Knowledge Bank

Infrared polarization spectroscopy is used to determine the alignments of the photoreagent and photoproducts after the ultraviolet photolysis of acetyl chloride. Matrix-isolated (argon) and neat samples of acetyl chloride are irradiated with polarized 266 nm light, producing complexes of ketene $(H2C=C=O)$ and HCl through a concerted elimination reaction. The alignment of the vibrational dipoles of the HCl and ketene products, and the remaining acetyl chloride is measured. The $\cos 2$ angular distributions allow the orientation of the molecules to be calculated relative to laboratory axis. Possible geometries for the reaction coordinate and the HCl-ketene complex are deduced form the determined orientations.