The molecular structure of 2-chloroacrolein studied by gas electron diffraction, vibrational spectroscopy and ab initio MO calculations

The molecular structure of 2-chloroacrolein was reinvestigated by means of gas-phase electron diffraction, ab initio MO calculations and vibrational analysis. From the MO calculations (MP2/6-31G*), the molecule was found to exist as two conformers: an anti form which is more stable than its syn counterpart by 0.3 kcal mol(-1). The corresponding populations at room temperature are 70% and 30%, respectively. An analysis of the experimental vibrational spectra was carried out. The ab initio force field was scaled to fit the experimental spectra. The data obtained were used to calculate vibrational amplitudes and perpendicular corrections required for further electron diffraction analysis. In the electron diffraction analysis, the bond length and bond angle differences predicted by ab initio calculations of the conformers were assumed. The vapour was assumed to consist of two conformers: exact anti form and another close to syn form. The torsional angle of rotation about the C-C bond in the latter was refined in two ways: the first refinement with perpendicular corrections and fixed amplitudes gave a dihedral angle in the range 160-180; the second, including both geometrical parameters and some amplitudes, resulted in a value of 160 for the dihedral angle. Conformational composition and geometrical parameters obtained from the electron diffraction analysis are in agreement with the results from ab initio calculations.