Molecular structure and conformational composition of 2-chloropropanal, a gas-phase electron-diffraction and ab initio investigation

Gas-phase electron diffraction (ED) data obtained at 72°C together with results from ab initio calculations have been used to determine the structure and conformational composition of 2-chloropropanal. The majority (66 ± 26%) of the molecules have a conformation where the CH₃ group is eclipsing the C=O bond (C-C-C=O torsion angle ∠φ₁ = 27 (7)°). Smaller amounts of a form (23 ± 28%) where H is eclipsing C=O (∠φ₁ = 109 (20)°) and a form (11 ± 26%) where Cl is eclipsing C=O (∠φ₁ = 240°) probably also is present in the gas phase. These results for the conformational composition are in fairly good agreement with the ab initio results calculated at the MP2/6-31G*//HF/6-31G* level. The results for the principal distances (r_g) and angles (∠_α) from a combined ED/ab initio study for the conformer where CH₃ is eclipsing C=O, with estimated 2σ uncertainties, are r(C-H)_methyl = 1.115 (10) Å, r(C-CO) = 1.529 (4) Å, r(C-CH₃) = 1.527 (4) Å, r(C=O) = 1.205 (4) Å, r(C-Cl) = 1.805 (6) Å, ∠C-C-H_methyl = 110.4° (ab initio value), ∠H-C-Cl = 108.1 (39)°, ∠OC-C-Cl = 108.9 (7)°, ∠C-C-C = 112.4 (15)°, ∠C-C=O = 124.3 (18)°, and ∠C₃-C₂-Cl = 109.2 (7)°.