

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 ΔE = Eg − Ea = 3 ± 1 kJ mol−1, and an entropy difference ΔS = 4 ± 3 J mol−1 K−1. The geometry of the molecule at 20°C is described by the following ra and ∠α parameters (error limits are 2σ including corrections for correlation among observations and other experimental uncertainties): r(CC) = 1.360(14) Å, r(CC) = 1.491(16) Å, r(CBr) = 1.911(11) Å, r(CCl) = 1.803(9) Å, = 1.139(30) Å (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)°, <∠CCH> = 112.8(4.2)°, τ (gauche torsional angle) = 109.5(3.5)° and <(Δτ)2>12 (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°.

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