Mode-Selective Excited-State Proton Transfer in 2-(2‘-Pyridyl)pyrrole Isolated in a Supersonic Jet

Excitation of 2-(2‘-pyridyl)pyrrole, a molecule with a weak intramolecular hydrogen bond, leads to proton transfer from the pyrrole to the pyridine nitrogen atom. The reaction occurs even for a cold molecule isolated in a molecular beam. The process is highly vibrational-mode-selective. Only those vibrations that strengthen the hydrogen bond are observed in the excitation spectrum of the tautomeric emission. On the contrary, excitation of out-of-plane modes hinders the reaction. Excited-state intramolecular proton transfer (ESIPT) in the jet occurs via tunneling. H/D isotope effects of 30−60 or higher are observed, with the values crucially dependent on the mode of vibrational excitation.