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, H 2ClC C( O) O CH 3, 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 C Cl eclipsing C O and gauche with C H approximately eclipsing C O. In both of these conformers O CH 3 is also eclipsing C O. The experimentally observed conformational composition at 25 °C was 36(8)% syn and 64(8)% gauche (parenthesised values are 2 σ), corresponding to a free energy difference between conformers of Δ G exp ° = 1.4(9) kJ/mol. The corresponding theoretical values obtained for Δ G° are 1.1 kJ/mol (HF), 2.3 kJ/mol (MP2), and 2.4 kJ/mol (MP3). The results for the principal distances ( r h1) and angles ( ∠ h1) for the major gauche conformer obtained from the combined GED/ ab initio study (2 σ uncertainties) are r(CO CCl) = 1.502(9) Å, r(C H) = 1.084(6) Å (average value), r(C Cl) = 1.782(4) Å, r(C O) = 1.213(4) Å, r(CO O) = 1.346(4) Å, r(CH 3 O) = 1.468(10) Å, ∠ C C Cl = 110.0(6)°, ∠ C C O = 124.7(6)°, ∠ C C O = 108.3(10)°, ∠ C O C = 115.9(8)°, ϕ(Cl C C O) = 111(2)°, ϕ(C O C O) = 3(3)°.