Molecular structure and conformation of phenylacetyl chloride as obtained by gas-phase electron diffraction and theoretical calculations

The structure and conformation of phenylacetyl chloride, PhCH2(CO)Cl, has been determined by gas-phase electron diffraction (GED), using results from ab initio calculations (HF and MP2/6-311 + G(d,p)) to obtain restraints on some of the structural parameters. The molecules exist in the gas-phase (118 °C) as a mixture of two stable conformers; syn with CO eclipsing CalkylCphenyl and gauche with CO eclipsing CH. For both conformers the phenyl ring is about 90–120° out of the plane of the acetyl group. The experimentally observed conformational composition was 75(8)% gauche and 25(8)% syn (parenthesised values are 2σ). Using theoretical values for the entropy difference between conformers, including the fact that there are two identical gauche forms, this composition corresponds to an enthalpy difference of ΔH°(gauche–syn) = −0.8(12) kJ mol−1. The results for the principal distances (ra) and angles (∠h1) for the gauche conformer obtained from the combined GED/ab initio study (2σ uncertainties) are: r(CH)phenyl = 1.078(6) Å, r(CH)alkyl = 1.089(12) Å, r(CO) = 1.180(6) Å, r(CC)phenyl = 1.396(4) Å (average value), r(CphenylCalkyl) = 1.509(16) Å, r(CalkylCcarbonyl) = 1.513(12) Å, r(CCl) = 1.780 (6) Å, ∠CphenylCalkylCcarbonyl = 110.6(12)°, ∠CCO = 125.5(8)°, ∠CCCl = 113.3(6)°, ∠CCHphenyl = 119.9(2)° (average value), ∠CphenylCalkyl H = 110.8(2)° (average value), ϕ(CCCC) = 118(4)°, ϕ(CCCO) = 95(2)°.