Upon X-ray irradiation of o-nitrotoluene isolated in an argon matrix a
n aci-nitro isomer of the radical cation, characterized by UV-VIS and
IR spectroscopy, is formed as a result of spontaneous intramolecular h
ydrogen atom transfer between the o-methyl and nitro groups in the ion
ized molecule. Relative conformer distribution of the aci-nitro radica
l cation can be changed by photolysis with visible light. Along with t
he ionic species traces of neutral products were also seen.