Photochemistry of 2-(methylamino)pyridine in a low-temperature argon matrix: Amino–imino tautomerism and rotational isomerism

Photoreaction of 2-(methylamino)pyridine in a low-temperature argon matrix was investigated by infrared spectroscopy and density functional theory calculation. The two amino tautomers with nearly isoenergetic conformation (TA and CA) around the C–N(HCH3) bond for 2-(methylamino)pyridine change into N-2(1H)-pyridinylidenemethanamine as the methyl-imino tautomers (TMI and CMI) by intramolecular hydrogen-atom (or proton) transfer upon UV irradiation (320 > λ ≥ 300 nm). The reverse tautomerism occurs by longer-wavelength light irradiation (370 > λ ≥ 340 nm), where only the amino tautomer TA is reproduced from the methyl-imino tautomer TMI.