3-Bromo-2-methyl-1-propene: molecular structure and conformation in the gas phase as determined by electron diffraction and molecular mechanics calculation

Abstract

A gas phase electron diffraction study of 3-bromo-2-methyl-1-propene shows that there is predominantly a gauche conformer present. Data recorded at 20 and 180°C show 4(8) and 5(4)% respectively of a second confomer with a planar heavy atom skeleton. The gauche structural results in terms of ra distances and ∠α angles at 20°C were found to be: r(CC) = 1.331(9) Å, r(CCH2Br) = 1.484(6) Å, r(CCH3) — r(CCH2Br) = 0.017 Å, (assumed), r(CBr) = 1.965(6) Å, ∠CCCH2Br = 121.5(0.7)°, ∠CCCH2Br — ∠CCCH3 = 0.7° (constraint from molecular mechanics calculation), ∠CCBr = 112.2(0.5)°, torsional angle = 112.5(2.2)°. Uncertainties are given as 2σ, where σ includes uncertainties due to correlation among observations, electron wavelength and other parameters used in the data reduction. The results obtained from the 180°C data agree very well with those given above. The molecular mechanics calculations yield information consistent with the experimental results.