Intramolecular hydrogen-atom tunneling and photoreaction mechanism of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices

► Photoreaction of 4-bromo-2-chloro-6-fluorophenol was investigated. ► Isomerization occurred by intramolecular hydrogen-atom tunneling. ► A five-membered ring ketene compound was produced by detachment of HCl. ► Conformation of a complex between the ketene and HCl was determined.