Molecular structure and conformational composition of methyl chloroacetate: An electron-diffraction and ab initio molecular orbital investigation

The molecular structure and conformational composition of methyl chloroacetate, H2ClCC(O)OCH3, have been determined by gas-phase electron-diffraction (GED), using results from ab initio molecular orbital calculations (HF, MP2 and MP3/6-311+G(d,p)) to obtain constraints on some of the structural parameters. The molecules exist in the gas-phase at 25 °C as a mixture of two stable conformers: syn with CCl eclipsing CO and gauche with CH approximately eclipsing CO. In both of these conformers OCH3 is also eclipsing CO. The experimentally observed conformational composition at 25 °C was 36(8)% syn and 64(8)% gauche (parenthesised values are 2σ), corresponding to a free energy difference between conformers of ΔGexp° = 1.4(9) kJ/mol. The corresponding theoretical values obtained for ΔG° are 1.1 kJ/mol (HF), 2.3 kJ/mol (MP2), and 2.4 kJ/mol (MP3). The results for the principal distances (rh1) and angles (∠h1) for the major gauche conformer obtained from the combined GED/ab initio study (2σ uncertainties) are r(COCCl) = 1.502(9) Å, r(CH) = 1.084(6) Å (average value), r(CCl) = 1.782(4) Å, r(CO) = 1.213(4) Å, r(COO) = 1.346(4) Å, r(CH3O) = 1.468(10) Å, ∠CCCl = 110.0(6)°, ∠CCO = 124.7(6)°, ∠CCO = 108.3(10)°, ∠COC = 115.9(8)°, ϕ(ClCCO) = 111(2)°, ϕ(COCO) = 3(3)°.