The molecular structure of 2-bromostyrene and (E)β-bromostyrene obtained by gas-phase electron diffraction and ab initio molecular orbital calculations

Abstract

The molecular structures of 2-bromostyrene (C6H4Br–CHCH2) and (E)β-bromostyrene (C6H5–CHCHBr) have been studied using gas-phase electron diffraction at 378 and 373K, respectively, together with ab initio molecular orbital calculations (HF/6-311G(d) and MP2/6-311G(d)). The experimental data for 2-bromostyrene are consistent with a non-planar model in which the vinyl group is rotated away from the bromine atom (τ(C8C7–C1–C2)≈155°). The presence of a small amount of a second conformer (τ(C8C7–C1–C2)≈54°) cannot be excluded. The ab initio calculations show the latter conformer to be a stable form, but 2.52kcal/mol higher in energy (MP2/6-311G(d)). The principle bond distances and valence angles obtained from least squares refinements of the electron-diffraction data are (rg and ∠α with 2σ uncertainties, including estimates of systematic errors and correlation in the experimental data): 〈r(C–H)〉=1.109 (13)Å (average value), r(C7C8)=1.328 (21)Å, 〈r(C–C)ph〉=1.405 (3)Å, r(C1–C7)=1.478 (11)Å, r(C2–Br)=1.913 (6)Å, ∠Br–C2–C3=117.2 (5)°, ∠C1–C7C8=130(4)°, ∠C7–C1–C2=122(1)° and τ(C8C7–C1–C2)=152 (20)°. (E)β-bromostyrene also exists in a non-planar form with the vinyl group rotated about 40° out of the plane of the phenyl group. The principle bond distances and angles (rg and ∠α) are: 〈r(C–H)〉=1.080(13)Å, r(C7C8)=1.344 (23)Å, 〈r(C–C)ph〉=1.396 (2)Å, r(C1–C7)=1.475 (12)Å, r(C8–Br)=1.884 (9)Å, ∠Br–C8C7=122 (2)°, ∠C1–C7C8=124 (2)°, ∠C7–C1–C2=123° (assumed value) and τ(C8C7–C1–C2)=40 (10)°.