Molecular structure of 1,3-dichloropropanone by gas-phase electron diffraction combined with ab initio calculations

Abstract

The molecular structure of 1,3-dichloropropanone (ClCH 2) 2CO, was determined by gas-phase electron diffraction (GED), augmented by ab initio MO calculations. The nozzle temperature in the GED experiment was about 365 K. Vibrational amplitudes and shrinkage corrections for the data analysis of GED were calculated from the harmonic force constants given by normal coordinate analysis. The C–C torsions were treated as large-amplitude vibrations in the data analyses. The potential function for the C–C torsional motion was approximated as V(φ 1,φ 2)=(V 1/2)(1− cos(φ 1))+(V 1/2)(1− cos(φ 2))+(V 3/2)(1− cos(3φ 1))+(V 3/2)(1− cos(3φ 2))+V 11 sin(φ 1) sin(φ 2)+V′ 11 cos(φ 1) cos(φ 2), where φ 1 and φ 2 are the dihedral angles of ClCCO for the two ClCH 2 groups. The values of V 1, V 3, V 11 and V′ 11 were determined to be −3.6, 4.8, −5.5, and 2.8 kJ mol −1, respectively. It was found that the anticlinal– anticlinal ( ac– ac) conformer was dominant, with a small population of the anticlinal– synperiplanar ( ac– sp) conformer present. The ac– ac conformer possesses C 2 symmetry, where φ 1 and φ 2 are both 116° and the ac– sp conformer has C 1 symmetry with the ClCCO dihedral angles of 128 and 12°. The principal bond distances and angles ( r g (Å) and ∠ α (°)) of the ac– ac conformer are: r(C–C)=1.526(3); r(CO)=1.210(2); r(C–Cl)=1.789(1); ∠CCO=121.0(4); ∠CCCl=110.2(4). The values in parentheses are three times the standard deviations. The ac– ac conformer is much more abundant than the other conformers in the gas phase, although the synperiplanar– synperiplanar conformer, where the two ClCCO angles are nearly equal to zero, is dominant in the crystal.