Conformational Structure of Gaseous 3-Chloropropanoyl Chloride by Electron Diffraction, Normal Coordinate Analysis, and ab Initio Molecular Orbital, and Density Functional Theory Calculations

The molecular and conformational structures of 3-chloropropanoyl chloride (CH2Cl−CH2−C(O)Cl) have
been studied by using gas-phase electron diffraction (GED) data obtained at 22 °C (295 K) and ab initio
molecular orbital (MO) and density functional theory (DFT) calculations up to the levels of MP4(SDQ) and
B3LYP using larger basis sets. Normal coordinate calculations (NCA) taking into account nonlinear vibrational
effects were also used in the analyses. The title compound may have up to four low-energy conformers in the
gas phase, labeled according to the position of each of the two chlorine atoms in relation to the CCC propanoyl
backbone, labeling the carbonyl chlorine torsion angle first:  AA, AG, GG, and GA; where A is anti (ideal
C−C−C−Cl torsion angle of approximately 180°) and G is gauche (ideal C−C−C−Cl torsion angle of
approximately 60°). It has been judged from the experimental GED data and the theoretical calculations, as
well as from previously published infrared (IR) studies on the molecule in both the liquid phase and in argon-trapped matrices at 10 K, that the gas phase consists of a mixture of at least three conformers:  AA (most
stable), AG, and GG, with the possibility of a smaller contribution (<10%) from the higher-energy GA form.
The GA conformer cannot be ruled out by the GED experimental data. Relevant structural parameter values
obtained from the GED least-squares refinements, with calculated ab initio MO MP2/6-31+G(2d,p) values
used as constraints, were as follows (AA values with estimated 2σ uncertainties):  Bond lengths (rh1):  r(C−C(O)) = 1.505(4) Å, r(C−CH2Cl) = 1.520(4) Å, r(CO) = 1.197(4) Å, r(C(O)-Cl) = 1.789(3) Å, and
r(C−Cl) = 1.782(3) Å. Bond angles (∠h1):  ∠CCC = 111.5(11)°, ∠CCO = 127.0(5)°, ∠CC(O)Cl = 112.5(3)°,
and ∠CCCl = 110.3(3)°. Torsion angles (φ(C−C) = φ(ClCCC)): for AA, φ1(C−C(O)) = φ2(C−CH2Cl) =
180° (assumed for true Cs symmetry); for AG, φ1(C−C(O)) = −140(5)°, φ2(C−CH2Cl) = 76(13)°; for GG,
φ1(C−C(O)) = 46(8)°, φ2(C−CH2Cl) = 77(14)°; for GA, φ1(C−C(O)) = 67.9° (assumed), φ2(C−CH2Cl) =
177.8° (assumed). The non-AA conformers all have chiral C1 symmetry with twice the statistical weight
(multiplicity) of Cs. The MP2/6-31+G(2d,p) calculated composition (%) based on the zero-point energy (ZPE)
corrected energy differences, and the statistical weights for conformers:  AA/AG/GG/GA = 28/35/28/9 was
assumed in the final GED refinement. The more recent literature concerning the title molecule, as well as for
several related molecules, has been examined and a survey has been attempted in the present article. The new
experimental results for 3-chloropropanoyl chloride are discussed and compared with the previously published
findings.